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I TA L I A N H A B I TAT S
Rocky slopes and screes
13
Italian habitats
Italian Ministry of the Environment and Territory Protection / Ministero dell’Ambiente e della Tutela del Territorio
Friuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine
I TA L I A N H A B I TAT S
Scientific coordinators
Alessandro Minelli · Sandro Ruffo · Fabio Stoch
Editorial commitee
Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
"Rocky slopes and screes · The hazard of a precarious life"
edited by Alessandro Minelli e Fabio Stoch
Texts
Paolo Audisio · Lucio Bonato · Margherita Solari · Nicola Surian · Marcello Tomaselli
In collaboration with
Alessandro Petraglia
English translation
Elena Calandruccio · Gabriel Walton
Illustrations
Roberto Zanella
Graphic design
Furio Colman
Rocky slopes and screes
The hazard of a precarious life
Photographs
Nevio Agostini 7, 40, 43/2, 49, 63/2, 121 · Archive Museo Friulano di Storia Naturale 33, 35, 70, 87, 99 ·
Archive Naturmedia 32, 36, 71/2 · Archive Naturmedia (Ferrari-Montanari) 34 · Archive Naturmedia
(Tomaselli) 29, 39, 41, 42, 43/3, 50, 51, 61, 64, 65, 67, 69/2, 71/1 · Paolo Audisio 26, 48, 56, 75, 77, 80,
81/2, 89/1, 98, 106, 107, 110, 129, 132, 144 · Alberto Bianzan 59, 60, 73, 74, 128, 131 ·
Alessandro Biscaccianti 82, 83 · Stefano Bossi 96, 111, 136 · Marco Cantonati 28 · Carlo Càssola 52 ·
Carlo Corradini 8 · Ulderica Da Pozzo 58, 122 · Vitantonio Dell'Orto 16, 22, 44, 72, 112, 116, 117/2, 118,
120, 124, 125, 126, 138 · Angelo Leandro Dreon 114/2 · Paolo Fontana 89/2, 91, 90, 100/2, 102 ·
Gianluca Governatori 66, 81/1, 103/2, 104, 113, 134 · Luca Lapini 114/1, 127/1 · Andrea Liberto 94 ·
Giuliano Mainardis 84, 101 103/1 · Riccardo Marchini 10, 18, 21, 43/1, 55, 62 · Michele Mendi 78 ·
Giuseppe Muscio 13 · Pierluigi Nimis 30, 31 · Paolo Paolucci 127/2 · Parodi Roberto 115, 123, 139 ·
Ivo Pecile 57, 79, 130, 135, 141 · Ermanno Quaggiotto 85, 86 · Nicola Surian 11, 15, 19, 20, 24 ·
Elido Turco 6, 27, 76 · Augusto Vigna Taglianti 46, 54, 63/1, 63/3, 68, 69/1, 88, 92, 109, 117/1, 137, 145, ·
Roberto Zucchini 133
© 2006 Museo Friulano di Storia Naturale, Udine, Italy
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or
by any means, without the prior permission in writing of the publishers.
ISBN 88 88192 27 1
Cover photo: High Cimoliana valley (Carnic Pre-Alps, Friuli Venezia Giulia, photo Ulderica Da Pozzo)
M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O
M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E
Contents
Italian habitats
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Paolo Audisio
Climatic and geomorphological aspects . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Nicola Surian
1
Caves and
karstic
phenomena
2
Springs and
spring
watercourses
3
Woodlands
of the Po
Plain
4
Sand dunes
and beaches
5
Mountain
streams
6
The
Mediterranean
maquis
Flora and vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Marcello Tomaselli
Animal life on cliffs and screes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Paolo Audisio · Lucio Bonato
7
Sea cliffs and
rocky
coastlines
8
Brackish
coastal lakes
9
Mountain
peat-bogs
10
Realms of
snow and ice
11
Pools, ponds
and
marshland
12
Arid
meadows
Invertebrates: taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Paolo Audisio
Vertebrates: taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Lucio Bonato
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Paolo Audisio · Lucio Bonato · Marcello Tomaselli
13
Rocky slopes
and screes
14
High-altitude
lakes
15
16
Beech forests The pelagic
of the
domain
Apennines
17
Volcanic
lakes
18
Mountain
conifer forests
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Margherita Solari
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
19
Seagrass
meadows
20
21
Subterranean Rivers and
waters
riverine
woodlands
22
23
Marine bioLagoons,
constructions estuaries
and deltas
24
Italian
habitats
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Introduction
PAOLO AUDISIO
Screes and cliffs in montane and
submontane areas are often such
inhospitable environments, that they
hinder or prevent colonisation by plants
and animals. Lack of nutrients in the
soil, the compactness and extreme
steepness of rocky slopes and cliffs, or,
alternatively, their opposite - loose
screes and rubble - all prevent most
plants from rooting.
The only exceptions are plants
adapted to life on vertical cliffs
(chasmophytes) and glareous plants
(living on debris slopes with roots in
gravelly, mobile soil).
Epigean terrestrial invertebrates
(those generally living on the surface
Scree with purple saxifrage (Saxifraga
oppositifolia)
of the ground) cannot find sufficiently
compact soil with enough water, and
therefore only a few specialised species are able to survive. However, many
vertebrates, especially birds, colonise these habitats permanently, settling
in cliff crevices and cracks, precisely to exploit the inaccessibility of these
areas, which protect them from potential predators and disturbing factors,
particularly during their mating season.
Several arthropods (especially butterflies, bugs and crickets), and a few coldblooded vertebrates (reptiles) visit these areas to bask in the sun and exploit
the high daytime temperature of the terrain.
Many phytophagous insects have adapted to developing on chasmophytes
and glareous plants, giving rise to very peculiar entomological communities.
The interaction of these complex and often extreme environmental conditions
enables only a few specialised, exclusive or preferential species to survive in
these habitats.
The Campanile (“Belltower”) in Val Montanaia (Carnic Pre-Alps, Friuli Venezia Giulia)
7
8
Moreover, in their recent evolutionary history, many of these species have
survived in isolated areas and gave rise to endemics; this fact have increased
their value for conservation.
To describe the importance of these habitats better, it is essential to
understand their role as fundamental ecological areas from the historicalbiogeographic viewpoint, and as true flora and fauna sanctuaries.
During the complex palaeoclimatic cycles of the late Tertiary and
Quaternary, particularly during the Ice Ages, the most important rocky
outcrops emerging in the montane and submontane areas of centralsouthern Europe were also essential and relatively xerothermic “islands”.
Here, large numbers of thermophilous animals and plants managed to
survive, often differentiating at species or subspecies level, taking refuge
from otherwise cold and inhospitable habitats extensively covered by ice
and tundra. This is the well-known nunatak effect, from the Inuit word
nun[ae]ttak used by Eskimo communities to indicate large, isolated rocks
that rise above perennial ice-caps.
Today, isolated rocky walls, crevices and screes are, in many areas, true
refuges for phytophagous insects and several species of birds. In addition,
due to the so-called “hedge effect”, many xerophilous insects and birds seek
Gruppo del Capolago in the Carnic Pre-Alps (Friuli Venezia Giulia)
shelter here when fleeing from open areas like meadows, arid grasslands,
magredi and xeric glades threatened or destroyed by man.
The bases of high cliffs and rocky peaks often contain rills and hygropetric
environments - tiny, often temporary springs trickling over surrounding rocks
- which, although minute, can support small but important hygrophilous and
mesophilous phyto- and entomocoenoses. In ecotonal environments, these
conditions may therefore diversify the local animal and plant communities
greatly.
Entomologists, ornithologists, herpetologists and botanists all know that
high, isolated cliffs and their screes, or rocky gorges near narrow river
valleys, historically spared from man’s intervention because of their
inaccessibility and lack of exploitable potential, can locally host surprisingly
large numbers of species, even though the environment surrounding them is
deeply affected by humans.
Despite the fact that mountain cliffs and screes are “difficult” environments,
they are very interesting to scientists, and are frequently subject to direct and
indirect man’s impact (such as quarries for the building industry, illegal waste
dumping, road building and excavations along rocky slopes, near mountain
passes and ridges, and including nets to protect roads and buildings from
falling rocky debris, and even sports like free-climbing). They therefore
deserve to be thoroughly analysed.
This book aims at providing a general view of the most interesting flora and
fauna living on montane and submontane cliffs and screes, focusing on the
role played by these habitats in Italian ecological networks, specific issues
regarding their conservation and management, and the severity of threats
jeopardising their biological quality, geomorphology and landscape.
The authors have chosen to treat animal and plant populations and their
slope and scree habitats, indicated as those rising from the upper mountain
horizon to higher altitudes (in Italy, between approximately 1000-1300 m and
2500-2800 m) i.e., above the tree line or at the upper mountain horizon itself.
Low-altitude isolated slopes, coastal cliffs (already treated in the Habitat
volume “Sea cliffs and rocky coastlines”) and all the magredi, taluses and
screes which are not in mountain areas are excluded, although they are
undoubtedly particular, interesting habitats from the naturalistic viewpoint.
However, the montane/submontane interface environment and high-altitude
submontane slopes in river gorges are at least partially treated, and a special
note is devoted to the highest river gorges, independently of their altitude, to
emphasise their great natural value and the essential role they play in the
conservation of an environmental mosaic of natural habitats.
9
Climatic and geomorphological aspects
NICOLA SURIAN
Cliffs and screes soon catch the eye
when we visit mountain environments.
They are easily identified because they
are well exposed, i.e., either poorly
covered by vegetation or totally bare.
At the same time, their size and
steepness are awesome, and we are
immediately attracted by them. They
are among the most inaccessible
areas to man, and while taluses require
all the trekkers’ attention, cliffs are only
accessible to expert climbers or along
specially equipped paths.
In mountain areas, although screes
and cliffs are found everywhere, in
some places they are certainly more
likely to form. Generally, they can be
Cliffs and screes are often closely associated
landforms (Lastoni di Formin, Ampezzo
found at both low and high altitudes,
Dolomites, Veneto)
but are more frequent near summits.
Screes and cliffs are often associated landforms, and deposits of debris are
frequently found just below steep rocky cliffs. These are not merely spatial
associations, but also genetic ones, as screes are the result of the crumbling
of the cliff above. Cliffs are forms of erosion, of rock weathering following
several processes, whereas screes are typical debris deposits. The concurrent
presence of cliffs and screes cannot be considered a rule, because debris may
be washed away by streams flowing at the foot of cliffs, or because cliffs can
be composed of rock that does not crumble into coarse rubble, thereby
hindering the formation of screes.
The formation and evolution of cliffs and screes depends on various
geological and geomorphological aspects. The differing types of rocks
which make up Italian mountains, and the variety of processes that can
affect rocks (gravitational, fluvial, glacial, etc.) give rise to a wide range of
Val Masino (Lombardy)
11
rocky faces of various sizes and shapes, and debris deposits with differing
morphological and particle size characteristics. It is essential to emphasise
here that many Italian rocky faces and debris deposits are the product of
human intervention, such as extensive quarrying and excavations of
materials for road building.
Lastly, a clarification on the words slope and scree, which are widely used
but which do not have equal geomorphological meaning. We shall see, for
example, that according to geomorphology, classic screes are called taluses
or debris slopes, whereas other screes, associated with different landforms,
are referred to as landslides and glacial deposits.
1
20
10
13
2
25
20
15
TEMPERATURE (°C)
30
TEMPERATURE (°C)
12
10
0
5
Debris slopes of glacial origin on Monte Canin (Julian Alps, Friuli Venezia Giulia)
-10
0
-2
TIME OF DAY
06.00
South
East
12.00
18.00
West
North
Air
temperature
TIME OF DAY
06.00
Air
temperature
12.00
18.00
00.00
Rock surface
temperature
3
1
Variations in air and rocky slope
temperatures according to
orientation of slopes
m
3000
2
Variations of rocky slope and
air temperature at 3050 m in the Alps
(1 = air temperature, 2 = slope temperature)
2000
3
Number of freeze-thaw cycles
in Pic du Midi de Bigorre (blue line)
and along the Italian versant of the
Western Alps (red line)
1000
0
100
200
300
CYCLES
■ Climatology
The climate in mountain areas with screes and cliffs is obviously similar to
that typical in mountains, with the addition of elements deriving from local
geographic factors and direct exposure to sunlight. The typical
characteristics of mountain climates are gradually lower temperatures (from
lower to higher altitudes), relatively abundant rainfall, which turns into snow at
high altitudes and, proceeding upwards, more intense sunlight. In other
words, the distinguishing factor is altitude, as atmosphere rarefies with
distance from sea level.
For a better analysis of the climate of rock faces and debris slopes, this
general climatic feature must also consider local geographical conditions,
especially those regarding the direction of flanks. For instance, valleys
facing east-west are exposed to very different sunlight from those facing
south or north.
When versants are particularly steep, this difference may become
considerable. North-facing versants are exposed to sunlight for far fewer
hours than south-facing ones, and there may even be areas which are never
14
directly illuminated by sunlight, especially in winter, when the sun’s rays are
more oblique.
In addition, screes and cliffs are covered by scarce vegetation because plants
find little soil in which to root, and may even be bare, with very low humidity.
This reduces their thermal capacity and exposes them to the action of winds.
Low thermal capacity causes rapid heating during the day and equally rapid
cooling at night, partly because mountain air is rarefied and therefore contains
less water vapour.
Just like desert environments, the mountains have a marked temperature
range between day and night. Their exposure to winds may not be important
as far as geomorphological processes are concerned, but it may be an
important climatic factor from the ecological viewpoint.
One particularly interesting climatic factor that explains the evolution of rocky
cliffs are freeze-thaw cycles, which deeply influence the disintegration of rock.
These cycles, which are marked by the periodic passage of ground or rock
temperature to the critical value of 0°C, depend on altitude and local
conditions (exposure of the slope and direct exposure of rock).
These cycles become more frequent as altitude increases, but only up to a
certain altitude - at about 2000-2200 m in the Western Alps - above which they
start decreasing because temperatures are already lower and thaw is unlikely.
Local conditions can give rise to greater or lower numbers of cycles. For
instance, on a north-facing versant that is frozen for longer periods than one
facing another direction, freeze-thaw cycles are less frequent, and the
situation is reversed along rocky cliffs exposed to sunlight, which heat up and
cool down very quickly during the day.
■ Geomorphology
Cliffs. Cliffs are subvertical or very steep rocky faces. They may look
homogeneous or very irregular, according to the rocks composing them. Very
resistant, compact rocks, such as granite in Mont Blanc or quartz-diorite and
tonalite in the Adamello Massif give rise to very uniform cliffs. Stratified rock,
i.e., alternations of rocks with different resistance to weathering, produce
irregular versants, like the tiers often found in the Dolomites. The formation of
cliffs depends on particular geological conditions and on various phenomena
and processes acting at different times and in different ways. Particular
geological conditions and, more precisely, the presence of certain types of
rocks, are essential requirements for the formation of rocky cliffs.
Rocks that can potentially give rise to cliffs are geomorphologically known as
hard rocks, because they withstand erosion better than so-called soft rocks.
Although the words hard and soft are relative, in Italian mountains hard rocks
are limestone, dolomite, sandstone, granite and gneiss. In the Apennines, cliffs
cannot be found on outcropping clay or sand, but they form on sandstone,
flysch (stratified rock formations composed of alternating resistant and easily
erodible materials), ophiolite (basalt, gabbro, etc.) and limestone.
■ Hydrology
Although rainfall is quite heavy in the Alps and Apennines, for different reasons
cliffs and screes almost totally lack surface water. Water cannot accumulate
on cliffs, due to their verticality. There may be small local areas containing
water, particularly in fractured surfaces like ridges, or runoff where rocks have
varying degrees of permeability. Screes are composed of coarse debris,
ranging in size from a few centimetres to a metre, which favours rain seepage
underground, thus hindering surface wash and accumulation of water at levels
just beneath the surface. The situation changes in landslide and glacial
deposits, i.e., screes composed of coarse debris mixed with finer material
(sand, silt and clay). Here, water may be found both on the surface and in the
underlying rock.
Rocky face recently affected by landslide
15
16
Cliff at Monte Vallassa (Ligurian-Piedmontese Apennines)
Slopes forming on ophiolite are found
in the Ligurian-Emilian Apennines, and
an example of a limestone slopes may
be seen at Pietra di Bismantova, in the
Reggio Emilia Apennines. Another
important geological factor is the
position of rocks, i.e., how they are
spatially arranged. Rock faces may
have different gradients, from
Evolution of a slope affected by overturn
subhorizontal to subvertical, and this
inclination can follow the gradient of
the versant or not. Favourable
conditions for the formation of steep
slopes are subhorizontal positions,
with alternating hard and soft rocks,
and
infacing
slope
positions.
Examples of the former type are very
frequent in the Dolomites, where
limestone and dolomite alternate with
Transverse cracks along a versant with head
an easily erodible rock like marl. Where
scarps
hard and soft rocks alternate,
weathering is very selective, giving rise to tiered versants with successions of
very steep cliffs produced by hard rock and gently sloping ones.
Of the processes that cause the evolution of a slope and the formation of a
cliff, a very important one in mountain environments is the freeze-thaw cycle,
as mentioned above. This gives rise to seasonal soil creep, whereby water
penetrates the rock as it thaw, then increases in volume as it freezes, so that
the pressure of ice increases, widening pores and fissures in the rock (frost
wedging). Rock fractures therefore depend not only on temperature variations,
but also on the amount of water in the rock and its fracture condition. Gravity
causes fragments produced by frost wedging to roll down the slope and
accumulate at its foot. Although this phenomenon occurs continually, it is not
visible to the naked eye, as the falling fragments are very small.
The situation is quite different when large pieces of material occasionally
detach and roll down. This is a true landslide. Landslides on subvertical or very
steep slopes are called rockfalls and overturns. When rockfalls occur, the
material detached from the slope travels in air before falling on to the slope
again, or at its foot. In overturning landslides, the rocky mass rolls from the
base. The effects produced by landslides depend on the volume of falling
17
18
rock, but they are certainly more visible than those produced by single
fragments or blocks of stone. A rocky slope which has recently been affected
by landslide is visually “new”, i.e., it has a different colour from the other
versants, the colours of which have been modified by alteration processes.
The evolution of a steep rocky face over a longer time-span, i.e., thousands,
hundreds of thousands, or even millions of years, depends not only on the
above-mentioned phenomena (soil creep, fall of debris, and true landslides),
but also on other processes. Tectonic activity, for example, causes rock
deformation and uplift, and is therefore the primary, necessary phenomenon
causing gradients and mountains containing rocky slopes. Mountains formed
by tectonic activity are moulded by gravitational, glacial and fluvial processes.
In the Pleistocene, for instance, great masses of ice constantly moved across
the Alpine valleys, moulding the versants of the valleys themselves. In warmer,
interglacial epochs, like the present one which started about 10,000 years ago,
streams flowed across valleys. The versants of Alpine valleys are therefore the
result of several gravitational, glacial and fluvial processes which occurred at
different times, either separately or together. This is why some transversal
Alpine valleys, which have steep versants, are called canyons, although they
are not only river valleys, but also the product of erosion by ice.
Because Italian mountains vary considerably from the viewpoints of geology
(type of rock, tectonics) and geomorphology (shaping processes), they feature
different types of cliffs. Unlike many Alpine cliffs, those in the Apennines, Sicily
and Sardinia are less well-known. In the Apennines, imposing cliffs can be
found in the Gran Sasso, examples being the northern versant of Monte
Camicia, with its 1200 m of rocky face, and Monte Porrara in the Majella.
Although Sardinia does not have very high mountains (they are generally less
than 2000 m high), it does have interesting rocky cliffs. The Supramonte
limestone contains several canyons (cordula in Sardinian dialect), and
imposing peaks like Punta Carabidda and Punta Cusidore.
Valley of obvious glacial origin with subvertical walls (Val Gardena, South Tyrol)
Steep rocky face with a debris slide at its foot
Screes. We have already mentioned the close association between slopes
and screes, the latter being deposits of material detached from slopes. The
characteristics of screes (their gradient, particle size of rocks, etc.) depend on
various factors, the most important of which are:
● type of process causing weathering, transport and accumulation of
material;
● characteristics of the rock composing the versant;
● pre-existing topography.
19
20
Debris slopes and cones. In mountain environments, these are typical
landforms at the base of rocky slopes. Debris slopes are masses of debris
accumulating at the foot of uniform, continual rock faces; debris cones derive
from fractured, crested slopes, and sediments scatter in fan-like shapes at the
bottom of these fractures.
The rocky sediments produced by the weathering of a rocky slope fall by
gravity and accumulate at its foot. The fragments then undergo particle size
selection, as larger stones gather more energy and roll further down the
versant. The surfaces of debris slopes and cones have gradients between 30
and 35°, also called the angle of repose. Gradients depend on the shape and
size of sediments which, in turn, depend on the type of rock from which they
come. For example, limestone produces rough, angular debris, and schist
flattened fragments. The deposited material is, however, very loose. Its
instability is perceptible when trekking along a path. Debris starts rolling
downwards at each step we take.
In addition to gravity, other processes explain the shape of debris slopes and
cones. In summer, surface soil wash, which derives from high-altitude
snowmelt or heavy storms, may carry finer sediments contained in debris
slopes (creep). Sand and gravel from the upper part of the debris slope are
deposited further down. Surface soil wash therefore flattens the profile of the
slope. In winter, when debris slopes are covered with snow, rocky debris may
glide smoothly downwards on the snow, giving rise to elongated ridges
parallel to the slope, which are called roches moutonnées.
Debris slopes contain various types of material, with different colours. The
hues are given by the types of rocks involved. The colours of debris slopes are
less uniform when they are covered by vegetation, indicating that a certain
portion of the debris slope has stabilised. This by no means indicates that the
slope will never become active again; it simply shows it has been inactive for
some time. Debris slopes can actually stabilise for quite long periods. To
determine the length of slope inactivity, methods like lichen monitoring,
dendrochronology (the study of tree-ring growth) and carbon dating are used.
Lichen monitoring is the study of the size of lichens, based on the assumption
that these symbioses of algae and fungi colonise surfaces as soon as they
stabilise, and that their growth is constant in time. Dendrochronology counts
Sediment of a debris cone: large clasts accumulate at base of slope
The smooth, steep surface of a debris slope
Listed below are the most typical screes, formed as a result of the collapse of
rock fragments, as well as other types caused by gravitational, glacial and
periglacial processes.
21
and analyses tree-rings, which annually grow in pairs, one darker than the
other. Carbon dating, which has frequently been adopted for Quaternary
deposits, is used when accumulated debris contains organic matter (wood,
peat, bones, etc.).
22
Instability of deeply fractured rocky slopes causes non-classified debris slides
Landslide deposits. Downslope movements involving large amounts of material
rather than single fragments or blocks, are called landslides. They affect both
rocky slopes and loose, coarse or fine material. Here, we deal only with
landslides giving rise to screes. When large amounts of rock detach from slopes,
there are several types of downslope movement: rockfalls occur when at least
part of the movement is in free fall; overturns when blocks of rock capsize, and
rockslides when the rock moves down along the slope. The occurrence of any
one of these movements depends on the type of rock and particularly on their
jointing and fracturing. For instance, deep scarps crossing the slope are likely to
produce overturning landslides, and inclined stratified bedrock generally causes
rockslides. Downslope movement crumbles the rock in various ways, producing
sometimes heterogeneous debris, which is usually coarse and angular,
occasionally compact. In the former case, landslide deposits are very similar to
debris slopes, although they are generally more irregular and composed of larger
fragments. In the latter case, debris deposits do not resemble screes at all, and
may easily be confused with local outcropping rock.
In the Alps, rockfalls and slides occurred in the late Pleistocene (between
15,000 and 10,000 years ago), following the retreat of the large glaciers that had
filled the Alpine valleys. These phenomena had a severe impact on the
environment, sometimes clogging valleys temporarily or even permanently.
These landslide deposits are still visible today, examples being the Lavini di
Marco in Val d’Adige, and the Masiere di Vedana near Belluno. Although these
large landslides are in fact huge screes, they look slightly different from debris
slopes. Lengthy debris alteration on some of these deposits has produced soil,
which in turn has favoured the development of vegetation.
Other types of landslides that produce screes are debris flows. In this case,
debris movement is not only caused by gravity, but also by water. Debris flows
generally occur after intense rainfall, even if it is short-lived (a few minutes of
heavy rain are sufficient to trigger movement). Debris flows forming screes
generally start in debris slopes and landslide deposits, that is, they simply
cause previously fallen material to slide even further down the slope. In the
lowest parts of the versants or, more generally, where topographic conditions
favour lateral expansion, debris cones form. An example of a large debris cone
is the Rivoli Bianchi of Tolmezzo (Carnic Alps).
23
Glacial deposits. Another type of scree can be found at higher altitudes in the
Alpine chain (seldom in the Apennines). It consists of debris carried and
deposited by glaciers. The Italian Alpine areas with the greatest number of
glaciers are the Mont Blanc, Ortles-Cevedale and Adamello-Presanella
chains.
Glaciers are “conveyor-belts” carrying till, a mixture of rocky fragments of
sizes ranging from huge boulders to fine sediments like clay and silt. The
transported material is therefore deposited in different areas, along the sides
of the glacier, at its foot, or along the slope. Sometimes till accumulated by
glaciers does not look like a scree, especially when fine sediment is involved;
at other times, deposits and glacial forms show typical scree features. The
latter may now be lateral moraines created by ancient glaciers as they
gradually moved along. Heavy materials were pushed to either side, leaving
what are now elongated, sometimes steep ridges, parallel to the original ice
mass. They are found both near glaciers and further down the valley
(sometimes several kilometres from the glaciers themselves). End-moraines
at lower altitudes formed during the so-called “Little Ice Age”, a period of
renewed glaciation which took place between the 16th and the early 19th
centuries. The moraines of the Little Ice Age are recent landforms barely
colonised by plants, unlike older, Pleistocene moraines, which are gently
sloping and covered with vegetation.
24
An example of a rock glacier, a typical high-altitude Alpine landform
Rock glaciers. At high altitudes, where periglacial processes are intense, rock
glaciers are very common. They are tongues or lobes of angular rocky debris
ranging in length from several metres to a few kilometres, with terminal
hummocks and ridges. Material contained in rock glaciers originated from
debris slopes and glacial deposits. One landform association often observed
is rocky wall - debris slope - rock glacier.
The surface of rock glaciers is uneven, undulating, with notches and
indentations. Coarse material is deposited on the surface, and fine sediments
are found underneath. Rock glaciers are particular not only because of their
typical morphology, but also because they contain interstitial ice, deriving
either from a melting glacier later covered by debris, or by frozen seepage.
The presence of ice causes rock glaciers to move, changing shape as their
ice contents deform and the water at their base is more or less abundant.
However, movement is slow, and the highest velocity of a rock glacier never
exceeds one metre a year. Rock glaciers become inactive (stationary) when
the ice within them melts. Evidence of inactivity is given by overall sunken
forms and plant cover.
25
Flora and vegetation
MARCELLO TOMASELLI
■ Plant life on cliffs
Although cliff environments are
certainly unfavourable to plant life, the
number of organisms living in these
habitats is surprising, thanks both for
their diversity and to the presence of
unexpected systematic groups. Cliffs
may host algae, lichens, bryophytes,
ferns and various angiosperms.
Unexpectedly, plant diversity is
particularly high in rocky mountain
environments, especially along rock
walls above the tree line, and in
overhangs at lower altitudes, near the
typical cirques of the Pre-Alps and
Apennines. It is surprising to see how
Yellow saxifrage (Saxifraga aizoides) and fairy’s
thimble (Campanula cochleariifolia)
plants manage not only to grow but
also to produce often brightly
coloured, vigorously blooming flowers. From the phytogeographic viewpoint,
these environments are sometimes more important than other high-altitude
habitats. This is due to the role they played as refuge habitats during periods
of glaciation, ensuring the preservation of ancient plant species. The various
plants that stably inhabit cliffs occupy true micro-environments, establishing
very close relationships with the rocky slope, according to the ways in which
they manage to grow on it.
It is therefore impossible to identify limiting environmental parameters
common to all cliff plant communities. It is easier to focus on the
characteristics of the rocky slope environment and the ways in which it
affects plant life by analysing the groups of organisms that live, grow and
reproduce in this seemingly homogeneous habitat, which is actually very
particular and diversified.
Alyssoides (Alyssoides utriculata) and Mt. Nebrodi joint pine (Ephedra major)
27
28
■ Algae and lichens
Epilithic algae. Epilithic algae, those
growing and reproducing on stone,
are not very numerous and are divided
into three main groups. The first and
most numerous includes prokaryotic
algae of the Cyanophyta division, also
called cyanobacteria, cyanophyceae
or blue-green algae. The second and
third groups are composed of
eukaryotic algae of the Chlorophyta
The chlorophyte Trentepohlia (200x)
division, also known as green algae,
and Bacillariophyta or diatoms.
Among blue-green algae, the most frequent epilithic species are those of the
genera Gloeocapsa, Scytonema, Stigonema, Calothrix and Nostoc. Among
green algae, one genus including epilithic species is Trentepohlia, and epilithic
diatoms are found among the genera Tabellaria and Melosira.
Unlike other lithophilous plants, algae can grow on bare, compact stone devoid of even minimal cracks - by adhering to the rocky surface (esolithic
algae) or penetrating it (endolithic algae).
Epilithic algae can be found on any rocky substrate. Some species are
associated with silica (Gloeocapsa ralfsiana), others are exclusive to
limestone (Gloeocapsa sanguinea). Colonisation by epilithic algae is caused
by seepage on to the rock surface. Seepage must last for at least a few
weeks, enabling the algae to complete their life-cycle. These conditions
generally occur along north-facing versants. Growing epilithic cyanobacteria
turn reddish or blue-green. When seepage ceases and the algae become
dehydrated, surviving in dormant conditions, their thalli become darker, and
they can be identified as vertical bluish-black stripes that groove the rocky
face along the seepage line. A few cyanobacteria are able to colonise southfacing versants because they can withstand drought as well as the great daily
temperature variations of such locations.
Blue-green and green algae (Gloeocapsa, Aphanothece and Trentepohlia)
living on limestone substrates are endolithic, and creep within the rocks,
although only to a few millimetres’ depth. This is due to their capacity for
“melting” rock by releasing carbon dioxide through respiration, thus
transforming the insoluble calcium carbonate of rocks into soluble
bicarbonate. Algae can therefore burrow microscopic niches inside the rock
walls, initiating the process of epigean karstification on limestone. A few
endolithic algae can tolerate excessive calcium by accumulating it in the
form of calcium carbonate crystals in the mucilaginous membrane that
envelops their cells.
Epilithic lichens. A few epilithic algae of the cyanobacterial and chlorophytic
divisions cannot colonise rocky surfaces autonomously, and need a close
structural and functional association with species of fungi. The result of this
association, which is quite frequent in nature also in other types of
environments, is called lichen symbiosis, and the organisms produced by this
close interaction are called lichens. In lichen associations, algae are the
partners that can use light as a source of energy (phycobionts), and fungi are
the components (mycobionts) which use the organic matter produced by the
algae as a source of energy.
Most epilithic lichens are crustose, and only a few are foliose. Their names
depend on their exterior form. Crustose lichens, which grow very slowly,
resemble thin crusts of varying shapes and sizes that adhere to substrates
with the underside of all their thalli. Foliose lichens have laminar, lobate thalli
which cling to surfaces by means of interlaced hyphae (stalks) called rhizinas.
Cliff lichen
29
30
Unlike epilithic algae, lichens that colonise slopes do not simply attach
themselves to the surface, forming patinas, but are able to enter through
microscopic rock fissures by means of their densely intricate rhizinas. They
are so firmly rooted that it is almost impossible to remove them barehanded. Lichenologists must resort to geologists’ hammers to extract them
for analysis.
Although epilithic lichen species are quite numerous, no census has yet been
carried out. Among epilithic crustose lichens are manna lichens of the genera
Rhizocarpon, Lecidea, Lecanora, Protoblastenia, Caloplaca, Umbilicaria and
Acarospora and, in the Verrucariales order, the genera Verrucaria, Polyblastia,
Staurothele and Thelidium. Most crustose lichens, despite their intimate
contact with surfaces, are esolithic (and therefore do not penetrate rocks).
Limestone is the only type of rock hosting lichens that dissolve calcium
carbonate by means of acid secretions and burrow niches underneath the
rock surface, from which only the fruiting body of the mycobiont emerges.
These crustose lichens are therefore called internal or endolithic. They include
the species Petractis clausa, Protoblastenia immersa and Staurothele
immersa, names which refer to their endolithic position. Lichens are among
the few living forms that can be found at very high altitudes, where
Protoblastenia incrustans
temperatures are extremely low and periods of drought are long, due to frozen
water. Some lichen species can survive at temperatures as low as -196°C
without particular damage and can still assimilate carbon dioxide at -24°C. For
a few species, the best temperature for photosynthesis ranges between 0 and
-10°C. Due to their great capacity for adaptation to low temperatures, lichens
are among the very few organisms that can be found on rocks in the snow
belt.
Cliff lichens carpet rocky surfaces with variable but continuous covers, often
forming closed communities or phytocoenoses which exclusively contain
these types of organisms. They are thus called cliff lichen vegetation, and have
been analysed and classified according to the principles and methods of
classical phytosociology.
Lichens are usually divided into two main classes. The first is
Protoblastenietea immersae, which includes crustose lichen communities
colonising limestone, and the second is Rhizocarpetea geographicae, which
comprises lichens growing on silica. Further down the classification hierarchy
(orders, associations), they are distinguished by their floral composition, type
of growth on rock (esolithic or endolithic) and formation (crustose or foliose).
Another distinguishing factor for lichen classification is rock humidity.
Rhizocarpon geographicum
31
32
Grimmia, a genus of epilithic moss
The genus Grimmia (fringe moss) is a
perfect example of epilithic moss. It
comprises 150 species (41 in Europe,
33 in Italy), generally found in areas
with temperate climate, but also in
Alaska, Tierra del Fuego, Siberia, and
mountain areas in Indonesia and
Central Africa.
Although the growth of fringe moss is
generally quite typical, it is not
determined genetically, and may
change according to the substrate,
shade, humidity and exposure where it
develops. Species that usually grow in
the shape of hemispherical
cushions (G. alpestris, G.
montana) may develop
tufts of hair near
crevices in the rock,
just as other
species (for
example, G. ovata),
which usually look
like scattered,
dense blackish hair
in south-facing rock
fissures, may sometimes
produce loose, green
cushions on humid, north-facing
versants.
Except for G. pitardii, which grows in
more evolved soil, all the European
species of Grimmia colonise rocks of
various types, acid (G. montana,
G. alpestris) or base-rich (G.
tergestina, G. anodon, G. crinita).
Species that grow on base-rich rocks
live in warm, dry habitats, and
acidophilic species prefer cold,
humid areas.
Fringe mosses grow in various
colours, and European species are
usually bluish-green, like G. alpestris
and G. caespiticia, reddish-brown
Alessandro Petraglia
(G. torquata, G. teretinervis) and
nearly black, like G. pitardii,
G. sessitana and G. atrata. Their
colour is due to exposure to sunlight,
as moss cushions growing in sunny
areas are normally yellowish-brown
or even bluish, whereas those in
shady habitats are dark green.
One of the most typical features of
Grimmia is the white hairs forming on
their leaftips. The hairs reduce
evaporation during drought, capture
dew, humidity and airborne particles,
thus carrying out an essential
function for species living in
exposed, dry habitats.
Slightly different
habitats may host the
same species with
hairy or non-hairy
forms, just as
different forms of
the same species
living in the same
habitat may grow
short or very long
hairs. Species such as
G. arenaria, G. curviseta and
G. crinita, which colonise dry, sunny
habitats, have apex hairs that are
even longer than their leaves.
According to geographic distribution
and habitat frequency, there are
seven known Italian species that
need to be protected. G. apiculata,
G. limprichtii and G. teretinervis are
found only in alpine areas and are
extremely vulnerable to pollution and
restriction of their habitats. G.
anomala, G. arenaria and G. atrata
are found in a few exposed habitats,
and G. pilosissima is a rare species
of the Mediterranean mountains
which, in Italy, grows only in Sardinia.
Epilithic bryophytes. Both mosses
and liverworts, i.e., bryophytes, are
well-adapted to life on slopes. There
are therefore several species of slope
bryophytes, divided into various
families. The two orders of mosses with
the greatest number of species living
on slopes are Grimmiales (genera
Grimmia, Racomitrium, Schistidium,
etc.), and Pottiales (genera Tortella and
Tortula). In the Andreaeales, the only
genus growing on rock is Andreaea.
Among liverworts, there are several
genera with species developing on
humid rocks in shaded areas, like
Preissia, Pellia, Metzgeria, Marsupella,
and others.
Tortella tortuosa
Bryophytes attach themselves to
rocks in three different ways. Some species closely adhere to compact rock
through their dense felts of rhizoids, just like epilithic crustose lichens. Other
bryophytes require very thin rock fissures. The base of the moss felt
containing rhizoids creeps into the fissures and roots the moss firmly. This is
a typical feature of chasmophytic mosses which, as we shall see later on, is
developed by superior plants. Lastly, there is a large group of bryophytes that
can attach themselves to rock only if it is covered by an even minimal amount
of debris.
When colonisation begins, debris is either carried by rocky fragments falling
from above, or by winds. Later, when the moss cover has become firmly
attached, the mossy carpet itself can cause disintegration of the substrate,
which gives rise to additional debris. As colonisation continues, the moss mat
grows, causing the formation of decomposing organic matter deriving from
the decayed parts of the mat itself. Bryophytes adopting this type of
colonisation are called comose.
The development of bryophytes on rocks is due to ecological parameters,
the most important of which is the chemical composition of rocks.
Bryophytes include many acidophilic cliff species, such as those of the
genera Andreaea and Grimmia, and the cushion moss Oreas martiana. There
are also species growing on base-rich substrates, like Hypnum dolomiticum
and Barbula bicolor.
33
34
Bryophytes and pteridophytes on wet cliffs
Plant species including bryophytes
and pteridophytes growing on humid
cliffs with trickling water belong to the
phytosociological class Adiantetea.
In these areas, rills contain
carbonates that favour the
development of typical tuff deposits.
They are colonised by dense,
compact bryophytic layers containing
pteridophytes, especially maidenhair
(Adiantum capillus-veneris). This type
of vegetation is typical of cirque
environments at low altitudes in the
Mediterranean area.
At higher altitudes, the class
Adiantetea is replaced by the class
Montio-Cardaminetea, whose
communities are better suited to
harsh conditions.
As already noted, humid cliffs are
colonised by communities of the class
Adiantetea, which replace the
chasmophytic class Asplenietea
trichomanis typical of dry
environments.
In addition to maidenhair, typical
Adiantetea species are the thalloid
liverworts Preissia quadrata,
Conocephalum conicum and Pellia
endiviifolia, and the mosses
Eucladium verticillatum and
Hymenostylium recurvirostre.
Communities of mosses such as
Eucladium verticillatum,
Hymenostylium recurvirostre and
Didytmodon tophaceus colonise
warm, sunny vertical cliffs with
trickling phreatic waters poor in
nutrients or streaming carbonate-rich
water.
These plants can be found on both
limestone and silica substrates and
occasionally tolerate even lengthy
periods of summer drought.
Alessandro Petraglia
Other communities of the same class
grow on shady schist cliffs in narrow
cirques with rills, or near waterfalls.
Such cliffs, which are wet even in the
driest summers, are colonised by lush
hygrophilous bryophytes and
pteridophytes. Acid substrates host
royal fern (Osmunda regalis), and
neutral, slightly alkaline cliffs rooting
chainfern (Woodwardia radicans).
The latter is a tree fern found in subtropical mountain areas and, in Italy,
grows in a few locations in Sicily,
Calabria and Campania. It is
extremely rare in the Mediterranean
area, where it colonises exclusively
humid cirques. Woodwardia radicans,
which is protected by international
conventions and is contained in the
list of Italian plants at risk of
extinction, is a relict of tropical flora of
the Tertiary in Mediterranean areas.
Maidenhair (Adiantum capillus-veneris)
Sunlight is another important factor
affecting temperature and, indirectly,
water supply, which in turn depends
on the type of substrate and its
gradient.
Some bryophytes are typically
sciaphilous (i.e., they thrive on shady,
cold, humid slopes), and live in areas
with less light than phanerogams,
penetrating into deep cracks, caves
and karst sinkholes. Those living along
south-facing slopes can withstand
very high temperatures (up to 70°C)
and lengthy periods of drought
(thermo-xerophilous species), and
during such periods they gradually
suspend their biological activity and
Barbula crocea
become dormant.
Bryophytes often have thick cushion shapes, which enable them to retain
water and protect their interior. In some cases (e.g., several Grimmia species),
the cushions are silvery due to the dead leaftips that reflect light and restrict
transpiration.
Just like epilithic lichens, cliff bryophytes tend to aggregate in exclusively or
mainly moss communities, which are classified according to the same criteria
used for vascular plants. The vegetation classes that mainly include mountain
slope communities of bryophytes are Racomitrietea heterostichi, Grimmietea
anodontis and Ctenidietea mollusci.
Racomitrietea heterostichi includes communities with several cushion
bryophytes that colonise siliceous rock along various versants and substrates
with changing humidity. The class Grimmietea anodontis has thermoxerophilous communities, living even at low altitudes in walls and rocks,
generally limestone. A few communities of this same class can be found in
the montane level, and are therefore examined in this Habitat volume. The
class Ctenidietea mollusci includes communities linked to limestone in cool,
shady areas.
The class Adiantetea must be analysed separately. It is a community of
bryophytes and ferns that colonises slopes and walls with rilling or streaming
water on limestone. It is typical of the Mediterranean area, and therefore grows
in central and southern Italy, especially in cirques.
35
■ Vascular plants
36
Numerous species of ferns and angiosperms colonise rocky slopes in very
different micro-environmental conditions. This is why they are the most
extensively treated in this book. Vascular plant biodiversity on slopes depends
not only on environmental characteristics like exposure to the sun,
temperature, water and nutrients, i.e., on variations in microclimatic and
substrate conditions, but also on the capacity of these environments to
preserve traces of plants of past epochs, acting like refuge habitats, especially
at lower altitudes.
We first analyse the variations in slope environment conditions, focusing on
those aspects that influence the life of vascular plants directly, and then pass
to the importance of slopes as refuge habitats.
Gruppo Del Vescovo (Pontremoli, Tuscan-Emilian Apennines)
Plants and cliff environments. Only vascular plants (those with specialised
tissues) have true rooting systems, i.e., those which enable them both to
anchor themselves to the substrate and to absorb mineral nutrients dissolved
in the soil. Although the rhizinas of epilithic and endolithic lichens and the
rhizoids of cliff bryophytes certainly carry out the former function efficiently,
nutrition through root absorption directly from the substrate is a characteristic
of vascular plants. It replaces the capacity of lower plants to absorb minerals
from streaming water along versants or from airborne particles. The physical
characteristics of rocky faces (that is, their morphology) therefore become
extremely important in determining the diversity and living conditions of cliff
vascular plants, as rooting depends on them. The lithology of cliffs is also
important because it influences the mineral nutrition of vascular plants.
Typical cliff plants grow on vertical or subvertical walls, the gradients of which
may exceed 90° in overhangs. Vertical cliffs have complex microclimatic
conditions that require specific adaptations on the part of vascular plants. If
gradients are less steep, the environment is also milder, enabling colonisation
by unspecialised vascular species.
However, vascular plants cannot survive on compact vertical walls that
provide no hold for their roots and are therefore impenetrable. Cliff flora can
develop on rocky faces that offer at least a few cracks, ledges and holds, no
matter how narrow and small they are. This condition is easily found on
limestone, in which dissolution caused by surface water flow combined with
meteorological events like temperature variations and freeze-thaw cycles,
give rise to rock weathering. This is why limestone cliffs are generally richer in
vascular plant species, and in the summer may look like natural rockeries.
37
38
The chemical nature of the rock influences plant diversity indirectly, through
variations in its morphology, and, more deeply, it also affects the quality and
quantity of mineral nutrients available for root absorption. The difference
between limestone and silica cliff flora is based not only on the richness of
species, but also on their quality. This means that limestone cliffs are
colonised by species different from those found on silica cliffs. One of the
reasons is the quantity of calcium, an essential plant nutrient found in the
scanty soil inside limestone cracks and ledges. This mineral is totally or
almost totally absent in silica cliffs. Plants growing on limestone are therefore
called calcicoles, or calcium-loving. In ecological terms, this means that they
can absorb - and tolerate when in excess - the great quantity of calcium in
the soil. Soils rich in calcium have neutral or alkaline pH, and this explains
why calcicolous species are also called basophilic. Instead, silica cliff plants
are calcifuge, i.e., they have adapted to minimal quantities of calcium in the
soil and cannot withstand its excess. Soils that are poor in calcium have acid
reactions, and calcifuge plants are therefore called acidophilic.
Alkaline reactions of the soil have specific consequences on the supply of
indispensable mineral nutrients for plants.
In base-rich soil, phosphorus, iron and manganese are generally fixed in
insoluble compounds and are not readily available to plants, whereas
molybdenum is easily found. In acid soil, some elements may hinder plant
growth because their quantity is insufficient. Among these is nitrogen, which
is mineralised very slowly in this type of soil, phosphorus, most of which is
linked to iron and aluminium oxides, and molybdenum, which is generally less
available. Concentrations of other elements (aluminium, iron, manganese)
may sometimes be so high that they are toxic to plants.
In addition to the micromorphology and lithology of rocky walls, cliff plant
populations are deeply affected by topographic parameters, the variations of
which cause the formation of microclimatic gradients that directly influence
the growth of vascular plants. The first of these is altitude, with which
temperature is associated. As altitude increases, air temperature decreases,
more regularly so at higher altitudes. At Italian latitudes, the annual average
temperature decrease is calculated at 0.55°C every altitude increase of 100
m. This may reach 0.70°C in summer, which coincides with mountain plant
dormancy. In eco-physiological terms, temperature is important in regulating
the metabolic activity of plant cells, especially carbon dioxide assimilation
and therefore the productivity of vascular plants. Among cliff plants there are
both eurythermal species, i.e., those able to live in a wide range of
temperatures and altitudes, and stenothermal species, which can only
withstand slight thermal and altitude variations. One eurythermal species is
Apuan globularia (Globularia incanescens), which grows at altitudes between
200 and 1900 m, from the Ligurian coast to the summits of the Apuan Alps.
One of the most impressive stenothermal cliff species is the crimped
bellflower (Campanula zoysii), typically found on limestone in the Carnic and
Julian Alps between 1700 and 2300 m.
In addition to micromorphology, lithology and altitude, cliff plant diversity is
due to exposure, another fundamental topographic parameter. The exposure
of rocky walls is important because its variation influences the quantity and
quality of light available to plants, air temperature and water supply. To
understand how exposure can produce strikingly different microenvironmental conditions, let us analyse the situations of two versants facing
opposite ways, one south and the other north.
During plant dormancy, cliffs facing south are exposed to intense, prolonged
daytime light. Plants can thus suffer light stress, jeopardising their
photosynthesis. A side-effect of intense sunlight is also the rising
temperature of the rock which, at altitudes above the tree line, may reach
50°C on the hottest summer days. At night, temperatures fall and at higher
altitudes may drop to 0°C, even in the warmest periods.
Apuan globularia (Globularia incanescens)
39
40
Cliff plants growing on south-facing
versants may therefore undergo daily
temperature variations of up to 60°C.
Intense sunlight and high temperature
favour evaporation from the meagre
soil and plant transpiration, which is
enhanced by windiness, a typical
feature of mountain environments,
especially at the summits. Therefore, in
addition to the dangers posed by
intense sunlight and heat, plants may
also undergo water stress.
Conversely, rocky versants facing
north are illuminated by direct light for
only short periods during the day, and
sometimes only during their dormancy.
If cliffs are overhanging, the plants
Crimped bellflower (Campanula zoysii)
growing on them, or protected by
them, are only illuminated by indirect light, even in summer. Obviously, these
conditions pose no threat of stress due to excessive light; on the contrary,
these species show the typical adaptations of plants living in the shade.
Temperature variations are also restricted in north-facing versants. During their
life-cycles, the species living in this habitat must withstand temperatures that
may drop below 0°C, incurring stress from cold as the temperature of their sap
falls below freezing point. In winter, this condition is the norm because lack or
scarcity of snow cover - due to cliff steepness - deprives plants of the
insulation provided by a layer of snow. To avoid winter stress, cliff species
resort to dormancy, a condition that enables them to withstand temperatures
far lower than 0°C.
Water supply for plants living along north-facing cliffs is sufficient, to the
extent it can be along vertical cliffs whose capacity for retaining water is
practically nil. Along snow-covered versants, flowing of melted snow can
guarantee sufficient water supply for most, even the whole vegetative season.
It should also be remembered that in the Alps summer is the period with most
rainfall. Restricted sunlight also contributes to limiting water loss through
transpiration, so that plants never suffer from water stress. The only
exceptions are species living in or under jutting walls where neither snowmelt
nor rain can reach them. These species, which live in conditions of total shelter
from rain, may feature adaptations typical of xerophytes. A particular vascular
plant species living in “shelter from rain” conditions is cobweb saxifrage
(Saxifraga arachnoidea), endemic to the Trentino Alps and mountains near
Brescia since before the glaciations. It is covered with thick hairs resembling
cobwebs that trap water, providing the plant with sufficient water supply.
Types of growth and adaptations. Like bryophytes, vascular species rooting
in rock crevices are also called chasmophytes. They are perennials that grow
very slowly due to the scanty nutrients contained in the meagre soil, which is
full of coarse material accumulating on the bottom of crevices. Some nutrients
derive from rock disintegration, some from the recycled organic matter of the
dead and decaying parts of the chasmophytes themselves, which are
mineralised by decomposing organisms (bacteria and fungi). The recycling
process is more evident in pulvinate chasmophytes.
The roots of chasmophytes may creep into even tiny rock crevices, enabling
these plants to cling to the walls very firmly. Sometimes the length of roots
penetrating into fissures in search of water and nutrients is greater than the
subaerial part of the plant (up to 1m).
The roots penetrating fissures look like elongated cones with apexes facing
the interior of the rock wall. The apex is covered by secondary, fan-like
roots creeping laterally into even narrower rock fissures. In larger, tubular
Cobweb saxifrage (Saxifraga arachnoidea)
41
42
crevices, secondary roots are thicker and parallel to the main root. When
fissures are deep and large enough, chasmophytes generally have a single,
long, straight root. The subaerial part of chasmophytes looks like a rosette
or a cushion (pulvinus).
In rosette-shaped or rosulate chasmophytes, the bud axis has limited growth,
and the spaces between the two successive leaves (internodes) are therefore
short. All the leaves emerge from the same point of the stem, forming what is
called a rosette. When flowering, the apex of the bud develops a scapus, on
top of which the flower or inflorescence forms (as in livelong saxifrage,
Saxifraga paniculata). Once flowering and fructifying are over, the plant
sometimes dies. These are monocarpic species, and the most typical cliff
example is the ancient king (Saxifraga florulenta).
Cushion-shaped or pulvinate chamaeophytes have early aerial shoots
growing from the stem and developing radially (in all directions). Growth is
uniform, and flowers develop at the same time at the apexes of several
stems. According to the shapes of their pulvini, chasmophytes are divided
into the moss-cushion type (like moss campion, Silene acaulis), with short,
central stems perpendicular to the rock, and longer, prostrate peripheral
stems. When growth is uniform in all directions, cushions may be
Livelong saxifrage (Saxifraga paniculata)
hemispherical, when the base grows
directly on the rocky surface, as in
Swiss
rock-jasmine
(Androsace
helvetica) and Vandelli’s rock-jasmine
(Androsace vandellii), or perfectly
round when the base is free because,
between the point where stems grow
and the pulvinate base there is an
elongated shoot, as in Alpine rockjasmine (Androsace alpina).
One very frequent morphological
characteristic of stems and leaves in
chasmophytes is their hairiness, which
is an adaptation to a series of factors.
Plants living on south-facing versants
need hairs for protection against
excessive sunlight and heat. These
species avoid dehydration stress with
special tissues that retain water, called
water-bearing parenchymas. These
tissues makes leaves and stems
typically pulpy and succulent. Stem and
leaf succulence are characteristic of the
stonecrop
family
(Crassulaceae),
especially the genera Jovibarba,
Sedum and Sempervivum.
Many species living on north-facing
versants also feature hairiness. In this
case, hairs prevent the subaerial part
of the plant from freezing when
temperatures fall below the freezing
point of tissues.
However, not all cliff plants can
develop roots inside rock fissures, and
their nutrient requirements may be
more stringent than those of
chasmophytes. These plants can
colonise rock walls only when their
seeds germinate near jutting stones,
43
Moss campion (Silene acaulis)
Alpine rock-jasmine (Androsace alpina)
Vandelli’s rock-jasmine (Androsace vandellii)
44
ledges or narrow terraces, which provide sufficient fine debris for them to root.
Their fan-like roots penetrate the thin debris cover from which they absorb
nutrients. Higher nutrient concentrations produce faster, lush growth. These
vascular plants are also called comose, like bryophytes with similar ecological
behaviour. Comoses are typically tufted, because many lateral shoots depart
from the central one, as in Alpine fescue (Festuca alpina), one of the few cliff
species of the grass family.
Phytogeographic aspects. In addition to “experimental models” of how
plants can adapt to very harsh environments, cliff plants also give us
retrospective information on the history of plant populations in Italian
mountains, because they preserve the “genetic memory” of their geographic
territory.
This is especially true of vascular plants colonising valley slopes, gorges in the
Alps, southern Pre-Alps and Apennines (from the Apuan Alps to Calabria), and
all mountains on the major Italian islands. These areas preserve ancient
species, some of which bear no strict resemblance to any of the living species,
and are therefore totally isolated from the systematic viewpoint. The reasons
for the older age and systematic isolation of cliff flora proceeding from the Alps
Cobweb houseleek (Sempervivum arachnoideum)
to mountains on the islands is explained by the effects of glaciations on Italian
mountain flora in general. The main, although not exclusive effect was floral
impoverishment and the extinction of many species. This process was more
severe on the Alps than on southern chains precisely because these imposing
mountains were deeply affected by glaciations.
However, the ice cover was not uniform along the Alps. Peripheral areas of the
south-western Alps and all the south-eastern Alps, from Lake Como to the
Julian Pre-Alps, were only marginally covered. Limited effects are shown by
palaeo-geographic and palaeo-climatic data, as well as by biogeographic
evidence, such as the large number of palaeo-endemic plants found in these
areas. Palaeo-endemics are species with diploid or more frequently polyploid
chromosome numbers; they date back to pre-glacial times and grow in
restricted geographic areas.
Many Alpine palaeo-endemics are cliff species. Incapable of genetic
adaptations, they have survived on rocky cliffs until today because there they
could avoid competition by other species which were more demanding from
the nutritional viewpoint and which had escaped man’s colonisation of their
territory. These Tertiary species may also be found on nunataks, islands and
rocky summits emerging from ice-caps.
The conservation value of rocky slopes of hill and montane areas should not
only be viewed retrospectively, but also for their present importance. This is
true of cliffs surrounding cirques, gorges, and narrow, confined valleys with
markedly colder, more humid microclimatic conditions than those found at
similar altitudes outside such environments. These conditions at low altitudes
favour the colonisation and development of cliff species that generally thrive
at higher altitudes. The phenomenon by means of which Alpine species move
to lower altitudes, where they are preserved in refuge habitats, is called
dealpinisation.
Cliff ferns. Cliff ferns on the Italian mountains are not very numerous. The
largest number of species is found in the Aspleniaceae family, with the genera
Asplenium and Ceterach. Other genera with strictly cliff species are
Cystopteris (Athyriaceae) and Woodsia (Woodsiaceae), which may be found up
to the snow line. The genera Adiantum (Adiantaceae) and Notholaena
(Synopteridaceae) include cliff species which do not live above the snow line.
A few cliff ferns are thermophilic. Among these are cloak fern (Notholaena
marantae), which grows on winding cliffs to 1400 m, and maidenhair
(Adiantum capillus-veneris), which colonises humid or trickling limestone walls
to 1500 m. Among the Aspleniaceae, rustyback (Ceterach officinarum) prefers
45
46
calcareous rock, but can live on any substrate to 2000 m, generally on
southern versants. The genus Asplenium (spleenwort) includes several cliff
species, some of which live exclusively on limestone or dolomite, like
Asplenium lepidum, which grows in shady areas with trickling water, wall-rue
(Asplenium ruta-muraria), the most frequent species found to 2900 m,
Dolomite spleenwort (Asplenium seelosi), restricted to shady, calcareous areas
in the central and eastern Alps, and Black Forest spleenwort (Asplenium
fontanum), which is also associated with shady, damp, calcareous cliffs in the
central and western Alps and northern Apennines to 1800 m.
Serpentine spleenwort (Asplenium cuneifolium), as its name suggests, is
associated with serpentine and other ophiolitic substrates. In Italy, it grows
between Val d’Aosta in the Alps and Val Tiberina in the northern Apennines.
Although forked spleenwort (Asplenium septentrionale) is found on ophiolitic
rock, it may also grow on acid substrates.
Other spleenwort species seem to be indifferent to substrate type, like
maidenhair spleenwort (Asplenium trichomanes), which is found everywhere
on the Italian mountains and is divided into several subspecies according to
distribution and ecology, and green spleenwort (Asplenium viride), found from
the Alps to Calabria, at both high and low altitudes.
Maidenhair spleenwort (Asplenium trichomanes)
The genus Cystopteris includes three typically cliff species that generally grow
on shady walls or at their base. Two of them are indifferent to substrate type:
brittle bladder fern (Cystopteris fragilis), which is found on all Italian mountains,
and Dickie’s fern (Cystopteris dickiaeana), which grows discontinuously along
the Alps and Apennines. The third species, Alpine bladder fern (Cystopteris
alpina), prefers limestone slopes.
Lastly, the genus Woodsia features three cliff species, two of which grow in
silica soil, alpine and oblong woodsia (Woodsia alpina, W. ilvensis); the third is
calcium-loving, smooth woodsia (W. glabella subsp. pulchella). All three are
extremely rare. The most common is alpine woodsia, which grows along the
Alps and northern Apennines. Smooth woodsia may be found in Valsesia as
far as the Julian Alps, and oblong woodsia is restricted to the South Tyrol and
Lombardy.
Cliff angiosperms. Although here we only describe plants growing at high
altitudes, from the montane to the snow levels, there are several chasmophytic
angiosperms on the Italian mountains. The angiosperm genera with the
greatest numbers of chasmophytes are Campanula, Primula and especially
Saxifraga, whose name clearly refers to cliff habitats. As this volume cannot
treat all the chasmophytic angiosperms in Italy, at the end of this chapter
readers will find a chart with the most interesting species from the
phytogeographic viewpoint (endemics and species with restricted or
discontinuous distribution). It lists species living on the Alps (from west to
east), as far as those in the mountains on the islands, across the Apennines
(see pp. 68-71). Only a few endemic chasmophytes that are very important
from the viewpoints of taxonomy, conservation and aesthetics are specifically
mentioned below.
For the above mentioned reasons, the Ligurian and Maritime Alps are very
important locations for endemics. Most of the endemics living there are cliff
species of pre-glacial origin. In the Ligurian Alps, they live on limestone and
dolomite, common substrates in these areas; in the Maritime Alps,
chasmophytes grow in the outcropping gneiss and granite forming the
structure of this Alpine chain. Among calcicolous chasmophytes, Allioni’s
primrose (Primula allionii) is perhaps the best-known species, for both its rarity
and beauty. It grows along the Roia basin on the southern versant of the
Maritime Alps in a few shady areas between 500 and 1900 m.
Among acidophilic chasmophytes in the Maritime Alps, the most impressive
species is the ancient king (Saxifraga florulenta), a mysterious, legendary plant
living exclusively on silica cliffs along northern versants between 1900 and
47
3240 m, usually in inaccessible areas. The ancient king is a monocarpic
species that flowers and fructifies only once before dying, as it cannot
withstand further reproduction. It has been calculated that single plants take
between 35 and 75 years to flower. Proceeding northwards to the western
Alps, there are other endemic chasmophytes, like Vaud saxifrage (Saxifraga
valdensis), endemic to the Cottian and Graian Alps, which grows on limestone
and schist between 2000 and 2900 m, and Adriatic bellflower (Campanula
elatines), found on shady siliceous cliffs between 300 and 1900 m. Proceeding
eastwards, there is another silica-loving species typical of the north-western
Alps, dwarf rampion (Phyteuma humile), which is replaced by the Rhaetian
rampion (Phyteuma hedraianthifolium) in the Rhaetian Alps.
The Italian south-eastern Alps and Pre-Alps, which are mainly composed of
limestone, are the most important Alpine locations for cliff endemics. In
phytogeographic terms, this portion of the Alps is divided into three districts,
each with its own cliff chasmophytes:
● Insubrian, including the mountains between the Lake of Como and Garda
and Monte Baldo;
● Dolomitic, with the Veneto Pre-Alps, Feltre Alps and Dolomites;
● Carnic-Julian, including the Pre-Alps and Carnic and Julian Alps.
48
Ancient king (Saxifraga florulenta)
Moretti’s bellflower (Campanula morettiana)
49
50
The Insubrian district, besides the already-mentioned cobweb saxifrage, hosts
Rainer’s bellflower (Campanula raineri), found in the Lombard Pre-Alps and in
isolated locations in Val Sugana and near Vicenza. This gaudy chasmophyte
with large, pale blue corollas is found between 700 and 2000 m. In the
Dolomites, there is another impressive plant, Moretti’s bellflower (Campanula
morettiana), whose bluish-violet flowers dot dolomitic crevices at 1700-2400
m. The list of endemic bellflowers ends with crimped bellflower (Campanula
zoysii) found on limestone in the Julian Alps.
In the south-eastern Alps, there are a few species of cliff endemics of the
genus Primula, all of which have only recently been discovered. In the
Insubrian district, there is Grigne primrose (Primula grignensis), described by
Moser in 1998, and Monte Alben primrose (Primula albenensis), described by
Banfi and Ferlinghetti in 1993. In the Dolomitic district, Recoaro primrose
(Primula recubariensis) was discovered in the Little Dolomites by Prosser and
Scortegagna in 1998.
Our itinerary of endemic species on the Italian mountains proceeds
southwards to the northern Apennines. Here, the most important endemic
location is in the Apuan Alps. One Apuan endemic is Apuan globularia, which
grows between the limestone cliffs near the Gulf of La Spezia and the peaks of
Apennine primrose (Primula apennina)
the Apuan Alps, on both acid and
base-rich substrates. It may also be
found on sunny versants of the
Tuscan-Emilian Apennines, where it
colonises acid fissures in sandstone.
The Tuscan-Emilian Apennines do not
host many endemics. The only true
chasmophyte is the Apennine primrose
(Primula apennina), which grows on
sandstone cliffs in the north-west, at
1300-1900 m.
The Ice Ages did not affect the central
Apennines particularly, so that these
areas preserve pre-glacial orophilous
flora with several endemics.
True chasmophytes are Neapolitan
bellflower (Campanula fragilis subsp.
Mathilda’s rock-jasmine (Androsace mathildae)
cavolinii), found on limestone in Abruzzo
and Latium at 500-1300 m. Most of the orophilous flora of the central
Apennines grows on the other Adriatic shore, in Bosnia and Montenegro.
Among these so-called amphi-adriatic species, the rare Mathilda’s rockjasmine (Androsace mathildae) is particularly important. It lives in limestone
crevices on the Gran Sasso d’Italia and Majella at 2100-2900 m, and has
recently been found on mountains in Montenegro.
The southern Apennines have less imposing peaks and therefore fewer
endemic chasmophytes. One of the rarest species is the yarrow Achillea
lucana, which creeps into fissures in limestone and conglomerate on the
Basilicata mountains. On the Sicilian mountains, the situation is very similar,
and orophilous cliff flora is scarce due to low altitudes. Imposing cliffs higher
than 1500 m can only be found in the Madonie. Among chasmophytic Sicilian
endemics there is Gussone’s woodruff (Asperula gussonei), which colonises the
dolomitic cliffs of Quacella in the Madonie (1400-1800 m), and Busambra
cornflower (Centaurea busambarensis) growing on the limestone cliffs of
several Sicilian mountains, to 1400 m.
Our floral itinerary across the Italian mountains ends in Sardinia, where
lithologic variety with limestone and silica enabled differentiation between
calcicole and calcifuge species. Among the former there is Moris’ thrift (Armeria
morisii), which colonises limestone cliffs in the Sopramonte di Orgosolo and
Oliena between 1000 and 1300 m. Two silicicolous species of the genus
51
52
Helichrysum are worthy of mention, Mount Linas everlasting (Helichrysum
montelinasanum), which grows on granite in Mount Linas in south-western
Sardinia, and Limbara everlasting (Helichrysum frigidum), a species frequently
found on the highest mountains in Corsica, but limited to Mount Limbara in
northern Sardinia.
Cliff vegetation. According to phyto-sociological taxonomy, cliff plants of the
Italian mountains containing large numbers of chasmophytes all belong to the
same class (Asplenietea trichomanis), which in turn is divided into two orders
(Potentilletalia caulescentis and Androsacetalia multiflorae), which include
phytocoenoses of limestone and silica cliffs. Each of the two orders is divided
into alliances.
The order Potentilletalia caulescensis, distributed along the Alps as far as
Sicily, is divided into a large number of well-defined geographical alliances,
and includes several communities. The first group contains heliophilous and
thermophilic plants generally growing in montane and sub-alpine belts, but
which are occasionally found at lower altitudes. The most frequent
community within this group is Potentilletum caulescentis, found along the
southern Alps. Its predominant species is lax potentilla (Potentilla
Pink cinquefoil (Potentilla nitida)
caulescens), often associated with
dwarf buckthorn (Rhamnus pumilus).
Another group is composed of
communities that are generally found
in the Alpine belt, in sunny areas. The
most typical is Potentilletum nitidae,
which grows on the highest limestone
cliffs of the south-eastern Alps.
It is clearly visible for the brightly
coloured flowers of pink cinquefoil
(Potentilla nitida), which range from
pale to dark pink.
This community often features the rare
Hausmann’s rock-jasmine (Androsace
hausmannii). Higher up in limestone of
the Alpine and snow levels, the
Androsacetum helveticae snow-bed is
clearly distinguishable by the pulvini of
Swiss rock-jasmine.
All the communities mentioned so far
do not live on shady, north-facing
cliffs, or on poorly lit jutting cliffs,
which is where communities with ferns
live, especially those linked to the
genera Asplenium and Cystopteris, in
both the Alps and northern Apennines.
The most common community is
Cystopteridetum fragilis, with brittle
bladder fern (Cystopteris fragilis).
In the south-western Alps and Apuan
Alps, cliff communities contain several
endemic chasmophytes. In the
Ligurian Alps, the most frequent
community is Saxifragetum lingulatae
with thick-leaved saxifrage (Saxifraga
callosa = Saxifraga lingulata), which
carpets limestone cliffs with its
drooping flowers.
Another important community from the
53
Saxifraga
vandelli
Telekia
speciosissima
Physoplexis
comosa
Moehringia
bavarica
Potentilla
caulescens
Saxifraga
arachnoidea
Primula
spectabilis
Daphne
petraea
Endemic vegetation in limestone crevices in
Lombardy
54
phytogeographical point of view, is Primuletum allionii, with Allioni’s primrose,
which grows on shady limestone cliffs in the montane belt. The most important
community of the Apuan Alps is Sileno lanuginosae-Rhamnetum
glaucophyllae, whose typical species are blue-leaved buckthorn (Rhamnus
glaucophyllus) and moltkia (Moltkia suffruticosa), which grow on compact cliffs
in the montane belt.
Limestone in the central Apennines and Sicily hosts several communities of
chasmophytes. Two of them may be quoted as examples, as they are typical
of the central-southern Apennines.
The first is Campanulo cavolini-Potentilletum caulescentis, which colonises
limestone cliffs and contains Neapolitan bellflower. The second is restricted
to the highest mountains of the central Apennines, where it lives on subalpine and alpine cliffs. This is Potentilletum apenninae, whose typical
species are Dolomite cinquefoil (Potentilla apennina) and rock alyssum
(Ptilotrichum cyclocarpum). The two communities are the Apennine vicariates
(substitutes) of the Alpine Potentilletum caulescentis and Potentilletum
nitidae, respectively.
Limestone on the Sicilian mountains has only one community of
chasmophytes, the Asperuletum gussonei, which grows on the Madonie
Thick-leaved saxifrage (Saxifraga callosa)
mountains and contains Gussone’s
woodruff and Madonie everlasting
(Helichrysum nebrodense).
Sardinian mountain chasmophytes
have not yet been thoroughly analysed.
The only data available concerns
limestone cliff plants, which are very
different from those of the Alps,
Apennines and Sicily, to the extent that
they constitute an independent order
(Arenario-Phagnaletalia sordidae).
The most common community at
altitudes over 1000 m is the Laserpitio
garganicae-Asperuletum pumilae, with
southern sermountain (Laserpitium
garganicum), Sardinian woodruff
(Asperula pumila) and heart-leaved
Hairy primrose (Primula hirsuta)
micromeria (Micromeria cordata).
The classification of siliceous cliff plants is less complex, as there are only
three main alliances and fewer communities.
The Alps host two communities, the Androsacetum vandellii, along siliceous
cliffs of the Alpine and snow levels with Vandelli’s rock-jasmine (Androsace
vandellii), and Asplenio-Primuletum hirsutae on the upper montane and
lower snow levels. Typical species of this community are hairy primrose
(Primula hirsuta) and the gaudy pyramidal saxifrage (Saxifraga cotyledon).
The northern Apennines host one silicicolous community, Drabo aizoidisPrimuletum apenninae, which can only be found on the north-facing summits
of sandstone cliffs. Typical species are yellow whitlow grass (Draba aizoides)
and Apennine primrose.
On the Italian Alps, another community colonises sunny siliceous mountain
cliffs, Sileno rupestris-Asplenietum septentrionalis, the characterising species
of which is rock catchfly (Silene rupestris).
The south-western Alps deserve special mention, because their siliceous cliff
plants are very different from those of the mountains. The Maritime Alps host
Saxifragetum florulentae, which colonises vertical walls of the alpine and snow
levels, with the ancient king and Piedmont saxifrage (Saxifraga pedemontana).
Siliceous cliffs in the Italian peninsula and Sicily are very rare and little is
known about them. Although they are more frequent in Sardinia, their plants
are unknown.
55
■ Plant life in screes
56
Debris slopes and cones, whether the
result of glaciation in past epochs or of
more recent origin, cover extensive
areas above the tree line in the Alps
and Apennines. At a distance, most of
them look perfectly bare, but as we
approach them, we soon realise that,
concealed between the rocks, are
several plant species, some with
colourful flowers. Most of them have
Debris with mixed lithology (Valle Aurina,
adaptations enabling them to survive
South Tyrol)
in these generally unfavourable
environments. These highly specialised species have a particular name: in
geobotany, they are called glareous plants.
Although cliffs may be colonised by plants belonging to various taxonomic
groups, debris can host only plants that anchor themselves to the mobile
substrate with specially adapted roots. This excludes algae, lichens and
mosses, which do not have true roots. Epilithic lichens and mosses may be
found on old, stable clasts, and terrestrial bryophytes may insinuate
themselves into the fine material that forms inside crevices between the larger
boulders at the base of debris slopes. In both cases, however, these species
are normally found on rocks or in communities in grassland and mountain
valleys, and therefore cannot be included in the glareous group.
Debris cone partially stabilised by vegetation (Massiccio del Gran Sasso, Abruzzo)
The debris environment. Debris slopes are extreme habitats for vascular
plants, due both to their unfavourable microclimatic conditions and the
constant movements of surface sediments. As regards the microclimate, it is
the same as that described for cliffs, and topographic factors are important in
producing different habitats according to sun exposure, temperature and
water supply. In addition, however, another important factor is the quantity and
duration of snow cover, which is negligible on cliffs because of their
steepness, but essential on debris slopes. Snow cover modifies the
morphology of debris slopes or cones as it changes the position of the debris
itself, which may slide freely on the snow or start moving when it melts. The
quantity and duration of snow cover also provides water in the layer of fine soil
that forms underneath the coarse surface debris and from which glareous
plants obtain their nutrition.
57
58
The type of rock the disintegration of which produces clasts, the difference
between limestone and silica, and how they affect supplies of nutrients to the
roots of glareous plants have already been analysed. The only feature to
examine now is the type of bedrock, which affects the sensitivity of the rock
itself to agents responsible for disintegration, and which also affects the size
and shape of clasts and therefore the overall look of the debris slope or cone.
The characteristics of the debris environment that most influence plant life are
substrate instability, scarce fine soil (silt and clay), and restricted water supply
in the surface debris layers. These factors give rise to the peculiar solutions
adopted by glareous plants to survive in these conditions.
According to the quantity and mobility of debris still falling from the
overhanging slope, debris slopes may be active, i.e., continually modified by
the addition of new material. They are inhospitable to vascular plants and are
therefore almost totally bare. Other slopes may not be actively fed by debris,
but are still unstable because clasts are set in motion by freeze-thaw cycles,
by streaming meltwater, or by passage of animals and humans. These slopes
are more suitable for glareous plants. Lastly, debris slopes may be inactive,
i.e., they are not fed by new debris and are completely stable. In certain
conditions, these slopes may host vascular species, even those not especially
adapted to these habitats. According to the average diameter of the clasts,
debris slopes are divided into block slopes when the diameter exceeds 25 cm,
Last surviving fragment of a conifer wood, surrounded by a debris slope (Carnic Pre-Alps, Friuli Venezia Giulia)
coarse debris slopes when debris
diameter ranges between 2 and 25
cm, and fine-grained slopes when the
diameter is 2-0.2 cm.
The most important factors for
colonisation by vascular plants are the
amount, distribution and content of
water in the fine soil between and
under debris. The most inhospitable
are block slopes because, even if they
are stable, they generally lack fine soil
and sunlight, which obviously cannot
penetrate the overlapping boulders.
Debris slopes at the base of rock walls
are not homogeneous environments
which may all be colonised by vascular
plants in the same way. The apexes of
Debris slope with large blocks
debris cones and the upper part of
debris slopes have fine, unstable sediments, and are constantly supplied with
new rock fragments falling from the overhanging rock wall. The debris moves
continually and vascular plants are thus incapable of colonising these areas.
Proceeding downwards along the slope, the clasts are bigger and their
mobility decreases. This environment favours colonisation by glareous plants
which, as we shall see in the next section, resort to several adaptive strategies
in order to inhabit such slopes. Towards the base of the slope, the clasts are
even bigger, and are completely stationary. Debris slope stabilisation gives rise
to favourable conditions for colonisation by non-specialised lithophilous
plants (those which cannot be considered glareous), which settle in interstices
between boulders where sunlight and fine soil are sufficient for their growth.
The cross-section of debris slopes or cones reveals a convex profile.
Proceeding from the margins towards the centre, there is a first sequence in
which the bedrock meets the debris cover, and where friction slows down
clast movement, enabling plants to develop. This is followed by the so-called
pediment, which is the less elevated slope boundary where coarser boulders
are deposited by gravity. Then comes the debris slope and, lastly, the most
elevated part, called crest, which contains the finest material, generally
covered by coarser, rolling rubble. The most favourable conditions for
colonisation by vascular plants are found along the debris slope, where
conditions are similar to those in the central areas of the cross-section.
59
60
Adaptive strategies of glareous plants. Most glareous plants produce large
quantities of seeds, which are then dispersed by wind. This enables them to
colonise areas of the same debris slope or other slopes far away. Many seeds
are needed to colonise debris slopes in the very few areas which have clayey
substrates - the only ones enabling germination. Experimental analyses have
shown that, although the germination rate of seeds of glareous plants is high,
the chances that plantules (embryo plants) may actually develop fully are very
small. This is due to the fact that glareous plantules have not yet developed
the indispensable adaptations to survive in these habitats. The vegetative
systems of fully grown glareous plants devise adaptations to anchor
themselves to mobile substrates, avoid being buried by falling rubble, and
reach the humid layer of soil underneath the surface debris cover. These aims
are achieved by the great capacity for reproduction and regeneration of both
buds and roots of glareous plants.
They secure themselves firmly to the substrate by means of a main root (or a
rhizome), which penetrates deeply through the debris cover and fine soil
underneath. The root is supported by a surface system of rootlets that reach
the layer of fine soil and absorb water and nutrients. The plants then spread
and avoid burial by putting out creeping shoots that develop parallel to the
slope. A distinction must be made between migrating and propagating
shoots. The former are extensions of the mother plant (for example, creeping
avens Geum reptans) and grow independently only when severed by falling
stones. The latter detach themselves autonomously and produce independent
plants.
In the first half of the 20th century, European botanists defined the main types
of growth of glareous plants. They are divided into five groups:
● Migrating glareous plants, which move passively along slopes
● Creeping glareous plants, which creep or “float” on debris surfaces
● Rooting glareous plants, which extend in depth
● Stabilising glareous plants, which block debris flow
● Mat-forming glareous plants, which cover and consequently block large
quantities of debris.
Migrating glareous plants can colonise areas of the debris slope where the risk
of burial is high. They avoid this by growing creeping shoots that root
themselves into the substrate and regenerate the mother plant should it be
buried, giving the impression that the plant is “migrating” along or across the
slope. This group can be divided into varieties that migrate by extension, by
vegetative multiplication, or both. Examples of migrating glareous plants are
French sorrel (Rumex scutatus), creeping avens, fairy’s thimble (Campanula
cochlearifolia) and round-leaved pennycress (Thlaspi rotundifolium).
Creeping glareous plants have a dense network of thin shoots that “float” on
the debris surface, and fascicular main roots with thin secondary roots which
Scree with grassy tufts in Vette Feltrine (Veneto)
Debris slope with parsley fern (Cryptogramma crispa)
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62
also reach the underlying fine soil. Alpine toadflax (Linaria alpina) and bladder
campion (Silene vulgaris subsp. glareosa) are two examples.
Rooting glareous plants extend their roots far down, through the debris cover,
by means of a strong, branched rhizome that secures them firmly to the
substrate. Examples are parsley fern (Cryptogramma crispa), mountain sorrel
(Oxyria digyna) and two species of leopardsbane (Doronicum clusii and D.
grandiflorum).
Stabilising glareous plants capture fine debris by means of shoots formed of
dense tufts or thick, intricate patches of roots that grow perpendicular to the
slope. Tufted meadow-grass (Poa laxa), silver oat-grass (Trisetum argenteum)
and T. distichophyllum are tufted. Glacier buttercup (Ranunculus glacialis) and
mountain hawkbit (Leontodon montanus) are easily identified by their thick root
mat.
Mat-forming glareous plants develop low, creeping undershrub along the
debris surface. They may be woody, like mountain avens (Dryas octopetala), or
grassy, like alpine gypsophila (Gypsophila repens), which stabilises fine debris
by trapping it in the dense tangle of its shoots.
Migrating, creeping and rooting glareous plants all grow in the mobile sections
of the slope, whereas stabilising and mat-forming ones colonise the stable
areas, and their tufts, mats and tangled shoots produce a dynamic type of
vegetation that evolves into pioneering grassland.
Fairy’s thimble (Campanula cochleariifolia)
Endemic glareous plants. The
numbers of endemic glareous plants
are
smaller
than
those
of
chasmophytes. Endemics are found in
the genera Alyssum, Campanula,
Saxifraga and Viola. Below is only a
short list of the most important species
(for details, see pp. 68-71).
In the Maritime Alps, there are two
species of the genus Viola: Maritime
Alps pansy (Viola valderia), which
colonises siliceous debris slopes
between 1200 and 2300 m, and
Argentera pansy (Viola argenteriae) actually not a true endemic, as it is also
found in Corsica - which grows in fine
siliceous debris at higher altitudes
(1800-1900 m).
Berardie (Berardia subacaulis) and
simple-leaved milfoil (Achillea erbarotta) are found across the Western
Alps. The former is the only plant of its
genus and colonises fine limestone
debris at 1800-2700 m; the latter is
typical of siliceous debris between
2000 and 2800 m.
Another glareous plant associated
with limestone debris produced by the
disintegration of limestone, calcite
and mica is large-flowered bellflower
(Campanula alpestris), found in the
Western Alps.
In the Eastern Alps, there is another
species of the genus Viola, Comolli’s
pansy (Viola comollia), which grows
only in the Bergamasque Alps,
colonising high-altitude siliceous
rubble (2000-2450 m). In the Insubrian
district, limestone slopes host Tonzig’s
63
Round-leaved pennycress (Thlaspi rotundifolium)
Alpine toadflax (Linaria alpina)
Alpine gypsophila (Gypsophila repens)
64
toadflax (Linaria tonzingii) in the PreAlps near Bergamo (1600-2400 m). In
the Dolomites, the tiny species (the
only one of its genus) alpine scurvygrass (Rhizobotrya alpina) grows on
fine, humid limestone debris at 19002800 m. In the Carnic-Julian Alps,
Traunfellner’s buttercup (Ranunculus
traunfellneri) colonises even stable
debris covered with snow at 15002300 m.
In the northern Apennines, debris
Alpine scurvy-grass (Rhizobotrya alpina)
slopes are less extensive and host
only two endemics. One is yellow
thistle (Cirsium bertolonii), which grows in fine limestone debris in the Apuan
Alps and on marl in the Tuscan-Emilian Apennines between 1000 and 2000
m. The other is Zanoni’s murbeckiella (Murbeckiella zanonii), found on fine
sandstone debris.
The situation is quite different in the central Apennines, where debris slopes
are very extensive. The most important endemics are the elegant Apennine
pheasant’s-eye (Adonis distorta), whose yellow flowers carpet debris covers
to 2000 m. It often grows with Apennean pennycress (Thlaspi stylosum), a
small, pink-flowered crucifer that is very similar to round-leaved pennycress
in the Alps.
The southern Apennines host another phyto-geographically important
glareous plant that grows in patches in the Sicilian Madonie, snowy thistle
(Ptilostemon niveus). It colonises limestone debris at 1200-1900 m.
In Sicily, endemic glareous plants generally develop on the lava slopes of
Mount Etna, like Sicilian soapwort (Saponaria sicula), whose pulvini with pink
flowers dot lava beds to 2000 m.
In Sardinia, there are few debris slopes and endemics. Hairy pearlwort (Sagina
pilifera) typically grows on siliceous debris at high altitudes, and on the
summits of the mountains in Corsica.
Debris vegetation. On Italian mountains, debris vegetation includes several
communities. In the Alps, Apennines and Sicily, these communities belong to
the class Thlaspietea rotundifolii. Sardinian debris vegetation is still little
known. The class Thlaspietea rotundifolii is divided into four orders
(Thlaspetalia rotundifolii, Galio-Parietarietalia officinalis, Androsacetalia
alpinae, Galeopsietalia). The first two include limestone debris vegetation, the
others plants growing on siliceous debris.
The order Thlaspetalia rotundifolii has the greatest number of communities,
which are arranged at various altitudes and latitudes. The differing flora found
within each community is due to the lithologic differences in clasts. From this
viewpoint, one particular group of communities stands out: it colonises
debris slopes produced by the disintegration of limestone, calcite and mica in
the Alpine belt. This first group contains the communities Campanulo
cenisiae-Saxifragetum oppositifoliae and Saxifragetum biflorae, both
distributed along the Alps. The typical species of the second community is
two-flowered saxifrage (Saxifraga biflora), which has recently been found in
the Swiss Alps at 4450 m, and is therefore the highest-growing vascular plant
in European mountains.
The other communities of Thlaspietalia rotundifolii colonise limestone debris
produced by crumbling limestone and dolomite, and may be grouped
according to their geographic distribution, in the Alps and northern
Apennines, or in the central-southern Apennines. In both groups, there are
communities growing in alpine and sub-alpine belts (sometimes reaching the
montane level).
Berardie (Berardia subacaulis)
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66
Many calcicolous glareous plants live in the Alps and northern Apennines.
Among those of the alpine and snow levels is Papaveretum rhaetici, typical
of limestone-dolomitic debris of the Southern Alps. Its characteristic species
is Rhaetian poppy (Papaver rhaeticum), with its yellow flowers. Another
community of this group is Leontodontetum montani, associated with finegrain debris of south-facing versants, with the typical mountain hawkbit.
The montane and sub-alpine levels in the Alps and Apuan Alps host
communities preferring south-facing debris slopes rich in fine sediments. In
the sub-alpine belt of the Alps, the most frequent community is the
Athamanto-Trisetetum distichophyllii, easily identified by the white
umbrella-shaped flowers of athamanta (Athamanta cretensis). The Apuan
Alps often host the community Heracleo-Valerianetum montanae, whose
dominant species is Apennine cat’s ear (Robertia taraxacoides), a glareous
plant found along the Apennines and on Sicilian and Sardinian mountains.
Other communities colonise calcareous debris with coarse boulders in
shady cirques of the montane belt and in the lower sub-alpine belt of the
Alps and Apuan Alps. The particular microclimate of these areas favours the
growth of ferns, like Dryopteridetum villarii, dominated by rigid buckler fern
(Dryopteris villarii).
Rhaetian poppy (Papaver rhaeticum) and white alpine poppy (P. julicum)
The montane and alpine levels of
the central-southern Apennines are
inhabited by numerous communities
of glareous plants. One of the most
particular is Drypido-Festucetum
dimorphae, which colonises mobile
limestone debris with fine sediments
with the thorny mats of spiny
carnation (Drypis spinosa) and the
large tufts of Apennine fescue
(Festuca dimorpha). Higher altitudes in
the central Apennines are the
Val di Fassa saxifrage (Saxifraga depressa)
preferntial locations of the CrepidoLeontodontetum montani community,
with several glareous endemics, like Apennine pheasant’s-eye and Barrelieri’s
sneezewort (Achillea barrelieri).
A few communities of glareous plants grow in isolation on thermophilic
limestone debris slopes of the montane level. Among them is Rumicetum
scutati, in which French sorrel is the dominating species, found in the Alps and
northern and central Apennines. In the southern Apennines and Sicily, the
same habitat hosts snowy thistle, which forms the thermophilic glareous
community Senecioni-Ptilostemetum nivei in the Madonie.
The list of debris vegetation ends with the orders Androsacetalia alpinae and
Galeopsietalia, which include the phytocoenoses of debris slopes formed of
crumbling silica.
The order Androsacetalia alpinae is found on silica and sandstone slopes in
the Alps and Tuscan–Emilian Apennines, especially between the sub-alpine
belt and the snow line. Among the most important communities of this order
are Androsacetum alpinae, Sieversio-Oxyrietum digynae and Saxifragetum
depressae. The last community was recently discovered on vulcanite in the
Dolomites and contains Val di Fassa saxifrage (Saxifraga depressa).
Lower altitudes of the sub-alpine and montane levels are colonised by
Allosuretum crispae, a community with various ferns, especially parsley fern.
Allosuretum crispae grows at the base of debris slopes, stable debris slopes
with large clasts, and rockeries in the siliceous Alps and Tuscan-Emilian
Apennines.
The order Galeopsietalia includes phytocoenoses of thermophilic, siliceous
montane and submontane debris. Shady debris slopes in Italy host only one
community, Galeopsio-Rumicetum.
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68
Endemics on cliffs and screes
Cliffs and screes are conservative
habitats par excellence.
As such, they obviously contain
larger numbers of endemics, which
preserve the genetic memory of a
territory better than species of other
chorological categories.
Mountain chains are considered the
best areas for the development and
formation of endemics and, since
Italy is a mountainous country, its
flora is duly rich in endemics,
generally found in the mountains.
Endemics are definitely more
plentiful in mountain systems which
have aboundant rocky slopes and
scress.
However, this is not the only and
perhaps not the most important
reason why such plants grow in
these areas.
Most important of all is the history
of the mountain chain itself, its
origin, and the fact that it was more
or less isolated from nearby
mountain ranges.
Palaeoclimatic events, and
glaciations in particular, also plaied
an essential role in the origin of
endemic plants.
As already mentioned, the areas
with the largest number of endemics
are peripheral mountains, and their
important role in cliff and scree flora
has prompted the compilation of an
updated, complete list divided by
mountain sector.
The list includes plants that are not
true endemics, but whose
distribution is nonetheless restricted
to two distinct phytogeographic
sectors.
Marcello Tomaselli
Ligurian and Maritime Alps
LIMESTONE SUBSTRATES
● Cliffs > Ballota frutescens,
Campanula albicans,
Helianthemum lunulatum,
Micromeria marginata, Moehringia
lebrunii, Phyteuma cordatum,
Potentilla saxifraga, Primula allionii,
Saxifraga cochlearis
● Screes > Galeopsis reuteri,
Galium saxosum, Iberis nana,
Ligusticum ferulaceum
SILICEOUS SUBSTRATES
● Cliffs > Saxifraga florulenta,
Silene cordifolia
● Screes > Viola argenteria (also in
Corsica), V. valderia
South-western Alps
LIMESTONE SUBSTRATES
● Cliffs > Alyssum ligusticum,
Asperula hexaphylla, Campanula
macrorrhiza, Moehringia sedifolia,
Silene campanula
● Screes > Isatis alpina, Leucanthemum
atratum ssp. ceratophylloides
SILICEOUS SUBSTRATES
● Cliffs > Galium tendae, Jovibarba
allionii, Saxifraga pedemontana,
Sedum alsinefolium
South-western Alps
and northern Apennines
LIMESTONE SUBSTRATES
● Cliffs > Primula marginata, Saxifraga
callosa
North-western Alps
SILICEOUS SUBSTRATES
● Cliffs > Phyteuma humile, Potentilla
grammopetala
Western Alps
LIMESTONE SUBSTRATES
● Limestone cliffs including calcschist >
Androsace pubescens, Minuartia
rupestris ssp. clementei, Saxifraga
diapensioides, S. retusa ssp. augustana,
S. valdensis, Sedum fragrans
● Screes > Allium narcissiflorum (also on
serpentinite), Brassica repanda,
Campanula alpestris, C. cenisia,
Galium pseudohelveticum, Oxytropis
fetida, Saussurea alpina ssp. depressa,
Viola cenisia
Mt. Cenis pansy (Viola cenisia)
SILICEOUS SUBSTRATES
Cliffs > Artemisia glacialis, Campanula
elatines
● Screes (including calcschist) >
Achillea erba-rotta incl. ssp. ambigua,
A. nana, Adenostyles leucophylla,
Campanula excisa, Coincya richeri,
Leucanthemum atratum ssp.
coronopifolium, Thlaspi rotundifolium
ssp. corymbosum
●
Piedmont saxifrage (Saxifraga pedemontana)
Eastern Alps - Insubric district
LIMESTONE SUBSTRATES
● Cliffs > Athamanta vestina, Campanula
elatinoides, C. petraea, C. raineri,
Daphne petraea, Minuartia grignensis,
Rock mezereon (Daphne petraea)
Moehringia bavarica ssp. insubrica, M.
dielsiana, M. glaucovirens, M. markgrafi,
Primula albenensis, P. grignensis,
Saxifraga arachnoidea, S. presolanensis,
S. tombeanensis, S. vandellii, Silene
elisabethae, Telekia speciosissima
● Screes > Galium montis-arerae, Linaria
tonzigii, Moehringia concarenae, Thlaspi
rotundifolium ssp. grignense
SILICEOUS SUBSTRATES
● Cliffs > Androsace brevis, Phyteuma
hedraianthifolium
● Screes > Viola comollia
Eastern Alps - Dolomitic district
LIMESTONE SUBSTRATES
● Cliffs > Campanula morettiana,
Minuartia graminifolia, Primula
recubariensis, P. tyrolensis, Spiraea
decumbens ssp. hacquetii
● Screes > Draba dolomitica, Festuca
austrodolomitica, Rhizobotrya alpina,
Saxifraga facchinii
SILICEOUS SUBSTRATES
● Screes > Saxifraga depressa
Eastern Alps - Carnic-Julian district
LIMESTONE SUBSTRATES
● Cliffs > Arenaria huteri, Athamanta
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70
turbith, Campanula zoysii, Cerastium
subtriflorum, Pinguicula poldinii,
Potentilla clusiana, Saxifraga tenella,
Spiraea decumbens ssp. decumbens
● Screes > Alyssum wulfenianum,
Festuca laxa, Ranunculus traunfellneri
Traunfellner’s buttercup (Ranunculus traunfellneri)
Eastern Alps
LIMESTONE SUBSTRATES
● Cliffs > Androsace hausmannii,
Campanula carnica, Minuartia
cherlerioides, Paederota bonarota,
Physoplexis comosa, Phyteuma sieberi,
Saxifraga burseriana, S. hostii, S.
squarrosa, Valeriana elongata
● Screes (including calcschist) >
Achillea atrata, Androsace vitaliana ssp.
sesleri, Aquilegia einseleana,
Cerastium carinthiacum, Crepis
Carinthian mouse-ear (Cerastium carinthiacum)
terglouensis, Doronicum glaciale,
Galium margaritaceum, G. noricum,
Leucanthemum atratum ssp. halleri,
Minuartia austriaca, Pedicularis
aspleniifolia, Pritzelago alpina ssp.
austroalpina, Saxifraga aphylla, Sesleria
ovata, Soldanella minima, Thlaspi
alpestre, Th. rotundifolium ssp.
cepaeifolium
SILICEOUS SUBSTRATES
● Cliffs > Androsace wulfeniana,
Jovibarba arenaria
● Screes > Androsace wulfeniana
Eastern and Dinaric Alps
LIMESTONE SUBSTRATES
● Cliffs > Festuca stenantha, Moehringia
bavarica ssp. bavarica, Paederota lutea,
Saxifraga crustata, S. petraea, Silene
veselskyi
● Screes > Alyssum ovirense, Campanula
thyrsoides ssp. carniolica, Laserpitium
gaudinii, Papaver kerneri, Silene
quadrifida, Trisetum argenteum
Eastern Alps and Apennines
LIMESTONE SUBSTRATES
● Cliffs > Artemisia nitida, Moltkia
suffruticosa, Potentilla nitida, Valeriana
saxatilis
● Screes > Papaver ernesti-mayeri,
Saxifraga sedoides, Valeriana supina
Northern Apennines
LIMESTONE SUBSTRATES
● Cliffs > Globularia incanescens,
Leontodon anomalus, Polygala
carueliana, Rhamnus glaucophyllus,
Salix crataegifolia, Silene lanuginosa
● Screes > Cirsium bertolonii
SILICEOUS SUBSTRATES
● Cliffs > Primula apennina
● Screes > Murbeckiella zanonii
Central Apennines
LIMESTONE SUBSTRATES
● Cliffs > Aquilegia magellensis,
A. ottonis, Artemisia petrosa ssp. eriantha,
Campanula fragilis ssp. cavolinii, C. tanfanii,
Centaurea scannensis, Cerastium thomasii,
Moehringia papulosa, Pinguicula fiorii,
Potentilla apennina, Saxifraga ampullacea,
S. italica
● Screes > Achillea barrelieri, Adonis
distorta, Alyssum cuneifolium, Androsace
vitaliana ssp. praetutiana, Cerastium
thomasii, Cymbalaria pallida, Galium
magellense, Ranunculus magellensis,
Aquilegia champagnatii, Campanula
pollinensis, Globularia neapolitana,
Hieracium portanum, Lonicera stabiana,
Pinguicula hirtiflora
● Screes > Leucanthemum laciniatum
Sicily
LIMESTONE SUBSTRATES
● Cliffs > Armeria gussonei, Asperula
gussonei, Aubrieta deltoidea, Centaurea
busambarensis, Draba olympicoides,
Helichrysum nebrodense, Minuartia
verna ssp. grandiflora, Silene saxifraga
var. lojaconoi
● Screes > Senecio candidus
● Lava Screes > Anthemis aetnensis,
Saponaria sicula, Scleranthus annuus
Saxifraga speciosa
Saxifraga speciosa, Thlaspi stylosum,
Viola magellensis
Central and southern Apennines
LIMESTONE SUBSTRATES
● Cliffs > Achillea mucronulata, Saxifraga
paniculata ssp. stabiana, S. porophylla
● Screes > Carduus chrysacanthus,
Ptilostemon niveus (anche sulle Madonie)
Central Apennines and Dinarids
LIMESTONE SUBSTRATES
● Cliffs > Androsace mathildae,
Malcolmia orsiniana
● Screes > Drypis spinosa, Heracleum
pyrenaicum ssp. orsinii, Papaver degenii,
Saxifraga glabella
Central Apennines and
south-western Alps
LIMESTONE SUBSTRATES
● Screes > Festuca dimorpha
Southern Apennines
LIMESTONE SUBSTRATES
● Cliffs > Achillea lucana, A. rupestris,
Sicilian soapwort (Saponaria sicula)
ssp. aetnensis, S. vulcanicus, Sedum
aetnense, Senecio aetnensis, S.
ambiguus
Sardinia
LIMESTONE SUBSTRATES
● Cliffs > Armeria morisii, Asperula
pumila, Campanula forsythii, Centranthus
trinervis, Cephalaria mediterranea,
Limonium morisianum
SILICEOUS SUBSTRATES
● Cliffs > Armeria sulcitana, Helichrysum
frigidum, H. montelinasanum, Herniaria
litardierei, Potentilla crassinervia,
Saxifraga cervicornis, Sedum brevifolium,
Silene requienii
● Screes > Sagina pilifera
71
Animal life on cliffs and screes
PAOLO AUDISIO · LUCIO BONATO
As we have already mentioned in the
introduction, conditions are very harsh
for animals living in cliff environments,
whether vertical, bare cliffs or
consolidated debris slopes like screes
and moraines. There are several
natural factors that limit and determine
the survival of animals living in such
areas, the most important of which are
described below.
The steep, sometimes vertical
substrates hinder the development of
Rocky walls and screes are often associated
mobile fauna living above ground.
with each other (Vette Feltrine, Veneto)
The mobility and instability of these
areas, where landslides and debris flows are frequent, makes the life of
non-flying animals extremely arduous. For instance, many terrestrial
mammals find it difficult to move over these surfaces, which their feet cannot
grasp firmly, preventing them from climbing up, down and over debris slopes
and jutting rocks.
These surfaces also thwart apodal (legless) species like snakes and a few
saurians, and many vertebrates that live underground or in litter, like
insectivores and rodents. But they are easily accessible to birds and indeed,
several species use them as food sources, breeding grounds, or simply as
refuges, favoured by little interspecific competition and low predation of
nests. Among these are birds of prey such as falcons, hawks, eagles, ravens
and crows. Among the vertebrates, only birds are truly adapted to life in these
environments.
Invertebrates are favoured by their small size and weight, which enable them
to climb steep surfaces easily. However, the animals with the most successful
adaptations are those that best adhere to the substrate - terrestrial molluscs whose tiny dimensions enable them to creep inside micro-crevices or under
small pebbles. Examples are many microphagous beetles, those associated
Chamois (Rupicapra rupicapra)
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74
with aerial or underground plant parts (which therefore avoid steepness and
direct contact with the ground, like many phytophagous insects), and insects
that fly only as adults, are phytophagous, phytosaprophagous and parasitic,
and adhere to the substrate, like many lepidopterans, dipterans and
hymenopterans.
The typical lack of nutrient supply in rocky montane substrates usually
supports ephemeral, variable plant communities, with low levels of seasonal
primary productivity. In these conditions, animal communities too are
ephemeral and variable.
Unlike fertile pastures, grass- and woodland that develop at high altitudes,
food resources on cliffs and screes are generally insufficient. This affects
phytophagans, which can only rely on scarce, seasonal plant biomass, and
predators, because prey is restricted and varies unpredictably. Nonetheless,
a few birds, rodents and carnivores are actually specialised in searching for
food on screes and debris deposits, and several insect species are even
exclusive to these areas. Another limiting factor is the aridity of substrates,
especially in summer. That is, apart from the phenomena of temporary
condensation and trickling water, the steepness of cliffs and the great
superficial permeability of debris slopes prevent water stagnation and flow.
True soil and significant plant cover that can even temporarily trap water or
hinder its seepage are almost totally absent.
Moreover, direct sunlight and winds accelerate evaporation and
desiccation. This is why mountain cliff environments and their associated
debris deposits do not include those aquatic or even humid habitats
necessary to animal life.
Among vertebrates, not only strictly aquatic species like fish, but also those
partially associated with freshwater, like most amphibians, are totally
missing. Among aquatic insects, flying imagoes of greatly vagile species
may occasionally rest on stones, together with a few hygropetric animals,
i.e., those associated with films of water colonised by algae which trickle
down humid cliffs. Intense sunlight, especially in summer, usually influences
substrates with low thermal capacity, i.e., those which warm and cool
rapidly. This gives rise to thermal stress, daily and seasonal temperature
variations that affect both heterothermic and homeothermic vertebrates. In
addition, the climate is normally microthermic, with cold winds and heavy
snow, which slides off steep cliffs.
Great windiness and exposure to meteoric events responsible for
avalanches, landslides and rockfalls may sweep away the animals living on
Scree in the Carnic Alps (Friuli Venezia Giulia)
Parnassius apollo, with wings damaged by flying in bad weather conditions
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the surface of cliffs and screes, especially tiny invertebrates - including
flying species - thus increasing their mortality and hindering potential
colonisation.
Low temperatures and windiness thwart olfactory and pheromonal
orientation and communication, favouring visual capacity. By contrast, the
scarce vegetation and evenness of structure and colour make most animals,
especially vertebrates, very conspicuous to predators. Regular visitors such
as rodents and song birds are therefore elusive.
Another important factor is the typical fragmentation of these often isolated
mountain habitats. The combination of this and all the above conditions
hinders the breeding and reproduction of many species, particularly the most
specialised ones with small communities and lower distribution capacity,
which are therefore liable to local extinction.
Conversely, there are other abiotic factors which actually favour animal life
and contribute towards mitigating the otherwise harsh conditions of cliff and
scree environments, thus fostering the development and specialisation of
animal communities.
For example, exposure to sunlight, together with restricted snow cover in
winter due to great steepness, favours a local spring-summer-autumn
microclimate. This enables many xerothermophilous Mediterranean animals
like insects, pulmonate molluscs and a few reptiles to live at unusual altitudes
and at very high northern latitudes.
However, when snow melts in summer, alpine and high-altitude screes with
sufficient debris cover provide the optimal conditions for the life of a few
lithoclase arthropods living on incoherent rock debris. The only way of finding
these animals in summer is by patiently digging into the scree surface to the
depth at which cold and condensation last into the summer.
Although wind, lightning and storms negatively affect plants in exposed cliff
environments, there is some beneficial influence on animals, especially
xylophagous insect species. They find ideal conditions for feeding on the
woody parts of suffering, decaying shrubs.
Many xylophages are also xerothermophilous, and find their ideal habitat on
sunny cliffs. Consequently, also the entire community of insects associated
with xylophages (especially hymenopterans and predatory beetles) is
favoured.
In addition, the “hedge effect” of cliffs and river valley inlets interrupting the
landscape profile, provides at its base areas of accumulation for many animals,
especially flying insects (and plant seeds), which are carried by wind. Similarly,
Steepness and exposure to sunlight restrict accumulation of snow on screes
The harvestman Mitopus morio
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Peregrine (Falco peregrinus)
high-altitude and crest screes often
host low-altitude invertebrates (flying
insects like dipterans, coleopterans,
hymenopterans and lepidopterans),
which are carried from valleys to
mountain summits, where they form
ephemeral, “drifting” communities.
Mountain microhabitats, with higher
average
summer
and
autumn
temperature,
favour
temporary
colonisation by these insect species,
increasing biodiversity in such harsh
surroundings.
Lastly, the typical fragmentation of
these montane habitats, which are
isolated from each other and host
small animal communities, may favour
Extensive debris deposits in upper Val
Cimoliana (Friuli Venezia Giulia)
genetic differentiation over time, giving
rise to possible micro-evolutionary
phenomena of speciation, like those that affected invertebrates during the
Plio-Pleistocene macroclimatic cycles. Although mountain cliffs and screes
are extremely selective environments, it is precisely for this characteristic that
colonising animal species, especially arthropods, have very particular
features.
There are three main categories of animals living in these areas:
phytophagans in general, which are variously associated with the aerial
parts of typical local plants, including insects which are anthophagous
(feeding on flowers), phyllophagous (feeding on leaves), xylophagous
(feeding on wood) and spermatophagous (feeding on seeds) associated with
chasmophytes and glareous plants. There are also some molluscs, rodents
and artiodactyls, microphagous and phytosaprophagous animals associated
with selected plant debris deposited in rock crevices and scarce soil
(roughly eroded substrates) underneath pebbles and gravel (such as insects,
a few isopod crustaceans, millipedes and several molluscs), and predators
(arachnids, insects, occasional centipedes, a few reptiles, song birds and
raptors).
The first group includes animals with specific adaptations to these habitats;
the second and particularly the third group include euryphagous animals
that require little food.
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River gorges
From the natural and landscape
viewpoints, few Italian habitats are as
fascinating as river gorges,
independently of their bedrock geology,
altitude, and the rivers or streams that
created them. Part of their charm is their
brusque interruption of the
geomorphological and vegetational
continuity of the landscape. In addition,
they have always been very difficult to
colonise by man, although some
populations did find refuge there, either
inside karst cavities or by widening
natural openings which had recently
been used by grazing sheeps and goats.
For centuries, therefore, gorges avoided
the typical destruction and uncontrolled
modification of natural environments,
thus becoming, especially at low
altitudes, true “natural islands” capable
of surviving severe anthropic impact.
Gorges are important natural “corridors”
between areas, and are also natural traps
in which animals living nearby fall. They
are refuge areas for many vertebrates
escaping predators and disturbing
factors, and some bird species also find
optimal safe nesting and breeding
grounds.
The deepest and narrowest gorges reveal
considerable vegetation inversion caused
by the different positions of the rocky
walls. Plants typical of the upper horizon
grow in the valley, which is more humid
and shaded, and those that normally
grow at lower horizons develop high up
on sunny versants. This makes unusual
stratifications possible, especially along
south- and north-facing versants, where
xerothermic, sub-Mediterranean and the
opposite sciaphilous, cryophilic flora and
fauna mix. This gives rise to surprisingly
high levels of local biodiversity, with
phenomena similar to those of the deep
Paolo Audisio · Lucio Bonato · Marcello Tomaselli
River cirque in the Abruzzi Apennines
dolinas (sinkholes) typical of karst
landscapes (see the chapter “Dolinas” in
the Habitat volume “Caves and karstic
phenomena”).
Many of the animals and plants that
colonise the vertical or subvertical walls
of gorges are the same as those living on
mountain cliffs, but the peculiar
conditions mentioned above enable
larger numbers of species from different
areas and with differing ecological
requirements to inhabit these gorges,
especially the deepest ones.
The mixture of xerothermophilous and
sciophilous elements is particularly
evident in the vegetation growing at
medium-low altitudes. Phytoclimatically
distinct species with similar behaviour
grow within a few metres of each other,
like evergreen oak (Quercus ilex) and
beech (Fagus sylvatica). There are also
rare plants that are exclusive to these
areas (palaeo-endemics, species with
fragmented distribution or which
survived as relics).
In the South-western Alps, rocky gorges
at medium-low altitudes host
phytogeographically important species
such as shrubby horehound (Ballota
frutescens) and Allioni’s primrose
(Primula allionii). Typical inhabitants of
these environments in the South-eastern
Alps are Carnic sandwort (Arenaria
huteri) on limestone mountains east of
the river Piave, hairy spiraea (Spiraea
decumbens) and, on cliffs with trickling
water, the recently discovered Poldini’s
butterwort (Pinguicula poldinii).
Cirques in the Apuan Alps frequently
contain Reichenbach’s butterwort
(Pinguicula reichenbachiana), those in the
central Apennines short-haired sandwort
(Moehringia papulosa), the Gola del Furlo
in the Marches and the Gole in Val Nerina
house Mt. Nebrodi joint pine (Ephedra
major), and cliffs with rilling water
columbine (Aquilegia ottonis) and Fiori’s
butterwort (Pinguicula fiorii).
Among phytophagous insects in
particular, there are highly specialised
species of great conservative value that
are exclusive to these relict areas.
Carnic sandwort (Arenaria huteri)
Mt. Nebrodi joint pine (Ephedra major)
This is the case of snout beetles like the
rare weevil Ceutorhynchus pinguis, an
Apennine endemic associated with the
crucifer diffuse alyssum (Alyssum
diffusum), C. verticalis, a southern
Apennine endemic associated with
vertical cliffs, as its name implies, and
living on Aurinia saxatilis, and particularly
Mesoxyonyx osellanus, another Apennine
endemic which was only recently
discovered and is associated with the
rare, relict Ephedra major, which are also
found at medium altitudes on vertical
limestone cliffs of river gorges.
Among nitidulid beetles, Meligethes
lindbergi is typical of Apennine, Sicilian
and Sardinian river gorges. It is
monophagous (feeding on a single type
of food) and associated with the
germander Teucrium flavum (a plant of
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River gorges
the mint family). The similar M. nuragicus
lives on Teucrium massiliense in
analogous inland environments in
Sardinia. Meligethes subfumatus is often
found on sunny cliffs in the Maritime and
Ligurian Alps on another member of the
mint family, Lavandula angustifolia, and
the Italian sub-endemic species M.
scholzi is typical of medium-low altitudes
on rocky cliffs and gorges of southern
Italy, feeding exclusively on Ballota
rupestris (again of the mint family).
A few xylophagous, xerophilous
Mediterranean and sub-Mediterranean
beetles live outside their usual areas on
exposed, sunny cliffs of river gorges, like
several long-horned beetles and a few
buprestids, the larvae of which develop
exclusively or preferentially within the dry
twigs of evergreen and sclerophyllous
oaks (Cerambyx scopolii, Anaglyptus
gibbosus, Stromatium unicolor,
Trichoferus holosericeus, Deroplia genei,
Latipalpis plana, etc.), on Genista
(Trichoferus spartii) and figs
(Stenhomalus bicolor). Other species
typical of mesophilous habitats colonise
shady versants and are associated with
broadleafs like linden (Exocentrus
lusitanus), hornbeam (Axinopalpis
gracilis), and even beech (Rosalia alpina,
a species of European Community
interest), and a few mountain pines (e.g.,
Arhopalus ferus).
Typical bugs of these environments are
several lace bugs (Tingidae) of the genus
Copium, associated with small cliff
germanders (Teucrium).
Among hymenopterans, sphecid wasps
of the genus Sceliphron attach their mud
nests to the cliffs. They hunt spiders on
which their larvae feed. A few species of
potter wasp (Eumenes) can use the same
areas as nesting grounds. Their jug-
Paolo Audisio · Lucio Bonato · Marcello Tomaselli
Cerambyx scopolii
Anaglyptus gibbosus
Stromatium unicolor
shaped nests have a central bulge, which
is moulded after the cells have been
stocked with caterpillars and their eggs
laid. There are also several lepidopterans,
especially the rare papilionid butterfly
Papilio alexanor, which lives in isolated
locations and is associated with
xerophilous umbellifers growing along
river gorges in the Maritime Alps,
Calabria and eastern Sicily. The
nymphalid butterfly Polygonia egea,
which is now very rare and localised,
generally grows on pellitories (Parietaria of the nettle family).
Among preying spiders, the three Italian
Segestria species (Segestria bavarica, S.
florentina, S. senoculata) can be found all
over Italy in cliff crevices and under
stones up to intermediate altitudes.
River gorges traversing limestone are
typically inhabited by hygrophilous and
xeric mollusc species, according to the
varying degree of humidity of the local,
cool microclimate. Among them are
several prosobranchs of the genus
Cochlostoma, the species of which live
in restricted areas, the pyramidulid
Pyramidula pusilla and P. rupestris, and
numerous species of chondrinids of the
genera Rupestrella, Condrina and
Solatopupa (all of which live in restricted
areas). There are also numerous pulmonate snails of the genera Clausilia
and Charpentieria, most of which live in
northern Italy, Delima (central Italy),
Papillifera, Leucostigma, Medora in the
Apennines, and Sicillaria and Muticaria
in southern Italy. There are many cliff
species of snails living on the cliffs near
river valleys, like Chilostoma in the Alps,
Marmorana in the Apennines and on the
major Italian islands, and Tyrrheniberus
in Sardinia.
Although for many vertebrates the
vertical, rocky cliffs of river gorges are
inhospitable environments, some species
actually find these areas optimal habitats
to shelter and reproduce. Cliffs are
inaccessible to predators and unlikely to
be disturbed by man, and they also offer
environmental (microclimatic) conditions
which differ from those of surrounding
areas. In southern Italy, these cliffs
(below 1000 m) are nesting grounds for
birds of prey like Egyptian vulture
(Neophron percnopterus), Bonelli’s eagle
(Hieraaetus fasciatus) and lanner (Falco
biarmicus). Although not exclusive to
river valleys, other cliff species regularly
breed in these areas, examples being
blue rock-thrush (Monticola solitarius),
black-eared wheatear (Oenanthe
hispanica) and raven (Corvus corax).
Unfortunately, river gorges have recently
suffered severely at the hands of man.
The greatest threat comes from roadbuilding projects, as valleys are obviously
natural shortcuts. Roads, motorways,
railways, pipelines and power lines have
often been built to pass through river
valleys, destroying or greatly jeopardising
the landscape and biological quality of
the valleys themselves and of the rivers
flowing through them. In particular, roads
and railways are often protected against
rockfalls and landslides by controlled
explosions, and nets, metal barriers and
reinforcements are then set up.
River valleys are also affected by the
uncivilised practice of many
Mediterranean countries, which use them
as dumps for civil, agricultural and even
industrial waste (especially rubble from
construction sites, obsolete household
appliances, sewage, animal carcasses,
etc.), which have obvious consequences
on the local animal and plant
communities.
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