Download CHAPTER 25 Mediterranean Environments

CHAPTER 25
Mediterranean Environments
Introduction
The regions of the world with a Mediterranean-type climate have very distinctive
physical environments. The climate is transitional between tropical arid climates, on the
one hand, and humid temperate climates on the other. The stresses imposed by the heat
and aridity of the summer months have striking influences on Mediterranean ecosystems
– vegetation, soils and wildlife. Physical and ecological processes are active in the winter
months, when winter precipitation provides the moisture necessary for: weathering;
erosion of soils and hill slopes; river action; processes of humification, leaching, clay
translocation and clay mineral formation in soils; and the net primary productivity of
vegetation. By contrast, the summer is a season of desiccation, drought and relative
inactivity in landscape processes.
Human activities have had a major impact on regions with Mediterranean environments.
The Mediterranean region proper has a long history of human modification of the natural
landscape, stretching back into prehistoric times. Impacts from agriculture and nonagricultural industries have continued to increase, so that at the present time they have
never been more intense. Although the Mediterranean areas of California, Chile, South
Africa and Australia have long-standing impacts from indigenous peoples, modern
impacts on a large scale have occurred for only a century and a half. However, in all
these regions there are problems of land degradation, fire destruction, desertification and
problems of water quantity and quality.
Chapter summary
Climate: present and past
•
Summer months are dominated by high-pressure cells, whereas in winter
depressions bring changeable weather and precipitation.
•
Winter depressions in the Mediterranean region proper come from the south, the
west and the north, with their tracks being considerably affected by relief.
•
Rainfall varies from 300 mm to 750 mm, with bimodal distribution in countries
along the northern Mediterranean shore and a single maximum in the eastern and
southern Mediterranean.
•
Rainfall totals are very variable from season to season, as is shown by values of
the coefficient of variation of the order of 30 per cent.
•
Intense storms in autumn are extremely erosive, as they fall on dry, dusty soil
surfaces.
Soil formation and distribution
•
Hydrolysis of rock-forming minerals in the winter season makes clay minerals
and sesquioxides.
•
Dehydration in the summer season leads to some formation of anhydrous iron
oxide (haematite) which gives the distinctive red colour to terra rossa soils.
•
Brown soils (terra fusca) have higher organic contents and are moister.
•
During the first rain of autumn clay-sized particles in the A horizon are leached
into the Bt horizon.
•
Organic matter is readily oxidized during the hot summer, leading to low contents
of nitrogen and phosphorus.
Development and adaptation of vegetation
•
During the last glacial maximum the Mediterranean climate was cooler and dryer
than at present, with trees on the uplands and dry steppe on the lowlands.
•
The sequence of development during postglacial times has varied from area to
area, but the general overall trend has been to produce a climax of Mediterranean
evergreen oak forest.
•
The development of vegetation has been much affected by human activities of
different cultural ages, the net result being a majority of scrub (maquis) and heath
(garrigue) communities.
•
Mediterranean plants, from trees to the smallest herbs, show a variety of
ecophysiological and structural adaptations to severe water stress during the hot
summer.
•
There are many legumes in the Mediterranean flora, which greatly helps growth
in the nitrogen-deficient soils.
Distribution of the plant communities
•
Exposure and ‘rain shadow’ effects lead to many regional differences in rainfall,
and hence vegetation, though the two general trends are for aridity to increase
from west to east and from north to south.
•
Vegetation zones change rapidly with altitude as precipitation increases and
potential evapotranspiration decreases.
•
With increasing altitude the climax vegetation changes from evergreen oak to
deciduous forest to a coniferous forest of pine and fir.
•
Aspect is an important control of Mediterranean vegetation, with south-facing
slopes having higher temperatures and higher radiation levels, and north-facing
slopes having higher soil-moisture levels.
•
The lime (calcium carbonate) content of soils has an important influence on the
distribution of plants at the species level.
Fire in the landscape
•
Fire is an important ecological factor in Mediterranean ecosystems, being a
common occurrence in the landscape.
•
Many plants are adapted to fire, whether by growth adaptations or by rapid
regeneration after fire by shoots or seeds.
•
It is difficult to get reliable information on the frequency and causes of fires, as
reporting varies in reliability from country to country and with time.
•
Fires are caused by natural means, by deliberate land management, by accident or
by arson.
•
Afforestation of Mediterranean lands has greatly increased the fire risk, owing to
the widespread planting of pine and eucalyptus.
Desertification and soil erosion
•
Desertification refers to the total degradation of land ecosystems, with serious
impacts on all aspects of society.
•
Once the vegetation cover falls below 50 per cent, run-off, sediment yield, surface
splash and surface sealing all increase dramatically, with a consequent rapid rise
in erosion rates.
•
The role of plants in lessening erosion rates is a result of root systems, higher
organic matter contents and the improved structure and infiltration beneath plants.
•
‘Badlands’ show dramatic effects of gullying, piping and linear erosion slopes,
and are frequently found on marls.
•
Surface crusting results from the destruction of soil aggregates by intense rain,
and the repacking of the mineral particles and chemicals into a ‘surface skin’.
Water supply problems
•
Mass tourism, urbanization and agricultural irrigation are making enormous
demands on the water supplies of the region.
•
Groundwater supplies from underlying rocks depend upon the specific yield and
permeability of the rock.
•
Sedimentary rocks are far more important as aquifers than are igneous and
metamorphic rocks, though weathered and fractured igneous and metamorphic
rocks can be good aquifers.
•
Along coasts, fresh water is normally in equilibrium with saline ground water, but
overpumping of the aquifer causes salt water to penetrate inland.
•
Many Mediterranean countries are reporting problems of sea-water intrusion into
coastal aquifers, a problem which either requires expensive reclamation measures
or is irreversible.
CASE STUDY Wildfires in California during August, September and October 2007
We have seen that two key factors in the spatial structure and ecology of Mediterranean
vegetation are the long summer drought and fire. Fires frequently make the headlines
when lives and property are threatened, but fire is more common than popularly
supposed, and in general vegetation burns every 10-30 years in the Mediterranean biome,
In fact the vegetation structure and the volatile oils in the plants promote burning, and
ecologists recognise that both shrublands (maquis, chaparral) and heaths (garrigue) are
fire-dependent systems Several species are unable to reproduce without fire, and species
diversity and productivity decline after long periods of time without fire.
In addition to the flammability of the plants and plant litter, there are factors of climate
and weather which promote annual fires. High temperatures and low humidity in summer
are clearly important, because fuel flammability is increased by long periods of hot, dry
weather. In addition, dry winds provide an important mechanism to increase the fire’s
intensity and rate of spread. Winds not only drive the fire forward, but bring the flames
down into contact with flammable fuel on the ground. For this reason, also, fires spread
more quickly and burn with greater intensity when moving upslope. The risk of fire is
enormously increased by the incidence of hot, dry winds, and fire-alerts are triggered
when such winds are forecast. The sirocco is a hot, dry and dusty South or South-easterly
wind blowing from the Sahara in the Mediterranean basin. The Santa Ana is a hot,
desiccating wind which blows south-westwards from the Sierra Nevada onto the coastal
plain of southern California. It is a föhn-type wind, heating and drying adiabatically as it
descends. Forecasts of Santa Ana winds during spring and summer trigger fire-alerts in
the State.
Human impact on the biome in Europe, Australia and California has radically changed
the fire-regime. Urbanisation has brought a greater risk of fire from non-natural causes,
both accidental and deliberate. Also, the use of flammable species such as Aleppo pine
and eucalyptus in forestry schemes is widespread. The hazard of fire is also greatly
increased by housing and commercial development. Towns and rural communities in
Mediterranean regions rightly demand protection from fire, but it is now recognised that
the policy of preventing and controlling fires has itself changed the nature of the fires. In
California, for example, the size of individual fires in the chaparral has greatly increased
due to fire-control measures. Before fire-control, natural fires were frequent but small,
with even the largest averaging less than 400 hectares in area. This resulted in a
patchwork of areas of mature chaparral with large amounts of fuel, scattered amidst areas
of more recently burned chaparral with lower fuel loadings, which acted as natural firebreaks controlling the spread of fire.
By excluding fires, authorities have produced a landscape with much larger areas of old
chaparral with huge amounts of fuel. It is scarcely surprising that large chaparral fires
over 10 000 hectares in extent are now much more common. Warnings about the
increased fire-risk and its consequences are dramatically illustrated by the disastrous
conflagrations which affected the chaparral-dominated parts of southern California in
August, September and October 2007.
Statistics on wildfires are notoriously imprecise. There are different agencies to whom
reports are made, depending partly on land ownership. Preliminary data for 2007 from
the California Department of Forestry and Fire Protection (CDF or CAL FIRE) are
shown in Table 25.1. Compared to 2006, the number of fires decreased by 32% but the
area burned increased by 36%, with mean fire-size increasing by 100%.
Table 1 Number and size of fires reported by CAL FIRE, 2006 and 2007.
Year
2006
2007
5-year mean
Number of fires
11,119
7,589
9,880
Total acres burned
272,566
371,168
219,264
Total hectares
burned
110,306
150,210
88,735
Mean fire size,
hectares
9.9
19.8
9.0
Additional statistics on wildfires in southern California in 2007 are given in Table 25.2.
The large category of ‘unidentified and indeterminate’ causes will decline as more reports
come in. Among ‘human’ causes are campfires, barbeques, smoking, and equipment
malfunction. The small number of fatalities is witness to the efficiency of alarm and
warning systems in place. The largest fire was the ‘Witch’ fire in San Diego county
which started on 21st October 2007 and eventually burned 80 000 hectares, destroying
1650 homes and 499 outbuildings. The fire was started by human action, but fortunately
there were no fatalities.
Table 2 Causes and damage reported by CAL FIRE in 2007.
Causes
Lightning
Human actions
Arson
Unidentified/indeterminate
Damage to property
Homes damaged
Homes destroyed
Fatalities
Fire department
Civilian
Population forced to move from their homes
16%
25%
5%
53%
574
3043
1
1
>1 million
Because fire suppression leads to a dangerous build-up of fuel loads, government
authorities in the United States have a programme of prescribed burns under controlled
conditions, in order to imitate natural fire regimes and decrease fuel loads. This has been
relatively successful, except when these prescribed fires have got out of control with
disastrous results. This happened when severe damage was caused to the town of Los
Alamos, New Mexico, in 2000.
Figure 1 Map of the distribution of fires in Southern California.
The map shows the distribution of wildfires in the Mediterranean biome of Southern
California which had been contained by the beginning of November 2007. Autumn is a
common time for wildfires in this biome, as the effects of summer heat and the build-up
of flammable organic matter lead to very high fire-risk. Fires can then be easily started by
autumn lightning storms or human carelessness.
Source: http://boxer.senate.gov/calfire.cfm#MAP
Credit: California Department of Forestry and Fire Protection (CALFIRE)
Essay questions
1. Discuss the positive and negative effects of fire in Mediterranean regions.
2. How do Mediterranean plants survive the summer drought?
3. In what ways do Mediterranean processes of soil formation differ from those of
cool temperate regions?
Discussion topics
1. Discuss the erosional processes operating in badlands. How would you measure
the rates of these erosional processes? Discuss why there have been different
estimates of erosion rates operating in badlands.
2. How will future environmental change influence the processes of desertification
seen today in many Mediterranean environments? What conservation measures
are likely to be successful?
3. How can Mediterranean countries solve their problems of water quantity and
quality?
Web resources
http://www.medalus.demon.co.uk
This 8-year interdisciplinary study of Mediterranean Desertification and Land Use
(MEDALUS) was funded by the European Commission (EC). It represents one of the
most important recent scientific studies of the Mediterranean environment in Europe, and
the pressures to which it is being subjected.
http://www.csic.es
Many countries in Mediterranean Europe have active government programmes of
research into, and where necessary, rehabilitation programmes for, their Mediterranean
ecosystems. The Consejo Superior de Investigaciones Cientificas (CSIC) in Spain is one
of the most famous. Follow the web site for two important scientific stations: Centro de
Investigaciones sobre Desertificatión (CIDE), Valencia, and Estación Experimental de
Zonas Aridas (EEZA), Almería.
http://cdfdata.fire.ca.gov
The website of the California Department of Forestry and Fire Protection.