Download Indirect assessment of natural values at the stand level

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Field Assessment of Forest Ecological Values
- a structures and substrates approach at the stand level
1. Species or structures?
Maintenance of natural biological diversity is a fundamental aspect of sustainable forest management. Various
tools must be combined to address this ambitious goal, such as conservation of high biodiversity areas,
restoration of biodiversity areas in regions where forest ecosystems have been substantially simplified due to
human activities, and mimicking of natural forest elements in everyday management.
Integral to all these approaches is the ability to identify forest areas, succession stages and ecosystem
characteristics of special importance to forest-living flora and fauna in general and red-listed species in
particular. Usually, identification takes the form of species inventories. The resulting lists of species found in
certain areas has obvious merits in many cases, as have the associated improvement of species distribution maps.
However, there are also a number of limitations that make species approaches less suited in many situations:
- Few biologists have the wide knowledge necessary to cover a broader spectrum of organisms. Thus, teams of
trained specialists are necessary to produce a comprehensive picture of the biodiversity in a certain area. Such
high quality species inventories are too time-consuming and expensive to be an option except in special cases.
- To become more feasible, inventories are frequently limited to certain organism groups that are relatively easy
to monitor, like birds, mammal “wildlife” or vascular plants, an approach that tends to neglect habitats important
for other, ecologically dissimilar organism groups. Even in the Scandinavian countries, where the scope of
inventories has lately been expanded to include wood-living fungi, lichens and bryophytes, habitats for insects,
the most species-rich organism group in the Nordic red data lists, receive little qualified attention mainly due to
methodological constraints. The other option is to focus on a sub-set of “indicator” species representing a
broader range of taxa. While this might become a more fruitful approach in the future, most relations between
the “indicator species” and the “indicated species” are as yet tentative and uncertain.
- Many species are inconspicuous and easy to overlook in areas where they occur sparsely. Some species are
very short-lived and can be detected only during very brief periods, sometimes with several year intervals. There
is thus considerable risk that inventories under-estimate areas where the species searched for were not found
(although they do live there), areas where these species are but temporarily absent, or areas that have yet not
been colonised.
- In spite of a recent substantial increase in knowledge, we still know little about the ecology and long-term
landscape requirements and habitat dynamics of most forest living species. Established search paradigms tend to
equalise habitats where species are found today with their optimum habitats in the natural landscape, even
though current sites might be sub-optimal surrogates for habitats that have largely disappeared in the managed
landscape, e.g. fire sites and post fire successions. As a result most attention is focused on “undisturbed” oldgrowth sites, and the biodiversity in e.g. logging sites and other habitats drastically affected by natural
disturbances and human activities is frequently under-estimated.
- Species approaches tend to exclude forest managers and largely fail to utilise the educational potential of the
value assessment process. Furthermore, lists of species by themselves provide little guidance for the everyday
decisions of ordinary forest managers and their possibilities to modify their ecological impact.
There is thus an imperative need to bridge the gulf between the biologic community and that of the forest
managers, each with their own specific but little over-lapping skills, and to develop operational and costeffective approaches to assess ecological values as a complement to traditional species inventories. This article
presents one such method, developed in a Swedish context and currently in wide use in the forest sector. It is
based on assessing the potential for biodiversity as it is reflected in occurrences of natural processes, structures
and substrates to which species have adapted through the evolution. Also taken into account are ecologically
important elements of ancient cultural forest landscapes shaped by human activities, e.g. large, solitary, trees in
open or semi-open woodlands.
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Assessing “structures rather than species” avoids a number of the limitations of traditional inventories - provided
that the set of processes and structures chosen for assessment represents a broad enough range of ecologically
significant elements. However, an underlying assumption is that species occur primarily in habitats that contain
certain visible features. A structural approach will not recognise e.g. a site where the single piece of dead wood
left in the stand contain a random occurrence of a very rare fungus. It will also fail to differentiate between two
otherwise identical stands where historical, invisible factors like very long “continuity” is invoked as the main
cause for a substantially higher biodiversity in one of them. However, while at least the first case certainly
occurs now and then, we do not believe that either situation is common enough to be a serious obstacle.
The main benefits of structural approaches, rather than species occurrences, are that they focus on the habitat as
such with its many niches and inherent dynamics. Furthermore since they are simple and fast they can be used,
with a minimum of training, by ordinary foresters and forest owners as an integral, operational planning tool
over large areas. If resources permit, stands and sites where high values are identified can be targeted for more
in-depth inventories as a second step to further consolidate the basis for planning and decision-making.
In the Nordic countries, with their relatively species-poor forest ecosystems and well-known flora and fauna,
approaches based on species and structures both have a role to play in conservation and restoration. In other,
more complex forest types, e.g. in the tropics where there is less knowledge about species, assessment of
processes and structures is frequently the only viable option.
2. Habitat categories and disturbance dynamics
The method developed by the authors can be described as a “check-list of elements in a matrix of disturbance
dynamics related forest categories”. It permits systematic and repeatable assessment of forest processes,
structures and substrates, e.g. traces of fire, tree-species composition, age structure, coarse dead woody debris,
ground vegetation, soil and water characteristics etc. While many elements relate to structures presumed to
function directly as potential habitats for certain organism groups, other elements are included because they
contribute to the general niche diversity, e.g. canopy composition, topographic variation or lime-rich soils. Some
other features, e.g. trees with fire scars or signs of former grazing, serve mainly as indicators of previous stand
history that might influence the present biodiversity potential.
The core of the model is a number of “yes-or no” questions, grouped together into six different forest categories,
primarily identified by their different natural, and to some extent cultural, disturbance dynamics. In order to
avoid mixing elements from e.g. stands shaped by recurrent fires with elements that are more characteristic of
sites where fires have been uncommon, all elements assessed are related to the specific category where they
would appear naturally. To give a couple of examples; we consider old stumps with several fire-scars to be a
positive value in forests on drier ground composed of pine or pioneer deciduous tree species, while the same
feature in an old spruce forest would indicate an interruption of continuity. On the other hand, while we regard a
thick moss carpet on stones and boulders to be a positive trait in spruce forests, such lack of natural disturbance
does not add value in fire-prone forest types.
2.1 Early succession stages after disturbance
Early succession stages after natural stand-replacing disturbances like forest fires, major storms, or beaver-dams.
These habitats are often characterised by various amounts of dead or dying standing and fallen trees, damaged
but surviving trees, and some minor areas that escaped the disturbance. Most harvesting sites are classified in
this category, and, where planned and executed to mimic a certain amount of naturally occurring elements, such
areas can score rather high. Most harvested areas in Scandinavia however, lack fire-damaged trees and firescarified soils.
2.2 Fire-adapted pioneer tree forests in mid- and late succession stages
Drier forest sites with a history of more or less frequent stand-level fires in their natural state. These sites used to
form the dominant part of European boreal forests at low and medium altitudes. In natural conditions, new trees,
germinated in pulses after each fire, add to the survivors and create multiple age pine forests. On fertile soils,
more even-aged succession phases of deciduous pioneers like birch, aspen, alder and sallow develop, often with
some pine. Today, in the absence of natural fire dynamics, pioneer tree forests on more productive sites tend to
become dominated by spruce.
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2.3 Spruce forests affected by gap-dynamics
High humidity sites, seldom affected by fires in their natural state. New trees germinate more or less
continuously in gaps created by dead or fallen ancestors. Where conditions limit canopy closure, there is
frequently a natural mix in of deciduous trees. Spruce forests with high biodiversity potentials represents a wide
variety of conditions, from slow-growing, semi-open stands at high elevations or on mires, to very productive,
herb-rich stands. In the absence of fires or human disturbance however, biologically diverse spruce forests can
also develop on drier, naturally more fire-prone sites.
2.4 Broadleaf forests affected by gap-dynamics
Closed-canopy, multiple aged, deciduous forests of shade-tolerant tree-species, mainly beech, often mixed with
elm, lime, hornbeam, ash and maple. Some large oaks might be present due to earlier disturbance. Regeneration
occurs mostly in smaller gaps, but in some cases fungi or insects calamities can affect whole stands. Grazing and
browsing by large herbivores and flooding due to beaver activity had a substantial forest impact in the natural
state. Though they previously covered extensive parts of continental Europe, in Scandinavia such forests are
restricted by climate to the southernmost parts. There some sites have a long history of continuous forest cover,
while others are the result of active planting or spontaneous reforestation of previously open or semi-open land
in relatively recent times.
2.5 Water level fluctuation-affected deciduous forests
Forest sites along rivers and lakes, seasonally inundated due to naturally fluctuating water levels. Stands are
usually composed of flood-tolerant alders, pubescent birch and osiers, in colder climates sometimes with their
bases scarred by floating chunks of ice. Similar forests occupy topographic depressions with temporarily
standing water due to e.g. melting snow. In contrast to most sites, where pioneer trees eventually get outcompeted by spruce, areas disturbed by seasonal flooding can maintain more or less perpetual pioneer tree
dominance. Stands of a similar character have developed as a result of human interventions, e.g. after draining of
lakes or in old clay pits. Here the pioneers have upheld their dominance due to regeneration from stumps or roots
after coppicing or logging for fuel-wood.
2.6 Cultural landscape forests and woodlands
This category represents a rather heterogeneous group of forests and woodlands, characterised by contemporary
or historic grazing / mowing for hay. More fertile sites were often used as open forest meadows with solitary
oaks or pollarded ash or linden trees. Grazing by domestic cattle disfavour regeneration of palatable deciduous
trees, and less productive, intensively grazed sites near farmsteads and villages often turned into open coniferous
forest, to juniper shrub-land or even to open heath. When no longer used for grazing, these sites are
spontaneously reforested with a mixture of pioneer deciduous trees and spruce. Frequently though, some
scattered, wide-branched trees bear witness of a previous open woodland era.
3. Field assessment
The assessment is based on a field form containing eighty different questions. For each forest category we have
selected the fifty that we consider most relevant for reflecting the biodiversity potential in that specific forest
category. These questions have all been marked with a ring in the matching column in the field form. Elements
that we consider important for several categories have rings in several columns, and some elements that add
value to all categories have rings in all columns. Elements that we regard as less relevant for a certain forest
type, or even negative, lack rings in the corresponding forest category column.
The sites assessed should preferably be influenced by similar disturbance dynamics over the whole area. We
suggest that sites that are obvious mixes of different forest types are subdivided into more homogenous sub-sites,
and that these are assessed separately on different field forms. Once the site is delineated, there is often little
doubt which forest category that best fits the actual situation. In such cases, the assessor focuses on the questions
with rings in the matching column and pass over the others. Sometimes however, especially in areas with long
history of human influence, stands have characteristics in-between two or more categories making the choice
less obvious. In such cases we recommend the assessor to answer all questions in the two or more relevant
columns. All rings that correspond to questions answered with a “yes” are ticked off, while rings corresponding
to questions answered “no” are left unmarked.
The first forty questions (the left part of the field form) address general site features and habitats, together with
indicators of dynamic processes and previous site history. These score even where they occur as single elements,
e.g. “spring / spring-brook in forested surroundings”, or “signs of former – recent fire on trees and stumps”. For
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simplicity and consistency we require that most special habitats, in order to score, attain a minimum size of 0.1
hectare.
The second set of questions, numbers 41-80, address stand characteristics. They are preceded either by “several”
or by “substantial amounts” and refer to averages per hectare. While “several” denotes an average of more than
two per hectare, “substantial amounts of” represents a higher threshold, even if it is difficult to relate it to an
absolute number. It should be interpreted as significant occurrences of a certain element, i.e. one should be able
to notice the element when assessing the stand without specifically looking for it. This normally means that the
element is present in greater quantity than in the average, managed stand.
The stand-based, average character of questions 41-80 allows for some “give and take”. Thus the assessor can
chose “several” even if parts of the site lack the element, provided that this is compensated by more in other
parts. In a similar fashion “substantial amounts of” can be chosen where an absence of the elements in some
parts is balanced by a great abundance in other areas.
The word “conspicuous” is used through the assessment to denote a certain magnitude or quality “that catch the
eye”, e.g. “several trees stand out as conspicuously older / larger / than the stand in general”. Such trees,
remaining from an earlier forest generation or woodland era, are both older and larger than the other, younger
trees that dominate the stand in terms of numbers. The word “conspicuous” is used in a similar fashion in
relation to certain features, e.g. “conspicuous forest brow with herbs / shrubs”.
All diameters of trees refer to breast-height values and Swedish conditions, and the centimetre figure, if stated at
the end, refers to all elements specified in the question.
4. Calculating and interpreting the final score
When all questions with rings in the relevant category columns have been answered, the number of “yes”
answers are summed up as the site score + the stand score = total biodiversity potential score for each considered
column. The sums are noted on a separate space on the front cover of the form, (together with a general site
description and additional information relevant to the site gathered during the assessment or from other sources,
e.g. occurrences of less common species, cultural artefacts etc).
Every “yes” contributes equally to the total score, i.e. all elements, features and habitats have been assigned the
same, basic value. This, of course, is a gross over-simplification – in reality there is no way to calculate, no less
compare, the ecological functions and values of entities as different as e.g. a brook, a certain area of rocky
outcrops, or a certain amount of very large trees. In practise however, this cause less problems than would appear
at first sight. For areas that score high, the overall sum represents a combination of many different aspects where
each specific element contributes but a little to the total score, levelling out some of the inconsistencies. Some
elements also tend to be naturally inter-linked and thus collect greater weight; e.g. the brook mentioned above
might get additional points by the “seasonally inundated area > 0.1 ha in forested surroundings”, by
“…meandering in sand or silt”, or by the occurrence of “white-water / rapids / waterfall in forested
surroundings”.
Furthermore, we have intentionally amplified a number of elements that we consider of special importance as
habitats or as habitat indicators for species-rich organism groups, e.g. various kinds of deadwood. As an
example, the question “several downlogs > 20 cm” is followed some rows below by the question “substantial
amounts of downlogs > 20 cm. Since all questions should be answered independently in the order they appear,
any site with “substantial amounts” of downlogs > 20 cm” will score twice for this element – there will
necessarily be “several” such downlogs as well.
While ordinary, mature managed stands generally score but a few points, areas with high values could in theory
get a total score of 50. In reality however, even quite outstanding areas rarely score more than 30. The reason is
that each forest category represents a simplified classification of the multiple-faced real world, with a large
number of related – but still rather different – forest types brought together under the same heading. As an
example, the gap-dynamic spruce forest column contains low-productive bog-forests, ravine forests, herb-rich
forests, and mountain forests, all with their specific assemblies of value-adding elements. In order to score the
theoretical maximum of fifty, an area would have to contain a number of contradictory or mutually exclusive
traits, e.g. an open stand structure with slow-growing trees, in combination with herb-rich ground vegetation and
ample supply of ground-water enriched by oxygen and nutrients.
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All value assessments - be they based on species or structures - are subjective processes adding basically
incompatible entities. Thus, all estimations, scores and figures should be treated with caution and good
judgement. The score should be seen as a general estimate of the site biodiversity potential, facilitating
comparisons between different sites mainly within the same forest category and the same region. Comparisons
between sites from different categories demand solid regional experience, so that conservation efforts focus on
regionally underrepresented forest types even where those sites score slightly lower than sites from other
categories, better represented in the region.
To form the basis of planning, the score must be evaluated against e.g. site size, location, relative rarity, and the
potential to restore, maintain or enhance the site conservation values. In most cases however, conservation
aspects are unlikely to be the sole determinants of planning. Thus, in reality forest managers will often have to
take decisions as to which sites to prioritise for conservation and/or restoration efforts. As long as comparisons
refer to sites within the same category and of roughly similar size, the total biodiversity potential score serves as
a useful point of reference for such decisions.
5. Utilisation
Assessment of potentials for biodiversity as outlined above, can be applied to all forest types, independent of age
and stand characteristics. Thus, the methodology is not limited to sites of old-growth, key-habitats or other areas
with high biological values. Since most of the questions are based on easily distinguished features and concepts
of dynamics already familiar to foresters and forest owners, little additional training is needed to perform the
assessment. According to our experience, most people are capable of performing reasonably consistent and
calibrated assessments already after two days of instructions and practise in the field.
The methodology is primarily intended as a tool to identify areas in the forest landscape that contain, or has the
potential to contain, high biological values, in order for these areas to serve as focus points of conservation and
restoration efforts. Assessment for such purposes forms an integral part of forest certification in Sweden, both in
accordance with the FSC, and as practised by some forest owners associations. The method is also applied as a
cost-effective tool to guide in-depth species inventories to certain sites. Other usage includes internal monitoring
of ecological performance in everyday management activities like thinning and logging.
Furthermore, since the rings that were left unmarked indicate natural value-adding elements currently missing
from the stand, assessments made prior to forest operations serve to guide planning and management designed to
restore or create future values. While the site score focuses on features to be maintained, (with the notable
exceptions of intentional use of fire!), the stand score contains a number of elements that could be restored or
enhanced. Such restoration activities could include e.g. to increase the amount of large diameter trees by
modified thinning, to restore shaded, former solitary trees to sun-exposed conditions, or to create certain kinds of
deadwood.
Finally, field assessments fulfil an important educational function. Everyone that regularly uses the method
learns to identify and pay attention to elements and habitats necessary to sustain the biodiversity of the forests,
and to develop a dynamic approach to nature and the processes that affects the natural forest landscape. To ask
the relevant questions is often as important as getting exact answers - as in many other cases, the road is as
important as the goal.
Today a large number of Swedish forestry actors, e.g. companies AssiDomän, Korsnäs and Holmen, the 2 largest
forest owner´s associations Södra and Mellanskog, the State Forestry Agencies and Forestry Society managers,
use regional versions of our method as part of their conservation planning procedures and as a tool for internal
competence-building. We have also elaborated similar field forms adapted to forests and disturbance regimes in
Denmark, Latvia, Armenia and in northern China. Future development includes integration of assessments into
computer-based, regional landscape gap-analysis and ecological planning.
Börje Drakenberg
Risvägen 23
S-132 00 SALTSJÖBOO
Ph: +46-8-7479234
Anders Lindhe
Armfeldtsgatan 16
S-115 34 STOCKHOLM
Ph: +46-8-157886