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Forest functions, ecosystem stability and management
Forest Ecology and Management 132 (2000) 29±38
Forest functions, ecosystem stability and management Erwin FuÈhrer* Institute of Forest Entomology, Forest Pathology and Forest Protection, University of Agricultural Sciences Vienna, Hasenauerstraûe 38, A-1190 Vienna, Austria
Abstract Various socio-economic functions are ascribed to forests, based on the differentiated needs of the human population. Apart from the de®ned forest functions human welfare bene®ts from the diverse environmental effects of forests. The capacity of an ecosystem to sustain a speci®c function depends on the characteristics of its individual dynamics. Sustainable forest management concepts must take into account the compatibility between forest function and ecosystem characteristics. Incompatibility causes either dysfunction and ecosystem degradation or the need of corrective management interventions which may exceed tolerable economic limits. A detailed understanding of the destabilising and stabilising processes intrinsic to the ecosystem is necessary, for their regulatory interactions, and their responses to exogenous disturbances and perturbations, which emerge from forest management and environmental conditions. The study of mechanisms involved in the dynamics of forest ecosystems and their sub-systems, the evaluation of these mechanisms in the light of forest ecosystem diversity, forest function and forest management, would help forestry to successfully cope with the obstacles arising from nature, changing environments and socio-economic forces. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Ecosystem disturbances; Self regulation; Corrective interventions; Long-term dynamics
1. Introduction Various socio-economic functions are ascribed to forests, based on the differentiated needs of the human population. In Europe, scienti®c inquiry into forest ecosystems, which is foremost devoted to support forestry, must take these differentiated needs into account Ð in fact, most scienti®c questions are raised in order to address the function-related differentiation of forest management. The economic function, i.e. the production of timber and non-timber products, for * Tel.: 43-1368-63-5221; fax: 43-1368-63-5297. E-mail address: [email protected] (E. FuÈhrer)
one's own requirements or commercial purposes, remains the dominant interest everywhere. Historically the recreational function of forests were appreciated only in special situations (e.g. mountains); now the support of aesthetic and recreational interests is generally and increasingly acknowledged as an important goal of forestry (Dieterich, 1953; Hanstein, 1972). In practice, the majority of forests are multifunctional in that they ful®l, to varying extent, economic and social functions simultaneously. Nevertheless, clear functional specialisation of forests also exists (e.g. protective forests, short rotation plantations, energy plantations etc.), requiring entirely divergent management strategies. Trends in commercial forestry are
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headed towards such specialisation. The diversity of criteria typically linked to different forest functions, justi®es that ecosystem aspects of forest management are considered in a function-related context. 2. Forest functions Throughout history the dominant function of forests has been to provide natural resource products. Forests were and are exploited for both timber and non-timber products everywhere; forest plantations also are primarily devoted to this purpose. Sustainability of exploitable biomass production depends on a set of criteria which determine the nutrient and energy budget and the balance of food chain interactions in the ecosystem (see Andersson et al., 2000). Sustained economic productivity as a forest function implies human impact on the ecosystem. These impacts have special characteristics, which differ dependent on the utilised product (e.g. timber versus resin versus pasture) and the method of utilisation (e.g. coppice versus high forest versus clear-cut). There must be a corresponding pattern of ecosystem characteristics that provides the capacity to withstand these impacts. In landscapes where the human population, their goods and their infrastructure are permanently endangered by destructive natural events, the protective function of forests has ®rst priority. Protection can be directed against very different types of hazard, torrents and avalanches in the mountains, soil erosion by water and wind, contamination of ground and spring water, deserti®cation etc. Each forest management scenario is characterised by a speci®c pattern of ecosystem impacts which may leave a forest more or less susceptible to further impingement from natural forces. To combat this susceptibility, the functionbased patterns of impact must be speci®cally equipped. Ecosystem characteristics, being essential for protective forests, may vary due to the jeopardy to be averted (Mayer, 1976) and differ signi®cantly from those important in forest ecosystems devoted to production. Forests are increasingly used by urban populations for recreational purposes. Frequent visits exert speci®c effects e.g. permanent behavioural disturbance of deer populations, compaction and pollution of soil, increased risk of forest ®res etc. A forest directly
devoted to recreational purposes must also be easily accessible and ful®l particular aesthetic criteria (Willis and Benson, 1989). In contrast to productive and protective forests, criteria determining visitor comfort and safety may take precedence over other management objectives. Last but not least, forests represent the habitat of a considerable part of our ¯ora and fauna, which must be sustained for the conservation of biodiversity (Boyd, 1987). Formerly managed forest areas are left to nature for the sake of protecting threatened plant and animal species. The characteristics and condition of the ecosystem determine what happens in natural forest reserves after the management regime has been abolished. Apart from the de®ned forest functions above, human welfare bene®ts from the diverse environmental effects of forests (e.g. climate, landscape, hydrology, water and air quality, CO2 sequestration and aesthetics). These general environmental functions are also affected by several natural and anthropogenic loads. Their carrying capacity depends again on ecosystem features, determining the pattern of ecological responses, and on the reserves of the system to buffer extrinsic in¯uences (e.g. `critical load' of acid depositions). There are ecosystem-speci®c characteristics, corresponding to each forest function, that must be maintained. Otherwise degradation or even the decline of forest ecosystems may occur (FuÈhrer, 1990). The failure to sustain forest functions in the past often was caused by the ecological over-exploitation of forests in the course of multifunctional use (Glatzel, 1991). For instance, the weak condition of many of the protective forests in the Alps originates in the multiple uses to which they were subjected, in the past. Timber production, agriculture (pasture), hunting (high deer populations) and, in part, winter tourism, practised simultaneously, together exceeded the capacity of the ecosystem to maintain the properties needed for its protective function (Mayer, 1976). It is obvious that a forest can be expected to ful®l only those functions which correspond to the speci®c capacity of the ecosystem. When an individual forest area is to be devoted to a special function, (1) the speci®c ecosystem-related requirements of that function should be identi®ed and (2) the ecosystem should be examined to see if the corresponding characteristics
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of these requirements are available. Incompatibilities can be eliminated in two ways, either by corrective management intervention, or by adjusting the planned forest function to the capacity of the ecosystem. Both management intervention and planning adjustment are practised in forestry but their success in sustaining forest functions is dependent on the degree to which both decisions and speci®c interventions are based on scienti®cally sound knowledge. It can be concluded that foresters should be provided with better knowledge of: Characteristics and capacities of ecosystems (in terms of scales and thresholds) which individual forest functions typically are based on and which are necessary for a sustainable performance of the function. Methodology to identify the essential ecosystem traits and to evaluate them in light of the attempted forest function. Methodology for corrective management according to incompatibilities between ecological debits and credits, and for the assessment of their economic and technical feasibility. Due to the great diversity of practical situations, numerous scienti®c questions are hidden behind these three objectives. A schematic distinction of forest functions would separate the productive function from others because only production is expected to yield surplus biomass. Nevertheless, all functions should be sustainable irrespective of the dynamics that the forest ecosystems are subject to. Thus, the capacity of an ecosystem to sustain a speci®c function depends on the characteristics of its individual dynamics under the impact load generated by that function. Understanding these interactions requires: 1. de®nition of function-related nature and impact loads exerted on the ecosystem; 2. knowledge of the ecosystem processes necessary for coping with destabilising (disturbing) influences; 3. identification of assessment indicators (measurement) of the respective ecosystem capacities; 4. understanding the ecosystem responses to interacting loads (e.g. synergy) of destabilising forces, both related and non-related to the specific forest function.
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3. Forest ecosystem dynamics A basic characteristic of forest ecosystems is their dynamics. Success of sustainable forest management depends on the ability to adapt the management strategy to natural dynamics, or to manipulate the natural processes according to the management goal. Forest management goals and methods must be based on a profound knowledge of the driving forces and laws of natural processes, which allow forest dynamics to be predictable under different management regimes. 3.1. Ecosystem stability Despite of the permanent impact of destabilising forces, the ability of forest ecosystems to persist, is based on their natural mechanisms for control or management of these forces. Ef®ciency of these mechanisms is the essence of development and stability of ecosystems (Bormann and Likens, 1979). Success of control or management of the destabilising forces, impinging on forest ecosystems, therefore determines the success or failure of forest management.Sustainable forest management is based on detailed knowledge of: Potentially destabilising forces and agents. Ecosystem control mechanisms directed against the former. Operational conditions for the control mechanisms and effects expected to be exerted by the mechanisms. The postulate is that when all control mechanisms function properly, a state of ecosystem `stability' results. Terminology used to describe system dynamics following disturbances often leads to confusion. According to Redfearn and Pimm (1987) the commonly used terms could be de®ned as follows: `Stability' describes the tendency of a system to return to its equilibrium values after a disturbance; `resistance' is the tendency to remain unchanged by a disturbance. `Resilience' is a measure of how fast a system returns to its equilibrium after a perturbation; resilient systems have a short return time. (According to Bormann and Likens (1979), resilience is the ability per se of a system to return to its original state after
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a perturbation.) `Persistence' is a measure of the time a system lasts before it is changed to a different one. These dynamic features are related to an ecosystem's ability to cope with disturbances/perturbations, i.e. with destabilising forces driving the dynamics of the system. Introducing the term `variability' as a measure of the variance (amplitude) of reversible ¯uctuations over time, `resistance', `stability' and `resilience' would form an array of ascendently variable systems. Consideration of these dynamic features must be incorporated into the management concept, because management implies control over the dynamics. On the other hand, if the ability to cope with disturbances is gradually related to different patterns of dynamics, the concepts for sustainable forest management could bene®t from this relationship. Wherever non-stationary (¯uctuating) systems are compatible with the forest function, the management concept should be directed to a state of resilience. Where predictability of the system's behaviour is essential, stationary (i.e. less ¯uctuating) conditions (Ulrich, 1987), as represented by stable or resistant systems, would be preferred. Expenditure of management interventions is related to the type of concept. Commercial forestry, directed to regular and predictable yield of timber, is preferably based on forests displaying little ¯uctuation, i.e. a stationary equilibrium with relative high resistance to disturbances. This stationary equilibrium is more or less arti®cially maintained, requiring often considerable corrective management interventions. They are necessary to cope with the common disturbances being in¯icted on the system. Resistance-based systems are considered more susceptible to perturbations and changing conditions than resilience-based ones (Holling, 1973). In contrast, resilient systems are `allowed' to respond to disturbances and perturbations by even strong ¯uctuations, by this way sustaining the reversibility of the induced change of condition. Such systems may not ®t under the strict economic regime of commercial forestry, but they seem appropriate for low-cost management concepts. Resilient systems are able to recover from strong disturbances by their own forces. When their ¯uctuations are tolerated, they do not require expensive corrective management interventions.
Stability, understood as controlled dynamics allowing persistence of forest ecosystems in a certain condition, ranges in between the resistance-based and the resilience-based models. Stable systems are considered as adapted to the prevailing environmental circumstances, moderately ¯uctuating due to the incidence of disturbances, but not easily perturbed by stronger episodic natural events. They could be sustained on a moderate management regime and serious health problems are not expected, provided that environmental conditions remain unchanged. Every forest management concept is related to a speci®c type of ecosystem dynamics, taking into account its advantages and disadvantages. In practice the disadvantages are often not completely foreseen, however, they eventually occur, giving rise to unexpected forest health problems or loss of productivity. On the other hand, sustainable forest management is often successful, when locally favourable ecological conditions are coupled to appropriate management concepts, based more on intuition than rational use of ecological understanding. 3.2. Processes driving forest ecosystem dynamics Natural development of forest ecosystems is driven by intrinsic regulation processes and can be in¯uenced by extrinsic factors. Ecosystem dynamics is the result of the dynamic processes in diverse sub-systems (i.e. primary production, herbivory, decomposition) (Packham et al., 1992). The overall dynamic trend depends on the balance of sub-system dynamics. Mechanisms installed in the diverse sub-systems provide the capacity of self-regulation, necessary to canalise and control the destabilising forces to which the system is exposed. Forest ecosystem stability depends on the ef®ciency by which the concerned sub-system is responding to disturbing interferences. Precision, speed and intensity of this response determine the type of dynamics induced by disturbances. Hence, type and degree of stability (dynamics) depend on the ef®cient functioning of the mechanisms responsible for intrinsic self-regulation. Instability of degraded or arti®cial forest ecosystems is understood as the lack of intrinsic self-regulation. System maintenance requires external regulation through management intervention. The amount of regulatory energy necessary for the stabilisation of
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such systems depends on the degree of incompatibility between management goals and ecosystem capacities. Forest management concepts are characterised by the degree of external regulation necessary to compensate the lack of intrinsic self-regulation. Since this characteristic is ecologically and economically relevant, respective evaluation of management concepts would be useful, and guidelines for developing concepts of sustainable forest management should take this into account. The basis for this approach is knowledge of the processes responsible for intrinsic regulation, which again must be seen in relation to the agents causing disturbance and perturbation as well as in relation to in¯uences emerging from management interventions. Arti®cial extrinsic regulation is corrective management intervention to assure the persistence of the system, to reach the prescribed functional goal. Recent forest management concepts imply immense demands of corrective intervention in wide parts of European forests. An example are the `forest health' measures prescribed as routine management in conifer forests to prevent bark beetle epidemics (Schwerdtfeger, 1981). Each forest community with species outside their natural sites or of unnatural species composition needs corrective management intervention to augment intrinsic ecosystem control mechanisms of self-regulation. Fluctuations of environmental and/or economic conditions bring about situations where the cost of corrective intervention, if at all feasible, exceeds the economic potential of the forest. In such situations foresters stand helpless facing the destructive dynamics of their pseudo-stable forest ecosystems. It is less a shifting of paradigms but more the hard economic reality that force foresters (at least in central Europe) to minimise the expenditure for management operations. Consequently, when extrinsic regulation by management is reduced, the need for intrinsic self-regulation is increased. Otherwise the commitment to sustainability could not be ful®lled and an acceleration of forest decline would be unavoidable. 3.3. Deterministic conditions Ð process control Nature, intensity and frequency of disturbing agents on one side and regulatory capacity, ef®ciency and precision of the intrinsic mechanisms on the other, form the deterministic conditions which de®ne an
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ecosystem as either unstable or stable, resistant or resilient (Holling, 1973). Deterministic conditions are represented by different arrays of physical, chemical and biological parameters, situated within and outside the ecosystem, human action included. The interactive processes among these parameters follow certain laws, which are only incompletely understood. In theory complex factors are involved: The ¯ux of energy and matter, substrate character, vegetative species composition, trophic and population dynamics of the fauna, as well as the micro¯ora and microfauna. There is also evidence that the nature, intensity and speed of these interactions are subject to the ¯uctuations of the physical circumstances. Although we have knowledge of many speci®c factors operating in certain sectors of forest ecosystems, it is not known, to what extent this knowledge can be generalised. The common opinion is, that potentially destabilising forces are usually extrinsic to the ecosystems (Bormann and Likens, 1979). In this context climatic stress factors and human impacts are addressed. But there is also evidence of considerable capacity for destabilisation within the system: pest and pathogen populations are components of the ecosystems. The processes governing the population dynamics of these populations and the mechanisms which control the repair of associated damage, are doubtless situated within the ecosystem or respective sub-system. If these control mechanisms are to be constructively manipulated, they must be better understood than they are now. Mechanisms for control and regulation in forest ecosystems must be more closely analysed from their causative-hierarchical network connections. General or factor-speci®c differences of valence between the mechanisms would appear. Knowledge of these differences could be used to determine state-diagnostic and design-strategic concepts for forest ecosystem management. To be useful, such concepts must be extended over the whole ecosystem, because stabilityrelevant regulatory processes take place on and between all trophic levels. Correspondingly, the potentially destabilising forces whether exogenic or endogenic, can impinge on all trophic levels (FuÈhrer, 1997c) (Fig. 1). Reliable, easily recorded indicators are often sought for the conditions of forest ecosystems. Thus, scientists must develop criteria which represent a
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Fig. 1. Epidemically relevant effects and processes triggered off by environmental factors in the trophic levels and between-level-interfaces of multitrophic consumer food-chains of forest ecosystems. Environmental effects to (1) physical, chemical, biological soil conditions; (2) ecophysiology of individual trees and to stand structure as related to density and age; (3) ecophysiology, propagative potential, interspecific competition of herbivores; (4) ecophysiology, propagative potential, interspecific competitiveness of natural enemies, supply of supplementary ecological requisites. (FuÈhrer, 1997b).
multitude of complex information on a high level of aggregation. The well-known crown transparency as an indicator for `forest health' is one example; the `tightness' or `looseness' of nutrient export characteristics (Bormann and Likens, 1979) is another. Neither `crown transparency', the indicative value of which is rightly criticised (Innes, 1993), nor nutrient export characteristics can themselves provide information of where in the system the ecosystem malfunction is situated. A detailed search for the malfunction is necessary in every individual case. The better is the understanding of ecosystem control, the easier the search will be. Forest ecosystems are dif®cult to understand due to the immense diversity of components. Attempts to de®ne fundamental principles are limited to overly general statements. Even the common opinion that biodiversity increases ecosystem stability, for example, might have only limited validity (FuÈhrer, 1978; Tamm, 1992; Bengtsson et al., 2000). Therefore, the understandable desire for generalisation in forest ecology should be met with healthy scepticism. This is also true with the interpretation of models, irrespective of their high value for developing a basic understanding of ecosystem processes. In applied science and management practice, generalisations have only limited value, because they do not suf®ciently touch upon
essential details. Management decisions require detailed knowledge of the local situation, which can only be drawn from speci®c knowledge of a diversity of ecosystem components. Here our knowledge often falls short. It therefore, is an urgent task of forest ecosystem research to further develop these skills. Modelling can be of great value in developing these skills (see also Tamm, 1992). The practical bene®ts of detailed knowledge on the network of ecosystem control mechanisms, lies in its diagnostic and constructive value. Diagnostic application can serve to identify the causes of disturbances which already occurred, or to assess the capacity of the ecosystem to withstand the loads inherent to a particular management regime (`critical loads', in a wider sense). This knowledge can be used to assess the sustainability of particular management concepts, and to facilitate their realisation. Such evaluation is essential in afforestation projects, forest transformation and forest ecosystem rehabilitation. Risk assessment is another application which includes identifying and evaluating the conditions, which predispose a forest to the incitement of acute damage. (FuÈhrer, 1997c; Netherer and FuÈhrer, 1998; Nopp and FuÈhrer, 1998). This knowledge could be used to develop tools for the optimal adjustment of forest management concepts to the local ecological situation.
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3.4. Sources of disturbances The sources of forest ecosystem disturbances/perturbations are natural, (e.g. climatic factors, abiotic site conditions, ®re and pest organisms), semi-natural (e.g. climate change) or exclusively man-made. Manmade interferences emerge from either the socioeconomic environment of forests or forestry itself (Schimitschek, 1969; FuÈhrer, 1990). Many such disturbances are side effects resulting from other land uses, agriculture, industry, settlement, traf®c, tourism etc. To reduce or avoid these side effects strong political conviction coupled with clear knowledge of the causality and `dosage-effect relation' of the disturbances is needed. Concerning harmful effects of air pollution or poor water management on forest ecosystems, political mechanisms are partially established on the national or international scale, but their improvement is still urgently needed. The `critical load' concept, mainly discussed in connection with acid depositions, could be (and partially is) applied to other factors affecting forest ecosystems both continuously and cumulatively (e.g. herds of wild and domestic ungulates, tourists etc.). For acid deposition, Tamm (1992) suspects, that the `critical load' concept is insuf®cient, because many destabilising effects can occur before the `critical load' is reached. Likewise many subtle or indirect impacts are dif®cult to recognise, posing a challenge to both forest ecologists and the management decision apparatus. Both direct and indirect disturbances on forest ecosystems emerge from forestry practice itself. The predisposing effects of ecologically unsuitable silvicultural concepts and management techniques must be particularly pointed out. They participate signi®cantly in the destabilisation of forest ecosystems and facilitate acute disease incitement (Schimitschek, 1969; Cramer, 1984). This could be avoided if, (1) the cause-effect-complex can be made clear enough to form reliable models for risk assessment, and (2) the management concepts can stand up to a comparative long-term cost-bene®t calculation. Forest ecosystem research is challenged to produce the necessary information. Sustainable management must also augment a forest's resistance to destructive forces wherever they may come from, i.e. to strengthen the stability of the
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ecosystem. In many cases of forest decline the causal interpretation postulates predisposing (conditioning) short-term or long-term effects of anthropogenic environmental in¯uences as a basis of a more likely incitement of apparent diseases by natural stress agents (FuÈhrer, 1990, 1997c, 1998; Manion and Lachance, 1992). Predictability of forest health problems depends basically on the extent to which their causality is understood. Unfortunately, despite some general features, there seems to be a great diversity in the genesis of apparent forest diseases. The gaps in knowledge are considerable. However, the signi®cance of a speci®c disease and the necessity of its ef®cient prevention may be relative, depending on the function and condition of the forest in question. Rehabilitation of forest ecosystems is an endeavour strictly devoted to this purpose in cases, when progress of degradation is already threatening the persistence of the system. Here as well as in all afforestation projects, particularly those on extreme sites (high altitudes, karst, semiarid sites, mining and industrial dumps, polluted soils etc.), when a persistent cover of forest vegetation is the goal, the strategic concepts require the highest level of expertise and a reliable understanding of the relevant ecosystem processes. 3.5. Managing forest dynamics towards functional goals Forest management decisions must be based on clearly de®ned management goals and methods. Both the de®nition of a reasonable goal and the choice of the suitable method require suf®cient knowledge of how a forest ecosystem functions. If management decisions are not based on this knowledge and are not made individually, they can not take into account the compelling eco-geographic or site-related circumstances, for which the decisions are to be made. Silvicultural dogmatism, which have determined forestry in the past, and have caused many serious problems, would be prolonged. 3.6. Goal-specific management The suitability of management methods depends on the ecosystem processes that they induce. The purpose of management intervention is to in¯uence ecosystem development in a direction determined by the manage-
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ment goal, i.e. the forest function. In the great majority of cases the intended function of forests postulates the maintenance of certain qualitative criteria, which would not arise through natural self-regulation. For instance, protection against soil erosion, torrents and avalanches in mountainous forests depends on structures and developmental features in particular spatial and temporal arrays. In mountainous regions a forester's decision can determine landscape hydrology for decades or even centuries. Silviculture here needs reliable hydrological knowledge in order to avoid serious errors. Prevention of catastrophic ¯oods has priority in mountains rich with precipitation, while the maintenance of a continuous and high quality water supply is a primary function of forests in dry climates (Mayer, 1976). Paradigm shifts and increasing labour costs have resulted in a trend to more `near-natural' forestry and extensive forest management. Retaining dead wood, resulting from natural disturbances, is one example, but it is questionable whether this is everywhere tolerable from the view of forest health (FuÈhrer, 1997a, b). It is evident that persistence of forest ecosystems in a functionally welcome state usually requires at least a minimum of corrective intervention. This is true with `near natural' forests which need, at least, `custodiary' management, and more valid with forests under a rigid economic management regime. The further from the goal condition the management starts, or the more ecosystem dynamics tends to deviate from the goal-directed course, the stronger the management intervention must be. The basis and objective of sustainable forest management is the persistence of the utilised forest ecosystems in a de®ned functional state. The conditions under which a particular forest ecosystem can persist are limited by site and climatic characteristics, which can change considerably through time and human impact. Throughout most parts of Europe, human impacts have been the dominant force shaping site speci®c characteristics. Persistence of a forest ecosystem implies pronounced developmental dynamics, including phases of aggradation, maturation, degradation, destruction, reorganisation etc. Under ideal circumstances the system can enter a `steady state' phase, i.e. a smallspaced mosaic of units in phase-shifted developmental states, thus giving the whole ecosystem the character-
istics of collective stability (Mayer, 1976; Bormann and Likens, 1979). It can be assumed that in such cases ef®cient mechanisms of self-regulation successfully control all destabilising exogenous and endogenous forces, thus keeping the developmentally destructive phases temporally and spatially within ecologically tolerable limits. 3.7. Management controlling natural dynamics Natural development of forest ecosystems is permanently subject to exogenous and endogenous disturbances and Ð from time to time Ð is exposed to thorough perturbations caused by extreme abiotic and biotic events; overmature stands are particularly susceptible to such disturbance. Long-term cycling in forest ecosystems regularly includes phases of disintegration, followed by phases of system reorganisation and vegetative rejuvenation. While perturbing events often represent obvious turning points in the long-term dynamics of the ecosystem, disturbances happen at shorter intervals and usually can be cushioned by repair mechanisms of a resilient (stable) forest ecosystem. An essential objective of forest management is to prevent undesirable ecosystem dynamics such that the system is kept stable or more or less subject to a prescribed developmental rhythm. Forest management is, therefore, also a ®ght not only against uncontrolled ecosystem perturbation but also against seemingly minor disturbances which cannot not be easily compensated in less stable ecosystems and thus could incite the destruction of the whole system. What causes disturbances or perturbations, relevant to forest ecosystem dynamics, depends on the identity, intensity and temporal/spatial pattern (i.e. the `incitement potential') of the acting factors as well as on predisposing traits of the target ecosystem. (FuÈhrer, 1997c). `Stability' in the sense of `absence of uncontrolled dynamics' depends on the degree to which the system is adapted to the range of potentially inciting factors. Suf®cient knowledge of (1) the disturbing forces/factors acting on/in an individual forest ecosystem, (2) the mechanisms involved in the control of such forces, (3) the ecosystem conditions enabling these control mechanisms to succeed and (4) the relevance of particular factors to the dynamics of the ecosystem, are all essential in forming a theoretical basis for sustainable forest management.
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The evaluation of a function-related management goal in the light of the inherent potential of ecosystem interference usually reveals the necessity of corrective management interventions. The more the intended function is subordinated to the prevailing functional capacity of the ecosystem, the smaller are the required corrections. In order to assess the nature and costs of corrective operations, and to ®nd a reasonable mode of harmonisation between management goals and ecosystem capacity, at least two facts must be compared: (1) the suitability of the forest ecosystem for the proposed function and (2) the long-term effects on the forest ecosystem caused of the function. This may be illustrated by a simple example. A decision to devote a forest area to recreational purposes, may have far-reaching consequences for the habitat quality for deer. Behavioural reactions of the deer population will probably cause severe damage to stands (peeling) and regeneration, thus inducing signi®cant detrimental changes to the further forest development. This damage could be avoided by applying alternative strategies of wildlife management, but at a signi®cant cost. Thus sustainable management involves understanding the systemic capacity, longterm trends, and associated costs in maintaining particular functional goals. It is dif®cult to develop multifunctional sustainable management concepts on an ecologically sound basis. As history demonstrates, competition between functional goals can provoke insoluble ecological con¯icts (Glatzel, 1991). To assess the multifunctional potential of a site/region the relevant ecosystem criteria and function-related impact interactions must be understood. Indicators of the forest ecosystem's capacity are needed to cope with interacting impacts. More knowledge in this ®eld could be obtained from ecological studies on successful and unsuccessful examples of multiple use systems. 4. Conclusions Consideration of forest ecosystems from the view of their ecological and socio-economic functions makes clear that sustainable forest management concepts must take into account the compatibility between forest function and ecosystem characteristics. Incompatibility causes either dysfunction and ecosystem
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degradation or the need of corrective management interventions which may exceed tolerable economic limits. A careful evaluation of ecosystem capacity in light of the destabilising impact of function-related forces could help to prevent an undesired development of the ecosystem condition. Sustainable forest management is the management of long-term dynamics of forest ecosystems, while coping with short-term disturbance, in order to sustain the function to which the forest is devoted. Management strategies must be focused on ecosystem persistence and a suf®cient degree of ecosystem stability. A detailed understanding of the laws and processes, which determine ecosystem dynamics, is an essential basis for developing well adjusted management concepts. This understanding must include the destabilising and stabilising processes intrinsic to the ecosystem, their regulatory interactions, and their responses to exogenous disturbances and perturbations, which emerge from forest management and environmental conditions. The pattern of impact on the ecosystem, typically related to individual forest functions (utilisation), and the effects on the ecosystem, typical to individual management regimes, represent details fundamental to this process. When ecosystem dynamics is expected to correspond with the intended forest function, existing incompatibilities between function and ecosystem characteristics must be corrected by management intervention. Intervention must compensate the lack of intrinsic self-regulation. Forest health problems indicate such incompatibilities. The cost of successful corrective intervention strongly depends on the degree of incompatibility and on the ef®ciency of the management concept. Therefore, decisions concerning goals and methods of forest management touch ecological, technical and important economic aspects. Professional decision making processes urgently need a stronger scienti®c basis, than is presently available. Forest ecosystem research must develop this scienti®c basis. The study of mechanisms involved in the dynamics of forest ecosystems and their sub-systems, the evaluation of these mechanisms in the light of forest ecosystem diversity, forest function and forest management, are challenging tasks. Developing methods for the goal-directed manipulation of ecosystem processes, in order to increase the capacity of intrinsic
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self-regulation and to minimise the necessary expenditure on exogenous regulatory measures, would help forestry to successfully cope with the obstacles arising from nature, changing environments and socio-economic forces. References Ê gren, G.I., FuÈhrer, E., 2000. Sustainable tree Andersson, F.O., A biomass production. For. Ecol. Manage. 132, 51±62. Bengtsson, J., Nilsson, S.G., Franc, A., Menozzi, P., 2000. Biodiveristy, disturbances, ecosystem function and management of European forests. For. Ecol. Manage. 132, 39±50. Bormann, F.H., Likens, G.E., 1979. Pattern and Process in a Forested Ecosystem. Springer, New York. Boyd, J.M., 1987. Commercial forests and the nature conservation baseline. Forestry 60, 113±134. È ber die Disposition mitteleuropaÈischer Cramer, H.H., 1984. U Forsten fuÈr WaldschaÈden. Pflanzenschutz-Nachrichten Bayer 37, 97±207. Dieterich, V., 1953. Forstwirtschaftspolitik. In: Eine EinfuÈhrung. Parey, Hamburg. FuÈhrer, E., 1978. Forstzoologie zwischen Theorie und Praxis. II. Abschied von vertrauten Vorstellungen? Forstarchiv 49, 66±69. FuÈhrer, E., 1990. Forest decline in central Europe: additional aspects of its cause. For. Ecol. Manage. 37, 249±257. FuÈhrer, E., 1997a. Biotopholz Ð Forstschutzrisiko oder WaldhyÈ sterr. Forstz., 108 (4) 21±23. giene? O FuÈhrer, E., 1997b. Biotopholz Ð Forstschutzrisiko oder WaldhyÈ sterr. Forstz., 108 (5) 24±25. giene? O FuÈhrer, E., 1997c. Implications of `forest health' aspects on forest ecosystem research. In: Proceedings of the 1st EFERN Plen. Meeting, Vienna, 1996. Forstl. Schriftenr. Univ. Bodenkultur, Wien, pp. 55±68. FuÈhrer, E., 1998. Oak decline in central Europe: a synopsis of hypotheses. In: McManus, M.L., Liebhold, A.M. (Eds.), Proceedings of Population Dynamics, Impacts and Integrated Management of Forest Defoliating Insects. USDA Gen. Tech. Rpt., NE-247, 7±24. Glatzel, G., 1991. The impact of historic landuse and modern
forestry on nutrient relations of central European forest ecosystems. Fert. Res. 27, 1±8. Hanstein, U., 1972. Die Sozialfunktionen des Waldes und die geltenden Vorschriften fuÈr ihre Erfassung. Allg. Forstz., 27. Holling, C.S., 1973. Resilience and stability of ecological systems. Ann. Rev. Ecol. Syst. 4, 1±23. Innes, J., 1993. Forest Health: Its Assessment and Status. CAB International, Wallingford. Manion, P.D., Lachance, D. (Eds.), 1992. Forest Decline Concepts. APS Press, St. Paul, MN. Mayer, H., 1976. Gebirswaldbau, Schutzwaldpflege. Ein waldbaulicher Beitrag zur LandschaftoÈkologie und zum Umweltschutz. Fischer, Stuttgart. Netherer, S., FuÈhrer, E., 1998. Konzipierung eines RisikoschaÈtzmodells zur Bestimmung der PraÈdisposition von Standorten und BestaÈnden gegenuÈber Gradationen von Pristiphora abietina (Christ) (Hym., Tenthr.). Allg. Forst-and Jagdz 170, 53±60. Nopp, U., FuÈhrer, E., 1998. Assessment of the predisposition of spruce-abundant forests to various disturbances. I. Proc. Int. Workshop on Spruce Monocultures in Central Europe Ð Problems and Prospects, 22±25 June 1998 Brno Czech Rep. EFI Proc., in press. Packham, J.R., Harding, D.J.L., Hilton, G.M., Stuttard, R.A., 1992. Functional Ecology of Woodlands and Forests. Chapman & Hall, London, 407 pp. Redfearn, A., Pimm, S.L.,1987. Insect outbreaks and community structure. In: Barbosa, P., Schultz, J.C. (Eds.), Insect Outbreaks. AP, San Diego, pp. 99±133. Schimitschek, E., 1969. GrundzuÈge der Waldhygiene. Parey, Hamburg. Schwerdtfeger, F., 1981. Die Waldkrankheiten. 4. Aufl., Parey, Hamburg. Tamm, C.O., 1992. The evolution of ecosystem thinking in European ecology. In: Teller, A., Mathy, P., Jeffers, J.N.R. (Eds.), Proceedings of a Symposium on Responses of Forest Ecosystems to Environmental Changes. Elsevier, London, New York, pp. 5±15. Ulrich, B., 1987. Anthropogene VeraÈnderungen von WaldoÈkosystemen, Geschichte±Gegenwart±Zukunft. In: Glatzel, G. (Ed.), MoÈglichkeiten und Grenzen der Sanierung immissionsgeschaÈÈ sterr. Forschungsinitiative geg.d. digter WaldoÈkosysteme. O Waldsterben, Wien. ISBN 3-900 491-02-3, pp. 1±33. Willis, K.G., Benson, J.F., 1989. Recreational values of forests. Forestry 62, 93±110.
Forest functions, ecosystem stability and management Erwin FuÈhrer* Institute of Forest Entomology, Forest Pathology and Forest Protection, University of Agricultural Sciences Vienna, Hasenauerstraûe 38, A-1190 Vienna, Austria
Abstract Various socio-economic functions are ascribed to forests, based on the differentiated needs of the human population. Apart from the de®ned forest functions human welfare bene®ts from the diverse environmental effects of forests. The capacity of an ecosystem to sustain a speci®c function depends on the characteristics of its individual dynamics. Sustainable forest management concepts must take into account the compatibility between forest function and ecosystem characteristics. Incompatibility causes either dysfunction and ecosystem degradation or the need of corrective management interventions which may exceed tolerable economic limits. A detailed understanding of the destabilising and stabilising processes intrinsic to the ecosystem is necessary, for their regulatory interactions, and their responses to exogenous disturbances and perturbations, which emerge from forest management and environmental conditions. The study of mechanisms involved in the dynamics of forest ecosystems and their sub-systems, the evaluation of these mechanisms in the light of forest ecosystem diversity, forest function and forest management, would help forestry to successfully cope with the obstacles arising from nature, changing environments and socio-economic forces. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Ecosystem disturbances; Self regulation; Corrective interventions; Long-term dynamics
1. Introduction Various socio-economic functions are ascribed to forests, based on the differentiated needs of the human population. In Europe, scienti®c inquiry into forest ecosystems, which is foremost devoted to support forestry, must take these differentiated needs into account Ð in fact, most scienti®c questions are raised in order to address the function-related differentiation of forest management. The economic function, i.e. the production of timber and non-timber products, for * Tel.: 43-1368-63-5221; fax: 43-1368-63-5297. E-mail address: [email protected] (E. FuÈhrer)
one's own requirements or commercial purposes, remains the dominant interest everywhere. Historically the recreational function of forests were appreciated only in special situations (e.g. mountains); now the support of aesthetic and recreational interests is generally and increasingly acknowledged as an important goal of forestry (Dieterich, 1953; Hanstein, 1972). In practice, the majority of forests are multifunctional in that they ful®l, to varying extent, economic and social functions simultaneously. Nevertheless, clear functional specialisation of forests also exists (e.g. protective forests, short rotation plantations, energy plantations etc.), requiring entirely divergent management strategies. Trends in commercial forestry are
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 3 7 7 - 7
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headed towards such specialisation. The diversity of criteria typically linked to different forest functions, justi®es that ecosystem aspects of forest management are considered in a function-related context. 2. Forest functions Throughout history the dominant function of forests has been to provide natural resource products. Forests were and are exploited for both timber and non-timber products everywhere; forest plantations also are primarily devoted to this purpose. Sustainability of exploitable biomass production depends on a set of criteria which determine the nutrient and energy budget and the balance of food chain interactions in the ecosystem (see Andersson et al., 2000). Sustained economic productivity as a forest function implies human impact on the ecosystem. These impacts have special characteristics, which differ dependent on the utilised product (e.g. timber versus resin versus pasture) and the method of utilisation (e.g. coppice versus high forest versus clear-cut). There must be a corresponding pattern of ecosystem characteristics that provides the capacity to withstand these impacts. In landscapes where the human population, their goods and their infrastructure are permanently endangered by destructive natural events, the protective function of forests has ®rst priority. Protection can be directed against very different types of hazard, torrents and avalanches in the mountains, soil erosion by water and wind, contamination of ground and spring water, deserti®cation etc. Each forest management scenario is characterised by a speci®c pattern of ecosystem impacts which may leave a forest more or less susceptible to further impingement from natural forces. To combat this susceptibility, the functionbased patterns of impact must be speci®cally equipped. Ecosystem characteristics, being essential for protective forests, may vary due to the jeopardy to be averted (Mayer, 1976) and differ signi®cantly from those important in forest ecosystems devoted to production. Forests are increasingly used by urban populations for recreational purposes. Frequent visits exert speci®c effects e.g. permanent behavioural disturbance of deer populations, compaction and pollution of soil, increased risk of forest ®res etc. A forest directly
devoted to recreational purposes must also be easily accessible and ful®l particular aesthetic criteria (Willis and Benson, 1989). In contrast to productive and protective forests, criteria determining visitor comfort and safety may take precedence over other management objectives. Last but not least, forests represent the habitat of a considerable part of our ¯ora and fauna, which must be sustained for the conservation of biodiversity (Boyd, 1987). Formerly managed forest areas are left to nature for the sake of protecting threatened plant and animal species. The characteristics and condition of the ecosystem determine what happens in natural forest reserves after the management regime has been abolished. Apart from the de®ned forest functions above, human welfare bene®ts from the diverse environmental effects of forests (e.g. climate, landscape, hydrology, water and air quality, CO2 sequestration and aesthetics). These general environmental functions are also affected by several natural and anthropogenic loads. Their carrying capacity depends again on ecosystem features, determining the pattern of ecological responses, and on the reserves of the system to buffer extrinsic in¯uences (e.g. `critical load' of acid depositions). There are ecosystem-speci®c characteristics, corresponding to each forest function, that must be maintained. Otherwise degradation or even the decline of forest ecosystems may occur (FuÈhrer, 1990). The failure to sustain forest functions in the past often was caused by the ecological over-exploitation of forests in the course of multifunctional use (Glatzel, 1991). For instance, the weak condition of many of the protective forests in the Alps originates in the multiple uses to which they were subjected, in the past. Timber production, agriculture (pasture), hunting (high deer populations) and, in part, winter tourism, practised simultaneously, together exceeded the capacity of the ecosystem to maintain the properties needed for its protective function (Mayer, 1976). It is obvious that a forest can be expected to ful®l only those functions which correspond to the speci®c capacity of the ecosystem. When an individual forest area is to be devoted to a special function, (1) the speci®c ecosystem-related requirements of that function should be identi®ed and (2) the ecosystem should be examined to see if the corresponding characteristics
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of these requirements are available. Incompatibilities can be eliminated in two ways, either by corrective management intervention, or by adjusting the planned forest function to the capacity of the ecosystem. Both management intervention and planning adjustment are practised in forestry but their success in sustaining forest functions is dependent on the degree to which both decisions and speci®c interventions are based on scienti®cally sound knowledge. It can be concluded that foresters should be provided with better knowledge of: Characteristics and capacities of ecosystems (in terms of scales and thresholds) which individual forest functions typically are based on and which are necessary for a sustainable performance of the function. Methodology to identify the essential ecosystem traits and to evaluate them in light of the attempted forest function. Methodology for corrective management according to incompatibilities between ecological debits and credits, and for the assessment of their economic and technical feasibility. Due to the great diversity of practical situations, numerous scienti®c questions are hidden behind these three objectives. A schematic distinction of forest functions would separate the productive function from others because only production is expected to yield surplus biomass. Nevertheless, all functions should be sustainable irrespective of the dynamics that the forest ecosystems are subject to. Thus, the capacity of an ecosystem to sustain a speci®c function depends on the characteristics of its individual dynamics under the impact load generated by that function. Understanding these interactions requires: 1. de®nition of function-related nature and impact loads exerted on the ecosystem; 2. knowledge of the ecosystem processes necessary for coping with destabilising (disturbing) influences; 3. identification of assessment indicators (measurement) of the respective ecosystem capacities; 4. understanding the ecosystem responses to interacting loads (e.g. synergy) of destabilising forces, both related and non-related to the specific forest function.
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3. Forest ecosystem dynamics A basic characteristic of forest ecosystems is their dynamics. Success of sustainable forest management depends on the ability to adapt the management strategy to natural dynamics, or to manipulate the natural processes according to the management goal. Forest management goals and methods must be based on a profound knowledge of the driving forces and laws of natural processes, which allow forest dynamics to be predictable under different management regimes. 3.1. Ecosystem stability Despite of the permanent impact of destabilising forces, the ability of forest ecosystems to persist, is based on their natural mechanisms for control or management of these forces. Ef®ciency of these mechanisms is the essence of development and stability of ecosystems (Bormann and Likens, 1979). Success of control or management of the destabilising forces, impinging on forest ecosystems, therefore determines the success or failure of forest management.Sustainable forest management is based on detailed knowledge of: Potentially destabilising forces and agents. Ecosystem control mechanisms directed against the former. Operational conditions for the control mechanisms and effects expected to be exerted by the mechanisms. The postulate is that when all control mechanisms function properly, a state of ecosystem `stability' results. Terminology used to describe system dynamics following disturbances often leads to confusion. According to Redfearn and Pimm (1987) the commonly used terms could be de®ned as follows: `Stability' describes the tendency of a system to return to its equilibrium values after a disturbance; `resistance' is the tendency to remain unchanged by a disturbance. `Resilience' is a measure of how fast a system returns to its equilibrium after a perturbation; resilient systems have a short return time. (According to Bormann and Likens (1979), resilience is the ability per se of a system to return to its original state after
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a perturbation.) `Persistence' is a measure of the time a system lasts before it is changed to a different one. These dynamic features are related to an ecosystem's ability to cope with disturbances/perturbations, i.e. with destabilising forces driving the dynamics of the system. Introducing the term `variability' as a measure of the variance (amplitude) of reversible ¯uctuations over time, `resistance', `stability' and `resilience' would form an array of ascendently variable systems. Consideration of these dynamic features must be incorporated into the management concept, because management implies control over the dynamics. On the other hand, if the ability to cope with disturbances is gradually related to different patterns of dynamics, the concepts for sustainable forest management could bene®t from this relationship. Wherever non-stationary (¯uctuating) systems are compatible with the forest function, the management concept should be directed to a state of resilience. Where predictability of the system's behaviour is essential, stationary (i.e. less ¯uctuating) conditions (Ulrich, 1987), as represented by stable or resistant systems, would be preferred. Expenditure of management interventions is related to the type of concept. Commercial forestry, directed to regular and predictable yield of timber, is preferably based on forests displaying little ¯uctuation, i.e. a stationary equilibrium with relative high resistance to disturbances. This stationary equilibrium is more or less arti®cially maintained, requiring often considerable corrective management interventions. They are necessary to cope with the common disturbances being in¯icted on the system. Resistance-based systems are considered more susceptible to perturbations and changing conditions than resilience-based ones (Holling, 1973). In contrast, resilient systems are `allowed' to respond to disturbances and perturbations by even strong ¯uctuations, by this way sustaining the reversibility of the induced change of condition. Such systems may not ®t under the strict economic regime of commercial forestry, but they seem appropriate for low-cost management concepts. Resilient systems are able to recover from strong disturbances by their own forces. When their ¯uctuations are tolerated, they do not require expensive corrective management interventions.
Stability, understood as controlled dynamics allowing persistence of forest ecosystems in a certain condition, ranges in between the resistance-based and the resilience-based models. Stable systems are considered as adapted to the prevailing environmental circumstances, moderately ¯uctuating due to the incidence of disturbances, but not easily perturbed by stronger episodic natural events. They could be sustained on a moderate management regime and serious health problems are not expected, provided that environmental conditions remain unchanged. Every forest management concept is related to a speci®c type of ecosystem dynamics, taking into account its advantages and disadvantages. In practice the disadvantages are often not completely foreseen, however, they eventually occur, giving rise to unexpected forest health problems or loss of productivity. On the other hand, sustainable forest management is often successful, when locally favourable ecological conditions are coupled to appropriate management concepts, based more on intuition than rational use of ecological understanding. 3.2. Processes driving forest ecosystem dynamics Natural development of forest ecosystems is driven by intrinsic regulation processes and can be in¯uenced by extrinsic factors. Ecosystem dynamics is the result of the dynamic processes in diverse sub-systems (i.e. primary production, herbivory, decomposition) (Packham et al., 1992). The overall dynamic trend depends on the balance of sub-system dynamics. Mechanisms installed in the diverse sub-systems provide the capacity of self-regulation, necessary to canalise and control the destabilising forces to which the system is exposed. Forest ecosystem stability depends on the ef®ciency by which the concerned sub-system is responding to disturbing interferences. Precision, speed and intensity of this response determine the type of dynamics induced by disturbances. Hence, type and degree of stability (dynamics) depend on the ef®cient functioning of the mechanisms responsible for intrinsic self-regulation. Instability of degraded or arti®cial forest ecosystems is understood as the lack of intrinsic self-regulation. System maintenance requires external regulation through management intervention. The amount of regulatory energy necessary for the stabilisation of
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such systems depends on the degree of incompatibility between management goals and ecosystem capacities. Forest management concepts are characterised by the degree of external regulation necessary to compensate the lack of intrinsic self-regulation. Since this characteristic is ecologically and economically relevant, respective evaluation of management concepts would be useful, and guidelines for developing concepts of sustainable forest management should take this into account. The basis for this approach is knowledge of the processes responsible for intrinsic regulation, which again must be seen in relation to the agents causing disturbance and perturbation as well as in relation to in¯uences emerging from management interventions. Arti®cial extrinsic regulation is corrective management intervention to assure the persistence of the system, to reach the prescribed functional goal. Recent forest management concepts imply immense demands of corrective intervention in wide parts of European forests. An example are the `forest health' measures prescribed as routine management in conifer forests to prevent bark beetle epidemics (Schwerdtfeger, 1981). Each forest community with species outside their natural sites or of unnatural species composition needs corrective management intervention to augment intrinsic ecosystem control mechanisms of self-regulation. Fluctuations of environmental and/or economic conditions bring about situations where the cost of corrective intervention, if at all feasible, exceeds the economic potential of the forest. In such situations foresters stand helpless facing the destructive dynamics of their pseudo-stable forest ecosystems. It is less a shifting of paradigms but more the hard economic reality that force foresters (at least in central Europe) to minimise the expenditure for management operations. Consequently, when extrinsic regulation by management is reduced, the need for intrinsic self-regulation is increased. Otherwise the commitment to sustainability could not be ful®lled and an acceleration of forest decline would be unavoidable. 3.3. Deterministic conditions Ð process control Nature, intensity and frequency of disturbing agents on one side and regulatory capacity, ef®ciency and precision of the intrinsic mechanisms on the other, form the deterministic conditions which de®ne an
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ecosystem as either unstable or stable, resistant or resilient (Holling, 1973). Deterministic conditions are represented by different arrays of physical, chemical and biological parameters, situated within and outside the ecosystem, human action included. The interactive processes among these parameters follow certain laws, which are only incompletely understood. In theory complex factors are involved: The ¯ux of energy and matter, substrate character, vegetative species composition, trophic and population dynamics of the fauna, as well as the micro¯ora and microfauna. There is also evidence that the nature, intensity and speed of these interactions are subject to the ¯uctuations of the physical circumstances. Although we have knowledge of many speci®c factors operating in certain sectors of forest ecosystems, it is not known, to what extent this knowledge can be generalised. The common opinion is, that potentially destabilising forces are usually extrinsic to the ecosystems (Bormann and Likens, 1979). In this context climatic stress factors and human impacts are addressed. But there is also evidence of considerable capacity for destabilisation within the system: pest and pathogen populations are components of the ecosystems. The processes governing the population dynamics of these populations and the mechanisms which control the repair of associated damage, are doubtless situated within the ecosystem or respective sub-system. If these control mechanisms are to be constructively manipulated, they must be better understood than they are now. Mechanisms for control and regulation in forest ecosystems must be more closely analysed from their causative-hierarchical network connections. General or factor-speci®c differences of valence between the mechanisms would appear. Knowledge of these differences could be used to determine state-diagnostic and design-strategic concepts for forest ecosystem management. To be useful, such concepts must be extended over the whole ecosystem, because stabilityrelevant regulatory processes take place on and between all trophic levels. Correspondingly, the potentially destabilising forces whether exogenic or endogenic, can impinge on all trophic levels (FuÈhrer, 1997c) (Fig. 1). Reliable, easily recorded indicators are often sought for the conditions of forest ecosystems. Thus, scientists must develop criteria which represent a
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Fig. 1. Epidemically relevant effects and processes triggered off by environmental factors in the trophic levels and between-level-interfaces of multitrophic consumer food-chains of forest ecosystems. Environmental effects to (1) physical, chemical, biological soil conditions; (2) ecophysiology of individual trees and to stand structure as related to density and age; (3) ecophysiology, propagative potential, interspecific competition of herbivores; (4) ecophysiology, propagative potential, interspecific competitiveness of natural enemies, supply of supplementary ecological requisites. (FuÈhrer, 1997b).
multitude of complex information on a high level of aggregation. The well-known crown transparency as an indicator for `forest health' is one example; the `tightness' or `looseness' of nutrient export characteristics (Bormann and Likens, 1979) is another. Neither `crown transparency', the indicative value of which is rightly criticised (Innes, 1993), nor nutrient export characteristics can themselves provide information of where in the system the ecosystem malfunction is situated. A detailed search for the malfunction is necessary in every individual case. The better is the understanding of ecosystem control, the easier the search will be. Forest ecosystems are dif®cult to understand due to the immense diversity of components. Attempts to de®ne fundamental principles are limited to overly general statements. Even the common opinion that biodiversity increases ecosystem stability, for example, might have only limited validity (FuÈhrer, 1978; Tamm, 1992; Bengtsson et al., 2000). Therefore, the understandable desire for generalisation in forest ecology should be met with healthy scepticism. This is also true with the interpretation of models, irrespective of their high value for developing a basic understanding of ecosystem processes. In applied science and management practice, generalisations have only limited value, because they do not suf®ciently touch upon
essential details. Management decisions require detailed knowledge of the local situation, which can only be drawn from speci®c knowledge of a diversity of ecosystem components. Here our knowledge often falls short. It therefore, is an urgent task of forest ecosystem research to further develop these skills. Modelling can be of great value in developing these skills (see also Tamm, 1992). The practical bene®ts of detailed knowledge on the network of ecosystem control mechanisms, lies in its diagnostic and constructive value. Diagnostic application can serve to identify the causes of disturbances which already occurred, or to assess the capacity of the ecosystem to withstand the loads inherent to a particular management regime (`critical loads', in a wider sense). This knowledge can be used to assess the sustainability of particular management concepts, and to facilitate their realisation. Such evaluation is essential in afforestation projects, forest transformation and forest ecosystem rehabilitation. Risk assessment is another application which includes identifying and evaluating the conditions, which predispose a forest to the incitement of acute damage. (FuÈhrer, 1997c; Netherer and FuÈhrer, 1998; Nopp and FuÈhrer, 1998). This knowledge could be used to develop tools for the optimal adjustment of forest management concepts to the local ecological situation.
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3.4. Sources of disturbances The sources of forest ecosystem disturbances/perturbations are natural, (e.g. climatic factors, abiotic site conditions, ®re and pest organisms), semi-natural (e.g. climate change) or exclusively man-made. Manmade interferences emerge from either the socioeconomic environment of forests or forestry itself (Schimitschek, 1969; FuÈhrer, 1990). Many such disturbances are side effects resulting from other land uses, agriculture, industry, settlement, traf®c, tourism etc. To reduce or avoid these side effects strong political conviction coupled with clear knowledge of the causality and `dosage-effect relation' of the disturbances is needed. Concerning harmful effects of air pollution or poor water management on forest ecosystems, political mechanisms are partially established on the national or international scale, but their improvement is still urgently needed. The `critical load' concept, mainly discussed in connection with acid depositions, could be (and partially is) applied to other factors affecting forest ecosystems both continuously and cumulatively (e.g. herds of wild and domestic ungulates, tourists etc.). For acid deposition, Tamm (1992) suspects, that the `critical load' concept is insuf®cient, because many destabilising effects can occur before the `critical load' is reached. Likewise many subtle or indirect impacts are dif®cult to recognise, posing a challenge to both forest ecologists and the management decision apparatus. Both direct and indirect disturbances on forest ecosystems emerge from forestry practice itself. The predisposing effects of ecologically unsuitable silvicultural concepts and management techniques must be particularly pointed out. They participate signi®cantly in the destabilisation of forest ecosystems and facilitate acute disease incitement (Schimitschek, 1969; Cramer, 1984). This could be avoided if, (1) the cause-effect-complex can be made clear enough to form reliable models for risk assessment, and (2) the management concepts can stand up to a comparative long-term cost-bene®t calculation. Forest ecosystem research is challenged to produce the necessary information. Sustainable management must also augment a forest's resistance to destructive forces wherever they may come from, i.e. to strengthen the stability of the
35
ecosystem. In many cases of forest decline the causal interpretation postulates predisposing (conditioning) short-term or long-term effects of anthropogenic environmental in¯uences as a basis of a more likely incitement of apparent diseases by natural stress agents (FuÈhrer, 1990, 1997c, 1998; Manion and Lachance, 1992). Predictability of forest health problems depends basically on the extent to which their causality is understood. Unfortunately, despite some general features, there seems to be a great diversity in the genesis of apparent forest diseases. The gaps in knowledge are considerable. However, the signi®cance of a speci®c disease and the necessity of its ef®cient prevention may be relative, depending on the function and condition of the forest in question. Rehabilitation of forest ecosystems is an endeavour strictly devoted to this purpose in cases, when progress of degradation is already threatening the persistence of the system. Here as well as in all afforestation projects, particularly those on extreme sites (high altitudes, karst, semiarid sites, mining and industrial dumps, polluted soils etc.), when a persistent cover of forest vegetation is the goal, the strategic concepts require the highest level of expertise and a reliable understanding of the relevant ecosystem processes. 3.5. Managing forest dynamics towards functional goals Forest management decisions must be based on clearly de®ned management goals and methods. Both the de®nition of a reasonable goal and the choice of the suitable method require suf®cient knowledge of how a forest ecosystem functions. If management decisions are not based on this knowledge and are not made individually, they can not take into account the compelling eco-geographic or site-related circumstances, for which the decisions are to be made. Silvicultural dogmatism, which have determined forestry in the past, and have caused many serious problems, would be prolonged. 3.6. Goal-specific management The suitability of management methods depends on the ecosystem processes that they induce. The purpose of management intervention is to in¯uence ecosystem development in a direction determined by the manage-
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ment goal, i.e. the forest function. In the great majority of cases the intended function of forests postulates the maintenance of certain qualitative criteria, which would not arise through natural self-regulation. For instance, protection against soil erosion, torrents and avalanches in mountainous forests depends on structures and developmental features in particular spatial and temporal arrays. In mountainous regions a forester's decision can determine landscape hydrology for decades or even centuries. Silviculture here needs reliable hydrological knowledge in order to avoid serious errors. Prevention of catastrophic ¯oods has priority in mountains rich with precipitation, while the maintenance of a continuous and high quality water supply is a primary function of forests in dry climates (Mayer, 1976). Paradigm shifts and increasing labour costs have resulted in a trend to more `near-natural' forestry and extensive forest management. Retaining dead wood, resulting from natural disturbances, is one example, but it is questionable whether this is everywhere tolerable from the view of forest health (FuÈhrer, 1997a, b). It is evident that persistence of forest ecosystems in a functionally welcome state usually requires at least a minimum of corrective intervention. This is true with `near natural' forests which need, at least, `custodiary' management, and more valid with forests under a rigid economic management regime. The further from the goal condition the management starts, or the more ecosystem dynamics tends to deviate from the goal-directed course, the stronger the management intervention must be. The basis and objective of sustainable forest management is the persistence of the utilised forest ecosystems in a de®ned functional state. The conditions under which a particular forest ecosystem can persist are limited by site and climatic characteristics, which can change considerably through time and human impact. Throughout most parts of Europe, human impacts have been the dominant force shaping site speci®c characteristics. Persistence of a forest ecosystem implies pronounced developmental dynamics, including phases of aggradation, maturation, degradation, destruction, reorganisation etc. Under ideal circumstances the system can enter a `steady state' phase, i.e. a smallspaced mosaic of units in phase-shifted developmental states, thus giving the whole ecosystem the character-
istics of collective stability (Mayer, 1976; Bormann and Likens, 1979). It can be assumed that in such cases ef®cient mechanisms of self-regulation successfully control all destabilising exogenous and endogenous forces, thus keeping the developmentally destructive phases temporally and spatially within ecologically tolerable limits. 3.7. Management controlling natural dynamics Natural development of forest ecosystems is permanently subject to exogenous and endogenous disturbances and Ð from time to time Ð is exposed to thorough perturbations caused by extreme abiotic and biotic events; overmature stands are particularly susceptible to such disturbance. Long-term cycling in forest ecosystems regularly includes phases of disintegration, followed by phases of system reorganisation and vegetative rejuvenation. While perturbing events often represent obvious turning points in the long-term dynamics of the ecosystem, disturbances happen at shorter intervals and usually can be cushioned by repair mechanisms of a resilient (stable) forest ecosystem. An essential objective of forest management is to prevent undesirable ecosystem dynamics such that the system is kept stable or more or less subject to a prescribed developmental rhythm. Forest management is, therefore, also a ®ght not only against uncontrolled ecosystem perturbation but also against seemingly minor disturbances which cannot not be easily compensated in less stable ecosystems and thus could incite the destruction of the whole system. What causes disturbances or perturbations, relevant to forest ecosystem dynamics, depends on the identity, intensity and temporal/spatial pattern (i.e. the `incitement potential') of the acting factors as well as on predisposing traits of the target ecosystem. (FuÈhrer, 1997c). `Stability' in the sense of `absence of uncontrolled dynamics' depends on the degree to which the system is adapted to the range of potentially inciting factors. Suf®cient knowledge of (1) the disturbing forces/factors acting on/in an individual forest ecosystem, (2) the mechanisms involved in the control of such forces, (3) the ecosystem conditions enabling these control mechanisms to succeed and (4) the relevance of particular factors to the dynamics of the ecosystem, are all essential in forming a theoretical basis for sustainable forest management.
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The evaluation of a function-related management goal in the light of the inherent potential of ecosystem interference usually reveals the necessity of corrective management interventions. The more the intended function is subordinated to the prevailing functional capacity of the ecosystem, the smaller are the required corrections. In order to assess the nature and costs of corrective operations, and to ®nd a reasonable mode of harmonisation between management goals and ecosystem capacity, at least two facts must be compared: (1) the suitability of the forest ecosystem for the proposed function and (2) the long-term effects on the forest ecosystem caused of the function. This may be illustrated by a simple example. A decision to devote a forest area to recreational purposes, may have far-reaching consequences for the habitat quality for deer. Behavioural reactions of the deer population will probably cause severe damage to stands (peeling) and regeneration, thus inducing signi®cant detrimental changes to the further forest development. This damage could be avoided by applying alternative strategies of wildlife management, but at a signi®cant cost. Thus sustainable management involves understanding the systemic capacity, longterm trends, and associated costs in maintaining particular functional goals. It is dif®cult to develop multifunctional sustainable management concepts on an ecologically sound basis. As history demonstrates, competition between functional goals can provoke insoluble ecological con¯icts (Glatzel, 1991). To assess the multifunctional potential of a site/region the relevant ecosystem criteria and function-related impact interactions must be understood. Indicators of the forest ecosystem's capacity are needed to cope with interacting impacts. More knowledge in this ®eld could be obtained from ecological studies on successful and unsuccessful examples of multiple use systems. 4. Conclusions Consideration of forest ecosystems from the view of their ecological and socio-economic functions makes clear that sustainable forest management concepts must take into account the compatibility between forest function and ecosystem characteristics. Incompatibility causes either dysfunction and ecosystem
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degradation or the need of corrective management interventions which may exceed tolerable economic limits. A careful evaluation of ecosystem capacity in light of the destabilising impact of function-related forces could help to prevent an undesired development of the ecosystem condition. Sustainable forest management is the management of long-term dynamics of forest ecosystems, while coping with short-term disturbance, in order to sustain the function to which the forest is devoted. Management strategies must be focused on ecosystem persistence and a suf®cient degree of ecosystem stability. A detailed understanding of the laws and processes, which determine ecosystem dynamics, is an essential basis for developing well adjusted management concepts. This understanding must include the destabilising and stabilising processes intrinsic to the ecosystem, their regulatory interactions, and their responses to exogenous disturbances and perturbations, which emerge from forest management and environmental conditions. The pattern of impact on the ecosystem, typically related to individual forest functions (utilisation), and the effects on the ecosystem, typical to individual management regimes, represent details fundamental to this process. When ecosystem dynamics is expected to correspond with the intended forest function, existing incompatibilities between function and ecosystem characteristics must be corrected by management intervention. Intervention must compensate the lack of intrinsic self-regulation. Forest health problems indicate such incompatibilities. The cost of successful corrective intervention strongly depends on the degree of incompatibility and on the ef®ciency of the management concept. Therefore, decisions concerning goals and methods of forest management touch ecological, technical and important economic aspects. Professional decision making processes urgently need a stronger scienti®c basis, than is presently available. Forest ecosystem research must develop this scienti®c basis. The study of mechanisms involved in the dynamics of forest ecosystems and their sub-systems, the evaluation of these mechanisms in the light of forest ecosystem diversity, forest function and forest management, are challenging tasks. Developing methods for the goal-directed manipulation of ecosystem processes, in order to increase the capacity of intrinsic
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