Sustainable tree biomass production

Sustainable tree biomass production

Forest Ecology and Management 132 (2000) 51±62 Sustainable tree biomass production Ê grena, Erwin FuÈhrerb Folke O. Anderssona,*, GoÈran I. A a Sect...

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Forest Ecology and Management 132 (2000) 51±62

Sustainable tree biomass production Ê grena, Erwin FuÈhrerb Folke O. Anderssona,*, GoÈran I. A a

Section of Systems Ecology, Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, P.O. Box 7072, S-750 07 Uppsala, Sweden b Institut fuÈr Forstentomologie, Forstpathologie und Forstschutz, UniversitaÈt fuÈr Bodenkultur Wien, Hasenauerstrasse 38, A-1190 Vienna, Austria

Abstract Our understanding of forest production and forest growth is incomplete. Present yield tables have not predicted the recent changes in boreal and temperate regions of Europe. We argue that we need to have a causal understanding built upon a mechanistic knowledge of important processes. Using a carbon/nitrogen model as a framework, important components and processes of the forest ecosystem are discussed and the new knowledge needed is identi®ed. Present soil chemistry may not be fully relevant in describing plant/soil relationships. It has been shown that the near-root environment deviates from the conditions described by bulk chemistry. Modelling is discussed as an important tool for increased understanding and prediction. Sustainability of forest production is usually treated from a long-term perspective. Long-term changes can lead to de®ciency or even excess of mineral nutrients, which will affect the resistance of the tree or stand to drought, frost as well as attacks of insects and pathogens. Recent ideas of tree vigour or vitality in relation to insects and pathogens are reviewed as a component for understanding production stability in short- and long-term perspectives. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Sustainable production; Tree growth; Tree/soil interactions; Ecosystem analysis; Near-root environment; Modelling; Tree vigour/ pest relationships

1. Introduction Recent changes in the environment and in the management of our forests are often experienced in contradictory and ambiguous ways. Harvesting of timber and ®bre with different degrees of intensity and the acid precipitation are often seen as a threat to


Corresponding author. Prof. Folke O. Andersson, Karlsberg, Larv, S-530 10 Vedum, Sweden. E-mail address: [email protected] (F.O. Andersson)

the long-term productivity of the forest as there is an increased loss of nutrients. Weathering of soil minerals may not compensate for this loss. On the other hand, there is an increased deposition of nitrogen and there might also be a stimulating effect brought about by increased carbon dioxide concentrations and climate change, leading to a stable or increased production potential. The question remains whether these changes are of a sustainable character and will persist? It is well documented that boreal and temperate forests are growing better than would have been suggested using existing yield tables as a prediction

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tool. There is up to a 20% or more increased yearly tree production (Spiecker et al., 1996). It has also been documented that the site fertility or the site index is increasing. The effects of progressive over-exploitation, or episodic perturbation of forest ecosystems may not be compatible with sustainable forest management and the maintenance of productivity. Conditions increasing the production of biomass are welcome to the manager, whether as a result of controlled effects of management intervention or of uncontrolled environmental effects. However, the attraction of increased growth effects might seduce the forester to neglect the potential risks which may be associated with it. Forest management has long been based upon the concept of sustainability. In popular terms, sustainability can be understood as using the forest in a way where future generations shall have the same bene®t as the present generation of a sustained forest yield. Placed in an ecological perspective, we see production as a function of balanced nutrition of the plant cover and closed biogeochemical cycles. Sustainability can also be understood as the maintenance of the soil capacity for future production as well as the maintenance of the supply of water suitable for life support. Seen in a still broader perspective, the forest environment shall also be used for production of a wider range of values or amenities than wood and ®bre. Previously, there was an emphasis on sustainability of yield. Today's view of the forest is broader, emphasising sustainable forest management and embracing society-orientated values (Farrell et al., 2000). The aim of this paper is to identify areas of research for the near future in order to reach a causal understanding of forest growth in an ecosystem perspective. This causal understanding implies identi®cation of the relationships between nutrient availability and forest production. Furthermore, the importance of nutrition for maintaining production stability and sustainability of the forest ecosystem will be discussed; in particular, interactions between tree vigour or vitality and nutritional as well as water conditions of the trees. We will restrict this paper to mineral nutrition aspects. Although the increased CO2 concentrations in the atmosphere and changes in temperature and humidity are a part of the issue, these factors are omitted. We refer here to Cannell et al. (1999).

The area is wide and the guidance for selection of topics to deal with is that we apply a long-term perspective, decades and rotation ages of a forest rather than days or years. However, small-scale patterns with effects valid in a short-term perspective may be important for the understanding of plant/soil relationships and, thus, for the predictability of forest growth or production for the future. This holds also for the interaction between trees and pest organisms, such as insects and pathogens. In order to address the issue of sustainability, we need to have an insight into tree/plant nutrition, nutrient cycling and environment. Tamm (1995) gives a historical account of how the understanding of nutrients in the forest ecosystem has developed, while reviewing tree nutrition, biomass production, nutrient cycling, site studies and forest in¯uences. We refer to this paper for a more comprehensive background. 2. Sustainable tree biomass production 2.1. Some aspects on sustainable biomass production In order to discuss the issue of sustainability, we take as a starting point the current situation in European forests with increased growth rates and changes of site indices (Spiecker et al., 1996). We need some de®nitions to start with. The site fertility or site index is usually expressed as the culmination of the mean yearly production during a forest rotation. It is often explained in a multiple regression function, where a number of climatic and soil physical and chemical factors are used. This is also the way production or yield tables are constructed. The present increased yield in many European forests is not accurately described by these yield tables or functions as they are historical documents, established on the basis of measurements from earlier situations with different chemical and physical climate. To overcome this inadequacy, there is a need and a challenge to develop new yield tables founded on a causal understanding and mechanistic models. The nutrient budget approach is commonly used as the basis of sustainability. The export of nutrients from the forest ecosystem should not exceed replacements by deposition from the atmosphere, and weathering of primary minerals. Still, it will be a matter of discus-

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sion how intensively we will use the soils or forest land. There will be a whole array of intensity of utilisation. We can start with the native forest, where a selective cutting may take place. Here, the basic principle must be applied together with other restrictions governed by multiple-use strategies. In other cases, depending on previous land-use, semi-extensive forestry may be operated or even ®bre forestry or industrial pulp forestry will be realistic alternatives. In the latter case, management of the nutrition with amendments of nutrients is a realistic measure. The importance of weathering may then be seen as less important. Nutrient manipulation will still be restricted by environmental considerations. Groundwater quality must be protected through constraints on leaching and soil acidi®cation. The utilisation of the forest land as well as previous agricultural land at different degrees of production intensity will require an understanding of the factors regulating productivity. It is in many cases not only a question of basic soil properties alone, but also the impact of previous land-use. Fertility can also be builtup. Such an example is the present effects of the increased levels of deposition of nitrogen compounds in many parts of Europe. Increased height and biomass growth are observed and changes in fertility may be judged on the basis of changes in height growth. 2.2. Nature of the problem Ð research approaches Models in experimental sciences will always contain some element of empirical information to make them speci®c to the particular system studied. How much empirical information that is included and how that information is linked together varies considerably between models. At one end of the spectrum are the statistical models that rely on very large numbers of data and where the links between them are arbitrary in the sense that no attempts to interpret the functions connecting different data can or should be made. At the other end, we have theories that almost exclusively consist of functional relations between variables and the empirical world enters through constants of nature and a few parameters. Models in physics are typical of this category. Models in forestry or about forests fall between the two, but with a strong, historical tendency towards the statistical end of the spectrum. Statistical models are


limited to handling situations within the range of the database from which they are derived; extrapolation can be misleading. To go beyond existing data, we need means that allow us to extrapolate and interpolate with some con®dence. This we do by identifying mechanisms or rules that our system obeys and which do not allow arbitrary combinations of variables. The combination of a number of such mechanisms to provide a description of a system leads to what we call mechanistic models. The identi®cation of mechanisms is, however, not absolute, because while looking at a single mechanism we normally see that it, in turn, is made up of a number of underlying mechanisms and hence mechanistic models are really a hierarchy of mechanistic models. The dif®cult task of knowing where to stop in such a Ê gren and hierarchy of models is the art of modelling (A Ê Bosatta, 1990; van Oene and Agren, 1995; Andersson et al., 1997). However, more information is not necessarily better. Increasing the number of mechanisms in a model will initially improve the quality, but it also requires a certain amount of information about each mechanism. Because the information about mechanisms is not exact, an increasing number of mechanisms implies also an increasing uncertainty. There is, therefore, a trade-off between these two processes that, in general, leads to some intermediately complex model Ê gren, as the optimal one (Rastetter et al., 1992; A 1996). How detailed do we need our models to be in order to understand the changes in growth rates in European forests over the last decades? To some extent, this depends on what are the major driving forces which underlie these changes. If we attribute a large fraction of the growth changes to the increasing atmospheric carbon dioxide concentration, it is clear that we need models that can handle this factor and we may end up using biochemical models of photosynthesis (e.g. Friend et al., 1997). The price paid is that we simultaneously need a lot of detailed micro-meteorological information that can be dif®cult to obtain with suf®cient accuracy. If, on the other hand, we believe that the increasing nitrogen deposition is a major force, then we are likely to end up with some model that emphasises the interaction between trees and the soil (e.g. Andersson et al., 1997). Or improved management could be the most important factor, which would lead to yet another type of model. There is, of course,


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the possibility that all factors have to be reckoned with, in which case yet another model might be the optimal one. In summary: different problems require different models. Most mechanistic forest models do not treat individual trees, but place the same kind of tissue for all trees or other vegetation into one compartment. Some models (e.g. Pastor and Post, 1986; Friend et al., 1997) treat trees individually on a small plot, which is simulated repeatedly to create a statistical average. If the intention is to have models that not only will estimate biomass correctly, but also size distributions and information about wood quality, it is probably necessary to resort to models that explicitly describes the geometrical distribution of the trees in a stand. Such models have so far not been possible because of their extreme computational demand. We have earlier emphasised the need for an interaction between theory, hypotheses building and experimentation (Andersson et al., 1997). Without a base in ®eld experiments, often representing longterm efforts, even the best theories will be dif®cult to prove. 2.3. Identification of critical processes and research needs Ð long-term perspective Sound theories and hypotheses, supported by welldesigned and controlled ®eld experiments, form the basis for the development of our understanding. The generation of background data is exempli®ed by a recent study of eleven Nordic ®eld experiments (Andersson et al., 1998). In this work, we have analysed nutritional aspects of the production in terms of biomass and nutrient budgets (Fig. 1). The starting point for an increased understanding will be an ecosystem model, where the production of plant biomass is seen in the context of carbon and Ê gren and Bosatta, 1998; Andersson nitrogen cycling (A et al., 1997). The driving function of the production is the nitrogen content of the leaf biomass. In this case, the release of nutrients from the soil, which is dependent on the quality of the organic matter, as well as other inputs and losses from the ecosystem is critical. It is a carbon±nitrogen model, which also may handle other essential elements such as P, K and S, which in other parts of the world are limiting production. Over

large areas of Europe, nitrogen is still the most common limiting element for forest growth. 2.3.1. Plant production Plant biomass and production is one of the better investigated areas and qualitatively well understood in many respects. Belowground tree biomass and production, on the other hand, have received far less attention and have rarely been included in budgets of carbon and minerals. Furthermore, prediction of allocation in big trees is still weak Ð at laboratory scale there are less problems. We need a better understanding here, not only for the purpose of predicting quantities of total plant biomass or fractions of biomass, but also for the purpose of predicting harvestable biomass as well as organic matter recycled in the ecosystem. Changes in the relative distribution of litter components as well as the changes in chemistry of litter components can modify rates of mineralisation of carbon and nutrients in the soil. Hence, this can be an important feedback. 2.3.2. Litter fall This is a much neglected area. Although in a longterm perspective, whatever goes up has to come down, variability in litter production in the short term can be important for connections with climatic variability. The extension of study periods are also such that it is necessary to be able to distinguish variability from trends. It is also important to understand changes in retention times of different components of the ecosystem. Another aspect is the importance of litter for the decomposer community. It is essential to improve our understanding of the variability in quantity and quality of the litter over time and space. 2.3.3. Carbon mineralisation Substantial advances in understanding the decomposition process have been achieved. However, most of the advance in understanding has been on litter decomposition, and the late stages of decomposition products are not well characterised. Experimental techniques that allow us to follow changes in soil organic matter quality have to be combined with theories for progress to be made. A fundamental question is, which chemical and physical properties de®ne organic matter quality?

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Fig. 1. Nutritional and functional analyses of trees and forest ecosystems Ð collection of data, calculations, analyses and synthesis. From: Andersson et al., 1998.


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2.3.4. Nitrogen mineralisation Since nitrogen mineralisation is strongly coupled to that of carbon, the remarks made above also apply to nitrogen. In addition, there are some processes unique to nitrogen. A major concern is the effect of the availability of nitrogen on immobilisation and subsequent re-mineralisation. Inorganic fertiliser nitrogen applied to forests is, to a large extent, immobilised in the soil. What is the relative role of biological and abiotic, controls on this process and what role does the organic matter play? 2.3.5. Competition for nitrogen and other minerals Related to the previous point is the question of competition between plants and micro-organisms. There is no doubt that such competition exists, but is it an important problem or can we partition inorganic nitrogen between plants and microbes without involving competition? 2.3.6. Mineralisation and availability of other elements We need to consider all elements that can be limiting to production, in particular P as well as microelements. There is a need to understand and qualify their mineralisation as well as speciation into relevant forms and their availability for uptake by plant roots. The interpretation ought to be done on a mineral cycling basis considering input, release, uptake, immobilisation, and losses. 2.3.7. Deposition Atmospheric deposition of mineral elements has been recognised as an essential input. Nitrogen is today much discussed as well as its different pathways to the ecosystem. The input of all elements, in particular basic cations both in short- and long-term perspective need to be recognised. 2.3.8. Weathering Present conclusions concerning possible shortages of available mineral nutrients, such as magnesium and calcium, are usually built upon budget calculations. True weathering information is often absent. Even when present, there is a need to question how well the information re¯ects the available amounts and the availability to plant roots. Estimates of weathering rates usually apply to bulk soil, which is not repre-

sentative of the situation in the immediate vicinity of roots and mycorrhiza. The least understood process as well as the most dif®cult one to quantify is the weathering of minerals. It has a key role for the long-term productivity of the ecosystem. There seems to be a need to combine different methods, old as well as new ones, and consider the factors regulating weathering, such as mineralogy, physical variables like climate and quality of organic matter. Further, a number of biological in¯uences must be taken into account, such as the interaction of ®ne roots and mycorrhiza with soil particles. Exudation products and dissolved organic carbon are important in regulating the exchange and weathering processes. The availability of minerals to the roots will also require good insight into transport processes in the soil. 2.4. Spatial variability of soil properties implications for plant/soil relationships In works by Clegg (1996), Gobran and Clegg (1996), Guan (1997) and Gobran et al. (1998), it has been demonstrated that the near-root environment has properties which are signi®cantly different from those described by chemical analyses of the bulk soil There is a higher acidity, a higher cation exchange capacity and higher amount of organic matter in the near-root environment. It has also been demonstrated that the weathering rate is higher in this microenvironment. The implication of these ®ndings is that the active roots and mycorrhiza exist in an environment quite different from that in the bulk soil, with profound implications for nutrient availability and root-uptake. In other studies, it has been shown that mycorrhiza has direct contact with mineral particles and that a weathering of bare surfaces can take place. These ®ndings lead us to assume that budget analyses alone are not suf®cient in the description and quanti®cation of available plant nutrients in the soil. Therefore, an urgent research area is the nearroot environment and the mutual relations between plant and soil. This will be an important area in order to understand mechanisms of nutrient availability and its relation to production. A number of new techniques need to be tested in order to address these issues.

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3. Production stability and tree vigour; insect and pest interactions with mineral and water conditions of trees Besides the quantitative dimensions of biomass production, its qualitative aspects must also be considered, both for economic and ecological reasons. Ecologically, the qualitative aspects concern the ecophysiological and biogeochemical phenomena connected with the processes of production, because of their relevance for production and ecosystem stability (FuÈhrer, 2000). The living tree biomass has to successfully pass a long sequence of development phases, each of them being characterised by speci®c and detrimental in¯uences from limiting factors. Limitation of nutrients and water may exert different effects on growth and the physiological status of trees; depending on whether the limitations are disproportionate and/or temporary. Nutrient imbalances are assumed to cause severe physiological stress (Katzensteiner et al., 1992) and to predispose the tree to frost injury (Levitt, 1972; Thomas and Blank, 1996). Enhanced growth in¯uencing the root±crown ratio, may provoke excessive imbalances and drought stress in periods of limited water supply (Lyr et al., 1992). Abiotic effects, promoted by particular growth conditions, often trigger biotic diseases and pest epidemics. Therefore, it is commonly assumed that physiological weakness is a necessary prerequisite for pest organisms to successfully attack trees (Manion, 1981; Waring and Pitman, 1983; Christiansen et al., 1987). At a closer view, however, these relationships appear much less homogeneous and more contradictory, indicating that a general rule probably does not exist (Larsson, 1989). Misleading errors in this context might be that `growth rate' is often considered to be synonymous with `vigour' and that `resistance of plants to everything' is thought to be directly related to `vigour'. Divergent results of forest fertilisation (e.g. nitrogen) as well as experimental investigations give evidence of the limited predictability of plant growth-related interference of damaging agents, particularly insect herbivores (Mattson, 1980; White, 1984; Bryant et al., 1987; Schafellner et al., 1996). Herbivore responses to changing relative growth rates of the host plant, for example, seem to depend on different circumstances, the least of which being


the growth rate of the individual (Koricheva et al., 1998). Clari®cation of the underlying causes of such anomalies must take into account factors such as the following: Variety of plant characteristics: The trait patterns of the plant, which are relevant to the interactions involving potentially disturbing abiotic or biotic agents, vary significantly with the attack patterns of the agents. Physical, chemical, structural and biological peculiarities of the tree, all being involved in Ð or at least influenced by Ð the growth process, can be responsible for the tree's susceptibility, attractiveness, resistance, nutritional value and regenerative capacity. Factors essential for plant growth, such as CO2, minerals, water and light, seldom have a direct influence on the interaction with the attacking agent, although water may be an exception. Generally, the target sites in the plant, where the attack occurs, are characterised by products of plant metabolism, more or less complex organic compounds, differently arranged due to their functions (Fig. 2). During the process of biomass production, the inorganic and energetic basis of plant growth has experienced manifold transformations, leading to a high diversity of physical, chemical and biological trait patterns of the plant, when environmental stressors or biotic agents start their attack. Usually, it is difficult to uncover the causative chains involved in the capacity of a tree to successfully resist a certain attack (FuÈhrer et al., 1997, 1998). A more hypothetical example of such causal connections is shown in Fig. 3. Diversity of plant trait patterns vs. specificity of attack patterns: The influence of the supply of nutrients and water on ecophysiological reactions as well as metabolic and biochemical effects in the tree are dependent, to a large extent, on tree species, genotype and age (Lyr et al., 1992; Larcher, 1994). On the other hand, in the enormously species-rich complex of herbivores and plant-parasitic organisms' food preference, their specificity is essential to avoid interspecific competition. This specificity even concerns the distinction of subtle structural and biogeochemical traits of the host plant, due to the specific requirements of the consumers. The pattern of organic compounds in the plant biomass have a double function in the plant±herbivore system, as the carriers of behavioural and metabolic information and as a source of energy


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Fig. 2. Translation of plant growth-determining components into plant criteria relevant for plant resistance, invertebrate herbivore nutrition and their epidemic characteristics

(LunderstaÈdt, 1981). Thus, the overall effect that conditions determining plant productivity exert on the host plant Ð herbivore/pathogen interactions, depends highly on the organisms involved (Koricheva et al., 1998). It can confer an advantage on one herbivore species and a disadvantage on another. Relativity of plant quality-induced effects: Besides the species-related ambivalence of plant growth effects to the consumer communities, relativity of plant quality changes, appears to be essential for the influence on the plant±herbivore interaction. Generally, we depict the ecological valence of an environmental factor for an organism by the shape of an optimum curve over the factor continuum, with an ascendant and a descendent branch and the culmination in between. There is no plausible reason, why this model should not apply to the valence of food plant quality for herbivores. When beginning in this case near the lower end

of the scale, a stepwise increase of a food quality factor will first improve up to the optimum point and then deteriorate the nutritional valence. Most empirical or experimental results reported in literature, concerning food quality effects in insect herbivory, are obviously depicting only one side of the curve, the ascendant or descendent, indicating effects either positively or negatively related to food quality. It must be suspected that there the studied sector of the food quality continuum was one-sided limited. This assumption could explain contradictory results obtained in different studies on the same subject. An example could be the effect of excess nitrogen supply to Norway spruce on the performance of the little spruce sawfly (Pristiphora abietina Christ.). Merker (1963) achieved high mortality among larvae following deterioration of the food quality after application of large amounts of N-fertilisers. In contrast,

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Fig. 3. Possible paths of indirect effects, which extreme tree nutritional conditions can exert on the increase of susceptibility and food quality of the host tree for invertebrate herbivores. Disturbance of the carbon budget and carbohydrate allocation appear to have key function. They can be caused by magnesium deficiency in different ways, inhibition of foliar carbohydrate transportation, leading to accumulation of starch as described by Fink (1992). Disturbed allocation of carbohydrates may affect either the biochemical and herbivory-relevant features of foliage and stem phloem. Excess nitrogen causes additional changes in the leaves, for the benefit of animals feeding on woody plants (Mattson, 1980).

Schafellner et al. (1996) observed a significant improvement of chemical food quality criteria after moderate treatments with N-fertilisers. Even if only the ascendant part is depicted, it has often the shape of an exponential saturation curve. Schopf (1983) gives an example from spruce sawfly (Gilipina hercyniae Htg.). An integrated developmental index for sawfly performance is positively related to a chemical food quality index of the spruce needles, forming a saturation curve. Increase of that needle quality index by one unit causes a strong developmental effect on the insect, when this step is performed near the lower end of the scale, but only a slight effect when performed near the upper end. The dilemma of plants Ð to grow or defend: The position of plants in the ecosystem is well characterised by Herms and Mattson (1992) as a dilemma `to grow or defend'. The search for general patterns revealed several hypotheses, e.g. carbon/nutrient hypothesis and the growth/differentiation balancehypothesis (Bryant et al., 1983; Waring and Pitman,

1983; Lorio, 1988). None of the proposed hypotheses seem to cover the whole range of observed cases. But they all address the significance of environmental effects on budget and allocation of carbohydrates within the plant, partially under the aspect of source-sink interactions. Sink-limitation, i.e. growth-limitation, imposed by extrinsic factors such as moderate drought, low-temperature or nutrient deficiency will result in the accumulation of carbohydrates (starch) and products of the secondary metabolism, i.e. carbon-based semio-chemicals (terpenes, phenolics, lignins, etc.). Nutrient availability is a major factor limiting primary production on a global scale, and this applies to most forest ecosystems. Circumstances determining plant biomass production (growth) affect also secondary metabolism and, hence, the defence mechanisms of the plants. In this way, constitutive and induced plant resistance to herbivores and pathogens is closely related to plant growth. Influences, reducing or abolishing the sink-sided growth-limitation of trees (e.g. air-borne nitrogen


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deposition), tend to reduce more than to increase their defence capacity against biotic agents (Tuomi et al., 1990; Herms and Mattson, 1992). Management interventions, striving for the increase of forest productivity, may, therefore, be hazardous because they could raise the vulnerability of forests to pests and pathogens. However, with the present state of knowledge, this assertion must be considered somewhat speculative. The experimental data, serving as the basis of the theories mentioned, indicate much variation in the qualitative and quantitative criteria of plant responses due to plant species and ecological conditions. A differentiated pattern of growth-related expression of defence capacity in plants must be expected (see Herms and Mattson, 1992). On the other hand, changing source-sided growth conditions, such as CO2 concentration of the atmosphere, interact with sink-sided changes. Since the details of processes underlying these interactions are as yet unknown, a prediction of their effects on the consumer food chains becomes extremely difficult (Lindroth, 1996). Sustainable productivity of forest ecosystems is dependent on well-balanced nutrient and water budgets. Sustainability may also be threatened by major losses of production, caused by the interference of disturbing agents with the process of plant biomass production. Since susceptibility and resistance of the tree vegetation to such agents depend on characteristics of the growth process, the optimisation of growth and `resistance' should be a central aim of forest ecosystem research. For that purpose, the mechanisms for balanced interaction between host trees and potentially injurious agents (climatic, environmental, biotic) as well as the relation between these mechanisms and the growth characteristics require further study. Considering the diversity of interactions, one wonders if it will ever be possible to get suf®cient insight into this dif®cult sector of forest ecology. However, research in this ®eld has developed during the past decades, particularly concerning herbivory of insects and pathogenicity of air pollutants. Research efforts have yielded many important discoveries, conveying more realistic ideas about the complexity of biological processes. This encouraging progress gives rise to an optimism that research will yield improved understanding of these areas. In order to manage the problems associated with the high level of diversity, work

should be focussed on `key species associations', of relevance to the stability of widely distributed forest types (see Bengtsson et al., 2000). In order to elucidate the linkage of productivity and stability/resistance of the ecosystem, these studies must be designed on an interdisciplinary basis. 4. Conclusions The need of a deeper understanding of tree growth and nutrition has been emphasised in this chapter. An understanding of tree nutrition and its relation to tree vigour and resistance to drought and pest attacks are key areas. More ecophysiological insight is of high priority. Nutrient budgets are helpful in analysing sustainability, but are insuf®cient in themselves. An understanding of functional relationships is fundamental. An obvious task is to look for diagnostic variables of tree and ecosystem vigour or vitality. The work on ecosystem analyses, as illustrated in Fig. 1, gives an opportunity to look for functional relationships such as biomass and nutrient productivities or ef®ciencies. Tamm et al. (1999) discuss the possibility of using stem biomass or nitrogen productivity or ef®ciency of other elements as diagnostic tools of vigour or vitality of trees and stands. For example, the biomass or nitrogen productivity describes the relation between stem volume growth and needle biomass nitrogen content. A deeper understanding of nutritional conditions in the forest ecosystem will also allow us to diagnose conditions and to calculate and prescribe compensatory measures. We have also stressed the need for having adequate methods for determining nutrient availability and release in soils. There is a potential to develop and apply new methodologies to increase our understanding of the functioning of the forest ecosystem and its sustainability. Acknowledgements The authors acknowledge constructive criticism from two referees. Prof. E.P. Farrell, Dublin, Ireland, has likewise given constructive advice as well as having reviewed the language.

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