Ecological Stability of Forests and Sustainable Silviculture J

Home , Ecological stability, Resistance (ecology)

Fores;tdogy Management ELSEVIER Forest Ecology and Management73 (1995) 85-96

Ecological stability of forests and sustainable silviculture J. Bo Larsen Unit of Forestry, The Royal Veterinary and Agricultural University, Thorvaldsensvej 57, DK-1871 FrederiksbeTg C, Denmark Accepted2 November1994

Abstract

The ecological stability of forests is described and subsequently analyzed and discussed in relation to human impact. Forest management and utilization have a considerable influence on the stability and sustainability of forest ecosystems. Additionally, other human activities such as pollution and global climate change affect the present and future stability of our forests. The main components of stability are resistance (inertia, immovability) and resilience (recoverability). These are analyzed with respect to genetic diversity within and between species and in relation to the biogeochemical cycle. The possibilities and constraints of silviculture are then discussed in relation to sustainable management practices and strategies, i.e. choice of provenances and species, including species mixtures, tree breeding, harvesting practices, as well as the silvicultural system applied. Finally, forest decline is discussed in relation to stability by means of a stress integration model.

Ke.ywords: Forest ecosystem; Sustainability; Ecological stability; Silviculture; Forest decline; Management; Genetic variation; Biogeochemical cycle

1. Introduction system of autotrophs, heterotrophs, and the physical environment. Hence the concept of forest stability must be expanded to embrace the general framework of eco- Extensive forest declines during the last decade, logical stability in order to recognize and solve ecosys- especially in Europe but also in other parts of the world, have initiated the discussion on the stability of forest tem problems posed by present and future ecosystems. Initially the forest decline discussion anthropogenic impacts. focused on single causes such as acid rain, ozone, cli- We have to learn to manage ecosystems instead of matic extremes, pests, etc. Today most scientists look stands in order to assure sustainability in a broad sense, upon the so-called ‘novel forest decline’ as a decrease which implies securing long term productivity and in the stability of the ecosystem, caused by a number vitality, safeguarding biodiversity and protecting adja- of partly interacting stresses, some of which are anthro- cent ecosystems. Hence, it is of outstanding importance pogenic (Ulrich, 1987, 1989; Fiihrer, 1990). to gain more basic information about the interactions In forest management the traditional concept of sta- between silvicultural measures and stability. Silvicul- bility focuses upon the trees and is defined mainly in turists must be aware that by manipulating forest struc- relation to catastrophic events such as damage by snow tures, hey may permanently interfere with and wind. Thus it lacks an ecological basis. The com- fundamental, partly self-regulating processes of the partment ‘trees’ should not be considered in isolation ecosystem responsible for system stability and produc- but must be viewed as part of a complex interacting tivity.

Consequently the aim of this article is to define and as fluctuations beyond the ‘normal’ range of forcing discuss the basic principles of ecological stability, to functions defining the system. apply these principles in a forest ecosystem context, Perturbations may be natural or anthropogenic (man and to analyze silvicultural measures and the effect of made). Some natural perturbations include climatic other human activities such as pollution and global fluctuations, variation in density of herbivores, preda- change within this conceptual frame. tors, and pathogens, fire, wind, snow, and erosion. Anthropogenic perturbations may include system manipulation through utilization (clearing, harvesting. draining, use of pesticides and other chemicals, inrro- 2. Ecological stability duction of exotic species and unadapted populations j, pollution (acid deposition, SO,, ozone, and the CO, Ecosystems are open thermodynamic systems char- emission promoting a global change of climate). and acterized by input and output of energy and matter. the import of exotic (alien) organisms. Stability may be defined as the ability of a system to Numerous authors, working mainly with natural eco- remain near an equilibrium point or to return to it after systems, have defined stability in terms of buffer capac- a disturbance (Orians, 1975; Harrison, 1979). Hence, ity, constancy, persistence. inertia, resistance, ecosystem stability is characterized by a dynamic equi- elasticity, cyclicity, integrity and resilience (Lewontin, librium (steady state) achieved through interactions 1970; Holling, 1973; Orians, 1975; Zonneveld, 1977; among functional groups of organisms and the physical Haber, 1979; Harrison, 1979; Gigon, 1981: Ulrich, environment. For example, the nutrient cycling results 1987; Kay, 1991; Jargensen, 1992). However, the from the functional synchrony of autotrophs, hetero- same term is often used by different authors for differ. trophs, the atmosphere, and the soil compartment. ent aspects of stability, and a general agreement upon Ecosystems have numerous features, some are char- types and definition of ecological stability properties is acterized as relatively stable, others are highly unstable. still pending (Grimm et al., 1992). Hence it is important to define the feature (state vari- When thermodynamic systems are moved away able) in focus when discussing stability. These varia- from their local equilibrium, they shift their state in a bles can be divided into structural and functional way which opposes the applied gradients and moves features. Structural features include habitat diversity, the system back towards its equilibrium attractor. In floral and fauna1 diversity, biomass, age and size span, simple terms, systems have the ability to resist being spatial distribution, etc. Functional features include moved from equilibrium and a tendency to return to among others energy dynamics such as net and gross the equilibrium state when moved from it (Schneider production, production to respiration ratio, water use and Kay, 1993). The breakdown of stability into the efficiency, energy flow in trophic structure, element two principal components resistance and resilience ( as cycling, and degree of organism interaction, etc. suggested by Webster et al., 1975) seems therefore Furthermore, stability must be viewed as a function appropriate to gain a certain degree of simplification in of time and space. The persistence of species as a sta- order to discuss and visualize the most important bility feature is closely connected with the spatial aspects of ecological stability. Correspondingly, dimension of ecosystem (Grimm et al., 1992). The (Grimm et al., 1992) rank resistance and resilience impact of different silvicultural measures and manage- among the most important stability concepts related to ment systems on ecosystem stability suggests a certain specific properties and measures in ecological systems. spatial dimension (the mosaic cycle concept; Remmert, Resistance (inertia, immovability) comprises the 1991)) and that the temporal horizon should exceed ability of a system or the component in focus to resist more than one tree generation, thereby including the external stress. An ecosystem feature that is easily regeneration stage. changed has a low resistance, whereas one that is dif- Stability within an ecosystem context must be ana- ficult to displace is resistant and in this sense stable. lyzed in relation to perturbations (disturbances). Such Resilience (recoverability) comprises correspond- perturbations may be defined as events (forcing func- ingly the ability of a system feature, when changed by tions) which do not naturally occur in the system, or a perturbation, to return to its former dynamic state. A J. Bo Larsen /Forest Ecology and Management 73 (199.5) 85-96 87

Fig. 1. Four images of stability are shown as combinations of resistance (the size of the marble: large, high resistance; small, low resistance) and resilience (the size of the hollow in the ‘stability landscape’: deep, high resilience; shallow, low resilience). system that returns rapidly to its original steady-state or functional) focused upon will changepermanently is more resilient than one that respondsslowly. evolving into another dynamic equilibrium. In order to visualize resistanceand resilience, four In the following discussionstability is analyzed in images of stability are shown by meansof potential relation to its basiccomponents. In this context an eco- energy in Fig. 1. In this very simplified way of illus- systemmay be defined by the two main components: trating thesecomplex features, stability is shownas the ‘information’ and ‘matter’. Thereby, ‘information’ ability to resist external forces in order to keep the meansDNA and ‘matter’ the physical environment. By marble (the systemfeature or the statevariable focused meansof outsideenergy flux the information (the liv- upon) within a steady-state, respectively within its ing organisms) ‘organizes’ the system,creating func- amplitudeof recoverability or resilience (the hollow in tions and structuresin a locally reducedentropy state. the ‘stability landscape’). Resistanceis illustrated by Consequently,the discussionof the basicprinciples of the mass (graphically the size) of the marble. It ecosystemstability is related to (i) genetic diversity requiresa stronger perturbation to move the big marble (information) and (ii) the biogeochemicalcycle (mat- (the highly resistantsystem) within the areaof steady ter). state (the bottom of the hollow) and to displace it outside the area of resilience (lift out of the hollow), 2.1. Stability and genetic diversity than it doesthe smallmarble (the lessresistant system). Resilienceis denotedby the size and shapeof the hol- 2.1.1. Variation within species low in the ‘landscape’,illustrating the domainof attrac- Populations respondto spatial and temporal variation. More energy (a stronger perturbation) is required tion in the environment by selection and adaptation. to displace the marble out of the deep hollow (the Hence, the most important aspectof genetic variation highly resilient system), than is required to move the is the buffering it provides againstfluctuations in envi- marble out of the shallow hollow (the less resilient ronmental conditions (Hattemer, 1994). Adaptation system). involves physiological and evolutionary aspects(Lar- Under unperturbed conditions, all four simplified sen, 1988): the individual reacts to a perturbation ecosystems(hollow in the ‘landscape’ and marble) through physiological adaptation, which is limited by remain within their area of dynamic steady state (the the homeostaticcapacity of its genotype. If not all gen- bottom of the hollow). However, a perturbation might otypesin a population arephysiologically ableto buffer exceed the limits of local referential dynamics depend- the perturbation, selectionprocesses will induce adap- ing on the massof the marbleand the sizeof the hollow; tation at the population level (Hattemer and Ziehe, hence the system or the ecological feature (structural 1987; Hattemer and Mtiller-Starck, 1989). The adapt- 88 J. Bo Lursen /Forest Ecology and Management 73 (I 995) R5-96

ability of a population, i.e. the potential buffer capacity, species stands to insect attack including lack of natural is therefore closely related to the genetic variation enemies, high concentration of food (host plants), within the genotype (physiological adaptation) and absence of alternative hosts and the development of between genotypes within the population (evolution- closer coincidence between insect and plant phenolo- ary adaptation). gies. The evidence to support the opinion that insect Orians ( 1975) emphasizes the importance of spatial pest outbreaks occur more frequently in forest mono- environmental variability for several stability proper- cultures than in mixed stands is still inconclusive ties of ecological systems. This may reflect the impor- (Watt, 1992). However, the common comparison tance of maintaining genetic diversity at the population between natural diverse (mixed) forest ecosystem and level to increase the potential (evolutionary) adapta- man-mademonoculture is questionable,since the mon- bility and thus resilience. Genetic diversity of individ- oculture is not only characterizedby simplicity but also uals is correspondingly important in buffering by ‘unnaturalness’. environmental variability through time (Gregorius, In natural ecosystemsresistance is apparently asso- 1985). This reflects the homeostatic capacity of the ciated with diversity (speciesrichness); this suggests genotype, which is an important feature of resistance. that ecosystem response to perturbation primarily A reduction in the genetic resource base of species dependson the adaptive characteristicsof the popula- (genetic variation within and between individuals) tions in the system. Thesecharacteristics reflect histor- may consequently have a profound effect on both eco- ical perturbations and the continual evolution of system resistance and resilience. This is demonstrated associatedspecies. Selection acts to maximize fitness by Larsen (1986, 1994) by explaining the decline of of populationsin the systemand not directly on stability central European silver fir (Abies alba Mill.), pro- properties of the system as a whole. Ecosystem nounced during the last 15 years (and several other responseto environmental variability is consequently periods during the last centuries), as a result of genetic a complex product of coevolution. Coevolution may. variation lost during the last glaciation. These results depending upon the magnitude of perturbations and are supported by studies of genetic variation by means their distribution in time and space, therefore be of of polymorphisms of isozyme gene loci (Bergmann et outstanding importance for stability of ecological sys- al., 1990). Lagercrantz and Ryman (1990) postulate tems. similar genetic ‘bottle-neck effects’ in central and A stable (predictable) environment makes the southeast European Norway spruce (Picea abies (L.) development of a complex (mature) ecosystem Karst.). through succession and coevolution possible. The development of ecosystem maturity is a function of 2.1.2. Variation between species higher energy and matter cycling, higher average One of the central doctrines in population ecology is trophic structure and web complexity, higher that stability increases with the degree of interrelation- (bio)diversity, longer life cycles. In this context, com- ship in the food web. Increased trophic web complexity plexity might be associatedwith system resistance. leads to increased community stability (MacArthur, mainly owing to well developed biological population 1955; Elton, 1958; Pimentel, 1961). In nature popu- control mechanisms.A relatively simplesystem might lation stability is not always associated with fauna1 and develop under more unpredictableenvironmental con- floral diversity. Correspondingly, May ( 1973) conditions owing to the maintenanceof simple (early) cludes that the relation between complexity and stabil- successionalstages. Its robustnesscan in this context ity is substantially more complicated. be attributed to system resilience. Consequently,com- Floral diversity has been shown in many cases to plexity increasing resistanceseems to act negatively reduce pest problems. Conversely, monocultures have upon systemresilience, henceit is easierfor a relative often led to an increase in insect and disease problems simplesystem than for a highly complex (interactive j (Gibson and Jones, 1977). Consequently, many sci- system to return to its former state following a distur- entists recognize outbreaks of pests or diseases as an bance. ecological rather than a pathological problem Speciesrichness may additionally contribute to sta- (Vasechko, 1983). Several factors may dispose single bility through risk alternation, since different-species J. Bo Larsen/ Forest Ecology and Management 73 (1995) 85-96 89 respond differently to biotic as well as abiotic stresses. turnover and storage rates (Swank and Waide, 1980). Further, many insects and pathogens are spread more Further, the forest floor vegetation plays an important rapidly in homogeneous systems (monocultures) role in the cycling and retention of nutrients (Bormann owing to contact (roots, crown) between individuals and Likens, 1979), especially in phases where the dom- of the same (susceptible) species or clone (Heybroek, inant vegetation (the trees) is affected owing to con- 1982). trolled (thinning, clear-cuts) or uncontrolled disturbances (wind throw, etc) . The ground vegetation 2.2. Stabilizjr and the biogeochemical cycle promotes the percolation of water, minimizes erosion and contributes to diversity and activity of heterotroph The long-term productivity of forest ecosystems is organisms including decomposers, thus stabilizing the closely linked with the nutrient cycle (Webster et al., biogeochemical cycle by balancing production and 1975; Swank and Waide, 1980; Ulrich, 1987, 1989). mineralization. Therefore, the quantity and quality of For most natural ecosystems, recycling rates of nutri- the ground vegetation may significantly contribute to ents limit primary production and regulate, at the biogeochemical stability. source, biotic energy flow through the trophic structures. Major perturbations may disturb the biogeochemical 3. Silviculture and stability cycle through temporal and spatial aberration (decoupling) of biomass production and mineralization Most works dealing with ecological stability are (Ulrich, 1987). The stability of sites following distur- based on theoretical rather than experimental research. bance (natural or anthropogenic) is therefore closely Consequently, this theoretical knowledge has not been related to the ability of the system to recycle nutrients applied in management for stability in an ecological and maintain structural and biotic integrity. Hence, per- sense. The following discussion of possibilities through haps the most important aspect of stability in managed silvicultural measures to ensure stability of managed forest ecosystems is the ability to retain soil fertility forests will primarily focus upon the stability of the tree following pollution (soil acidification) and manage- species compartment in terms of structural features ment-induced perturbation (harvest, cultivation). (genetic variation, species diversity, age and size, life Monitoring nutrient loss rates may provide a useful span) and functional parameters (long term productiv- indication of ecosystem response to disturbance. ity including maintenance of biotic and abiotic Stability in this context is mainly due to resistance. resources). Key features include nutrient and water storage capac- In natural ecosystems there seems to be an inverse ity, the ability of the soil to buffer acidity formed during relationship between resistance and resilience. Hence net nitrification, and to prevent or diminish the leaching biotic factors which tend to increase resistance (diver- of nitrate and other nutrients. This type of stability sity, complexity, coadaptation) decrease resilience and corresponds to Ulrich’s ‘elasticity’ (Ulrich, 1987) or vice versa. This interrelation between resistance and is simply expressed as ‘buffer capacity’ (Jorgensen and resilience seems to reflect the development of the eco- Mejer, 1977; Ulrich, 1992). System resilience can be system as response to the types of perturbation com- conferred by mineral weathering, which provides the monly encountered by the system and their distribution system with available nutrients. Weathering of silicate in time and space (see also Section 2.1.) . However, minerals is an important stability parameter, especially man may influence resistance and resilience indepen- in managed forests characterized by a significant export dently, since they are not obligatory functionally of base cations due to harvest and deposition of acid linked. This emphasises the danger that management, compounds, since it is the main natural base cation unaware of the basic principles of stability, might source. Further, natural and anthropogenic (pollution) reduce both resistance and resilience and thus decrease airborne sources of elements may contribute to nutrient stability. Numerous examples exist, including intro- supply and thereby to system resilience. duction of unadapted species and provenances, reduc- Species composition might affect the stability of the tion of the genetic resource base through selection and biogeochemical cycle, mainly due to species specific breeding, large scale conversion of natural mixed for- 90 .I, Bo Larsen/Forest Ecology and Management 73 (1995) 85-96

ests into monocultures, the clearcutting system in com- might exceed earlier ‘normal’ levels, eventualty lead to bination with the use of herbicides, homogenization of permanent changes. structures beyond the ‘natural’ variation, shortening The question of adaptability is the main problem in rotation age in combination with whole-tree harvesting, clonal forestry. Stands with only one or a few genotypes etc. However, this also pinpoints the possibilities to have a greatly reduced adaptability on the population establish and to manage forests focusing on both resis- level (evolutionary adaptability) leading to lack of tance and resilience in order to increase system stabil- resilience. Forest stands with one or a few genotypes ity. especially selected for good performance in a particular environment, are unlikely to have enough buffering 3.1. The choice of species and provenances capacity in an uncertain future (Perry and Maghembe, 1989). Species selection is probably the most important sin- Artificial regeneration traditionally using a limited gle manageable factor in securing future stability. The number of plants raised in modern plant production main prerequisite for choosing plant material is to select systems limits the possibilities of natural selection species and to use provenances which are adapted to against undesirable (sellings) and poorly adapted gen- the biotic (pests) and abiotic (climate, soil) environ- otypes during stand development. This might lead to ment of the specific site including man made pertur- reduction in both resistance and resilience, thereby bations. affecting future structural stability. These problems are Local species and populations ensure a certain discussed by Hattemer and Miiller-Starck ( 1988), Hat- degree of adaptedness, since the population genetic temer ( 1994) and Ackzell and Lindgren ( 1994 ) _How- structure reflects the fluctuations in local environmental ever, the ecological significance of such limitations in forcing functions. In contrast, numerous examples the selection potential in artificial established stands is demonstrate the dangers of using unadapted species only insufficiently analyzed and far from understood. and provenances. Even comprehensive species and provenance trials may lead to erroneous conclusions, 3.2. Forest tree breeding since not all parameters affecting survival and productivity (mostly extremes) are encountered during the period of testing. A major weakness is that the testing Selection for certain phenotypic characters increases period is too short compared with the rotation age of the risk of an uncontrolled change of the genetic the species. Further, species and provenance trials nor- resource base, thereby promoting the possibility of mally cover only a small number of potential sites, diminishing the potential adaptability (Gregorius et al., making spatial extrapolation of the results rather uncer- 1979; Ziehe and Hattemer, 1989). This is of special tain. As a consequence, introduced species and prove- importance in relation to the present atmospheric pol- nances often contribute to a reduced system stability lution situation (Gregorius, 1986). Traditional forest due to lack of adaptedness (resistance). Changes in the tree breeding is based upon the assumption, that the environmental forcing functions caused by climate local environment and its amplitude of fluctuation in changes and the introduction of exotic alien organisms the past can be extrapolated into the future. Hence prog- (pests) may drastically reduce adaptedness of local eny testing, which provides the basis for future breed- species and populations. ing populations, is basically retrospective (Larsen, Fig. 2 illustrates the proposed concept of stability in 1990). relation to climate change. A system, which in the Hence, the breeding goal, in the context of stability, ‘good old days’ ( ‘1990’) was stable (high resistance aims to increase resistance by optimizing local adapt- and resilience), loses stability owing to decreasing cli- edness. The increasing anthropogenic impact upon the matic adaptedness of its organisms (populations) and global environment (pollution and the CO2 increase reduction in both physiological and evolutionary buffer promoting a climate change) makes the occurrence of capacity. Resistance and resilience decrease, and the unpredictable changes in the local growing environ- system develops into a susceptible state (‘2040’, and ment highly probable, even within the next tree gen- especially ‘2090’). Consequently perturbations, which eration. Traditional breeding for maximal growth and J. Bo Larsen /Forest Ecology and Management 73 (1995) 85-96 91

Fig. 2. Forced by climate change, the ecosystem resistance and resilience decrease over time as a result of a reduction in adaptedness and adaptability of its populations, and the system develops into a susceptible state. local adaptedness might, under these circumstances, continuous decomposition, thus preventing phases of develop into a ‘no through road’. humus accumulation and net mineralisation. Morgan et Increasing stability by breeding for potential adapt- al. (1992) and Brown (1992) demonstrated higher ability may therefore be the only possibility to ‘prepare’ mineralizationrates in litter of species mixtures (larch/ our forest tree populations for an uncertain future (Lar- pine and pine/spruce, respectively) compared with sen, 199 1) . The multiple population breeding concept pure species, reflecting higher activity among decom- (Namkoong et al., 1980) might be a promising strategy posers in litter mixtures. Other studies, however, con- to increase genetic variation at the population level. tradict these findings (Chapman et al., 1988). This should increase the evolutionary adaptive poten- Depending upon species-specific characteristics, tial, increasing resilience by promoting the buffer mixed forests may contribute to ecological stability by capacity of the population. However, since only small increasing resistance and resilience and additionally losses of plants owing to natural selection can be tol- decreasing the potential magnitude of perturbation erated, this strategy has a limited potential in practical owing to mineralization or acidification pushes as forestry. Breeding for the physiological adaptive poten- defined by Ulrich ( 1987). tial, i.e. increasing the homeostatic capacity and Mixed stands may have greater species diversity in thereby the resistance of the single tree, might in this other compartments of the ecosystem. This might lead context be more promising (Larsen, 1991). However, to an increased resistance, especially in relation to bet- adequate methods for testing physiological adaptability ter controls of pests. It must be emphasised, however, to uncertain environmental changes are still lacking. that this higher resistance is mainly due to the complexity of energy flow developed through coevolution 3.3. Species mixtures among functional groups of organisms. Species mixtures should support existing food web interactions The decomposer activity is reflected in the humus (Vasechko, 1983). Hence, the artificial creation of form, which is partly dependent upon the vegetation. more complex structures by mixing species, without Even aged conifer forests (spruce, larch and pine) have taking coevolutionary relationships into account, may a tendency to accumulate litter (raw humus), which therefore not lead per se to higher system resistance indicates a temporal decoupled matter cycle. Other (Roberts and Tregonning, 1980). more nutrient demanding species are characterized by Another aspect of system stability is the possibility a higher activity of decomposers, leading to better of increasing the potential depth of the rooting zone by humus forms (mull and moder) and more balanced introducing species characterized by a deeper rooting matter cycles. By choosing site specific appropriate ability (Ulrich, 1987). Such species might work as a species it is possible to a certain extent to improve the pump of nutrients (mainly base cations) from deeper biogeochemical cycle, thereby increasing stability. soil horizons, thus increasing the weathering potential By mixing different species, including trees, shrubs and the storage capacity of the system and thereby both and herbs, it is possible within certain limits to avoid resilience and resistance. Information about species- the temporal decoupling of biomass production and specific capability to expand the rooting zone in relation mineralization, as a mixture of litter types assures a to soil characteristics are still very sparse, however. 92 .I. Bo Larsen/Forest Ecology and Management 73 (1995) 85-96

3.4. Silvicultural systems the introduction of more adapted (tolerant) and adapt- able speciesoffers increasedpotential stability. The selection system maintains rather fixed struc- In managing forest ecosystemsit is important that tural features thereby promoting a functional steady stability also includes the regeneration stage.The sta- state. The systemis characterized by a good control of bility problems in the regeneration phaseare mainly the biogeochemicalcycle determined by the limited connected to the biogeochemicalcycle, due to accel- releaseof space for regeneration and by a rather high erated mineralization and reduced uptake leading to diversity, owing to a horizontal structure and individual lossesof nutrients. These problems depend upon the mixture. The selection system is probably the silvicul- silvicultural system adopted. tural systemmost closely relatedto the ‘shifting-mosaic To obtain a high production of fast growing species, steady state’ defined by Bormann and Likens ( 1979). forest managementoften promotes extreme structural except that old trees are not allowed to fall and decom- homogenization, usually even aged mono-specific sys- pose. tems. Such homogeneoussystems, however, also occur Group regeneration and strip regeneration exhibit in natural forest ecosystemsdue to large scaledestruc- ecological features intermediate between the clearcut- tion by natural causes(fire, wind, insects). These eco- ting and the selection system.Depending upon the size systemsexhibit cyclic stability through time (Orians, of the clearingsthe nutrient cycle remainsmore or less 1975). This type of stability is of minor importance in closed. Regenerationunder cover (canopy) implies a man managedforests, since they are characterized by mild disturbance with only small changes in matter major (uncontrolled) disturbances,and consequently balanceof the systemmaintaining control of the nutri- may not satisfy the requirement of sustainability in ent cycle. Further the micro climate created by the terms of production. forest cover remainsalmost unchanged. Clearcutting corresponds to a major disturbance leadingto heavy changesin biotic regulation and phys- 3.5. Harvesting practices and stand treatmerit ical environment (micro climate) and initiating a long period of recovery with releaseof energy and lost con- The effects of harvesting on long-term productivity trol of the hydrological and biogeochemical cycle by means of managementimpact evaluafion studies (Bormann and Likens, 1979; Vitousek et al., 1979; through modeling have attracted little attention until Nykvist and Rosen, 1985; Hornbeck et al., 1987; recently (Kimmins, 1977, 1986; Dyck et al., 1986). If SwankandCrossley, 1988; Emmettet al., 1991). Alter- the export of nutrients (mainly basecations, phospho- ationsin soil biology, including changesin the structure rus and micro nutrients) via harvesting exceedsdepo- of microorganism community (Jones and Richards, sition and inputs from weathering of minerals, the 1977) and declines in mycorrhiza (Amaranthus and deviation of the nutrient balancefrom the steady-state Perry, 1987) as well as nutrient lossesto streamand will indicate that the ecological carrying capacity has ground water (Bormann and Likens, 1979; Martin et been exceeded.Thus, sustainability is lost and stability al., 1985; Swank, 1988) are further consequencesof decreases.In many acid forest ecosystemsin Europe, larger clearings. The acceptability of clear-cuttings harvesting may remove more base cations than are should therefore be carefully analyzed according to the replaced by mineral weathering. According to Ulrich local conditions, especially in relation to the biogeo- ( 1987, 1991)) the existence of thesesystems is mainly chemical cycle (weathering rates, lossesof nutrients, due to atmosphericinputs from pollution and they are storage capacity, acidification, erosion, etc.). highly unstable. Even aged stands combined with large-scaleclear The common stem-only harvesting system limited felling offer little spatial variation in structural and the amounts of nutrients lost to the system, whereas functional ecological features promoting a low diver- whole-tree harvesting methodsmay lead to considera- sity and periodsof matter and nutrient imbalancesexag- bly more soil disturbance (erosion) and nutrient gerating these above mentioned ecological problems export. Substantial increases in nutriem losses by during the regeneration stage. Various silvicultural whole-tree compared with stem-only harvesting have measurescan shorten this period of acute entropy, and been calculated (Sterba, 1988; Mann et al., 1988; J. Bo Larsen /Forest Ecology and Management 73 (1995) 85-96 93

Fahey et al., 1991a,b). This indicates that harvesting method might be one of the most important single fac- STABILITY - STRESS INTEGRATION tors responsible for management induced nutrient losses. The actual effect upon long-term productivity depends upon the export-import balance and the nutri- STATE FACTORS ents stored. In many tropical forest ecosystems with STABIUTY FACTORS: - S~ectea and p rovanancas adapt4 to very limited nutrient storage and weathering capacity, the qwcifk watogkal alto condlUons. harvesting has a significant effect on system stability. - Spsclss adaptad stand structures and Hence, harvesting practices must be analyzed and traatment. reconsidered in relation to the biogeochemical status PRELMSPOSING FACTORS: - Wrong qsdss and provenance. of the actual ecosystem. The structural and functional features of the ecosystem depends upon the control of regeneration and upon the temporal regulation of space and relations between - Climatic extmmcw (trost, drought, etc). species. In this context, methods, frequency and inten- - Primary biotk stresses such as fungi sity of thinning play an important role by controlling the forest floor. Especially in plantation forestry, characterized by phases of pronounced build-up of raw humus (dense stands of spruce, larch and pine) a stabilizing effect on the biogeochemical cycle might be obtained by a thinning practice, which gives sufficient

light to assure the existence of a continuous ground Fig. 3. Forest decline is illustrated in relation to the stability concept vegetation. The use of herbicides may affect stability by means of a multiple stress integration model. The stresses are negatively (Likens et al., 1970), especially in phases grouped into predisposing, inciting and contributing factors as pro- of afforestation and recultivation after clear-cutting posed by Manion ( 199 1) where the nutrient cycle mostly is disturbed. The introduction of suitable ground vegetation in order to sta- stresses are grouped into predisposing, inciting and bilize the nutrient cycle might under such contributing factors as proposed by Manion ( 1991). circumstances be considered. Manion’s model, which was developed to explain tree Ecosystem stability can be increased by chemical decline, is in this context applied to ecosystems. A and biological soil amelioration (Ulrich, 1989). Such stable system (stand) depicted by high resistance and technical measures include soil cultivation and appli- resilience is, according to Fig. 3, characterized by well cation of fertilizers and lime. By improving the physical adapted populations, i.e. its species and provenances and chemical properties of the soil, mainly resistance are adapted to the specific ecological site conditions, (nutrient and water storage capacity) but also resil- biotic as well as abiotic. Further, stand structures and ience (weathering potential) can be increased. treatment are system and species adapted. In case the above mentioned prerequisites are not fulfilled, the system is predisposed, which in terms of 4. Stability and forest decline stability means that resistance and resilience are reduced owing to lack of adaptedness of its population Research during the last 10 to 15 years on the causes caused by diminished physiological and evolutionary of the widespread forest declines shows a rather het- buffer capacity. A similar reduction in the stability of erogeneous and complex pattern. It is therefore gener- the system may occur by long term changes in the ally accepted that these unspecific declines are caused ecological site conditions, such as climate changes and by a number of partly interacting stresses, some of changes in soil characteristics. Under these aspects the which are of anthropogenic origin. running changes in the soil chemical properties should Fig. 3 illustrates the concept of stability in relation be mentioned, which are forced by acid deposition and to a multiple stress integration model, where the lead to soil acidification, base cation depletion and 94 J. Bo Larsen /Forest Ecology and Management 73 (1995) 85-96

AI3 + -mobilization, influencing root vitality and distri- global ecological cycles. This stresses the importance bution and thereby affecting nutrient and water of expanding the meaning of sustainability by embed- relations. Harvesting practices leading to pronounced ding it within a general framework of ecological sta- export of base cations and stand management tech- bility. Further, the possibilities of an increase in niques causing excessive leaching phenomena will climatic variation within the next tree generation have similar long term effects upon the soil, thus pre- emphasize the importance of ecosystem flexibility and disposing the system. Finally, long term changes in the stability. air chemistry caused by pollution (SO*, ozone, PAN, Until now silviculture has been based upon infor- etc.) may be considered as changes in the site condi- mation gained partly through local management exper- tions, hence acting as predisposing factors. iments. Thus silviculture is mainly based upon A predisposed system, illustrated in Fig. 3 by empirical knowledge limited in time and space and reduced resistance (the small marble) and resilience lacking a basis of ecological understanding. This has (the shallow stability landscape), will show its reduced led to the development of management systems which stability when exposed to sudden external stresses or exceed the ecological carrying capacity and lead to perturbations. In accordance with Manion (1991) losses in sustainability and even to pronounced decline these are named inciting factors and may be of natural phenomena. (climatic extremes, fungi, insects, etc.) or anthropo- A new scientilic approach is needed to develop a genie origin (air pollutants, ozone, UV-radiation, etc.). program of forest management which satisfies sustain- In a predisposed system such perturbations might ability in its broad sense. Silviculture must develop exceed the limits of local referenial dynamics displac- from the empirical manipulation of stand structures into ing the marble outside the domain of attraction, which a science of ecosystem management founded upon in terms of decline means the occurrence of visible knowledge of system processes and interactions. Today symptoms. this knowledge is still very sparse, and much more A predisposed and, because of stresses, degenerating research in systems ecology is needed. system. might be further destabilized through a number of contributing factors such as secondary insects (bark beetles), pathogens, and the degeneration of important References symbiotic relationships (mycorrhiza) The marble is placed outside the domain of attraction and moves into Ackzell. L. and Lindgren, D., 1994. Some genetic aspects on human another dynamic equilibrium characterized by substan- intervention in forest regeneration-Considerations based on examples from an experiment in northern Sweden. Forestry, 67: tial changes in structural and functional features, which 133-148. means that the forestdeclines beyond the limit of recov- Amaranthus, M.P. and Perry, D.A., 1987. The effect of soil inocu- ery. lation on ectomycorrhizal formation and the survival and growth of conifer seedlings on old, non-reforested clearcuts. Can. J. For. Res., 17: 944-950. Bergmann, F., Gregorius, H.-R. and Larsen. J.B., 1990. Levels of 5. Sustainability in a broad sense and aspects of genetic variation in European silver fir (Abies ulbu). Are they future silviculture related to species decline? Genetica. 82: I-10, Bormann, F.H. and Likens. G.E.. 1979. Pattern and Processes in a Sustainability as a management goal was developed Forested Ecosystem. Springer, New York. in Germany (in German: Nachhaltigkeit) during the Brown, A.H.F.. 1992. Functioning of mixed-species stands at Gis- burn, N.W. England. In: M.G.R. CanneH, D.C. Maicomand P.A. 18th century, aimed at sustained production of wood. Robertson (Editors), The Ecology of Mixed-species Stands of Today the products and demands of forests are much Trees. British Ecological Society, Spec. Pubf. Il. BlackweB. more diverse, and include wood, shelter, recreation, Oxford, pp. 125-150. nature and habitat protection, species and gene conser- Chapman, K., Whittaker, J.B. and Heal, O.W.. 1988. Metaboiic and vation, etc. Consequently the criteria for sustainable fauna1 activity in litters of tree mixtures compared with pure stands. Agric. Ecosyst. Environ., 24: 33-40. forest management as defined at the CSCE Conference Dyck, W.J., Messina, M.G. and Hunter, I.R., 1986. Predicting the in Montreal in 1993 include biodiversity, production, nutritional consequences of forest harvesting on s&productivity. vitality and health, soil and water conservation, and In: G.I. Agren (Editor), Predicting Consequences of Intensive .I. Bo Larsen /Forest Ecology and Management 73 (1995) 85-96 95

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