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 November 1994 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. 0378-l 127/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO378-1127(94)03501-6 86 J. Bo Larsen/Forest Ecology und Management 73 (1995) 85-96 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 change permanently 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 means of potential relation to its basic components.In this context an eco- energy in Fig. 1. In this very simplified way of illus- system may 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 outside energy flux the information (the liv- upon) within a steady-state, respectively within its ing organisms) ‘organizes’ the system, creating func- amplitude of recoverability or resilience (the hollow in tions and structuresin a locally reduced entropy state. the ‘stability landscape’). Resistanceis illustrated by Consequently,