Range of Variability of unmanaged subalpine forests
Dominik Kulakowski, Peter Bebi
Eidg. Institut für Schnee- und Lawinenforschung SLF, Flüelastrasse 11, CH-7260 Davos kulakowski@slf.ch, bebi@slf.ch
ecosystems to form hypotheses about the natural structure and function of these forests. In the present paper, we discuss the concept of NRV, briefly review components of NRV of the sub- alpine forests of the Colorado Rockies and discuss the potential relevance of NRV of those forests to forming hy- potheses about natural forest processes in the Alps.
Because important differences in ecosystems can exist even over small distances, lessons from other forests can only be considered as untested hypotheses. However, such hypothesis and related general principles can help to form a basis for the understanding of the natural functioning of an eco- system.
Much research in the forests of the Rocky Mountains has aimed to describe and understand the NRV of those eco- systems. We now consider some of the findings from the Colorado Rockies that may be relevant to a desire to
manage the forests of the Alps within a natural state. The purpose of this paper is not to argue whether or not it is desirable to attempt to manage forests within their natural range of conditions, but rather to provide a first attempt to hypothesize how such conditions may look.
2 Natural Range of Variability
Modern management increasingly aims to maintain patterns and processes within their Range of Historical, or Natural, Variability. Because this con- cept has been mostly applied in western North America, where natural ecosystem development has only recently (e.g. 19th century in the Colorado Rocky Mountains) been affected by modern society, it is often termed the Historical Range of Variability. However, for ecosystems that have had a long history of intense human modification, the term Natural Range of Variability may be more appropriate. The fundamental premises of natural variability are that natural conditions and processes provide the best guideline for sustainable eco- system management and that natural disturbances are a vital attribute of most ecological systems (LANDRES et al. 1999). This dynamic picture portrays how ecosystems have perpetuated and sustained themselves and thus it is the best, if not the only, true model of sustainability.
Dendroecology and other lines of inquiry can describe key components of NRV if evidence of natural ecological conditions still exists and can be collec- ted. If evidence of NRV no longer exists in a particular ecosystem then, by necessity, one can consider general 1 Introduction
The best, if not the only, true model of ecological sustainability is based on restoring or maintaining the Natural, or Historical, Range of Variation (NRV or HRV) of ecosystem patterns and processes (LANDRES et al. 1999). For reasons of economic viability and soci- al values, there is increasing interest in managing the subalpine forests of the Alps in a more natural state (KRÄUCHI
et al. 2000; SCHÖNENBERGER 2001).
Furthermore, there is an associated need to understand resistance and ela- sticity of forests of the Alps in relation to natural disturbances (BRANG2001).
Managing ecosystems within NRV requires an understanding of how eco- systems function in the absence of major anthropogenic influences.
Because almost no information is available on the NRV for the subalpine forests of the Alps, it is necessary to draw on knowledge from comparable
Managing ecosystems within their natural range of variation is a promising approach for promoting ecosystem sustainability. In some forests, such as those of the European Alps, human influences have been so longstanding and intense that almost no evidence exists of natural ecosystem structure and function. In such cases, out of necessity, one may draw on research from analogous ecosystems to formulate hypotheses about the natural range of variation of forest patterns and processes. The subalpine forests of the Swiss Alps and the Colorado Rocky Mountain share important similarities as well as important differences. Natural forest development and succession in both ecosystems occurs over the course of centuries, and thus includes a broad range of patterns, processes, and stages devel- opment. Natural disturbances are important in both ecosystems and contribute to heterogeneity at landscape scales. Both systems are adapted to recovery following natural disturbances. Increasing human settlement within these ecosystems results in hazards to human endeavors and infrastructures. In the subalpine forests of the Rockies, protecting human settlements from natural stand-replacing fires requires a departure from natural conditions around the settlements in order to reduce fire hazard. In the subalpine forests of the Alps, protecting human settlements and infrastructures of high value from avalanches and other disturbances also requires forests to be outside of a natural state in some areas. In other areas, however, forests that are shaped primarily by natural processes can also serve a protective function.
principles from similar ecosystems elsewhere as hypotheses for the natural range of variability.
Decades of research on ecosystem development have resulted in older approaches to resource management that formerly stressed long-term stab- ility rather than dynamic change to be strongly criticized, if not discredited.
Modern ecosystem-based management recognizes that ecosystems are not static and that change is an inherent part of systems (SWANSON et al. 1993;
MORGAN et al. 1994). Management practices based on attempts to main- tain stable ecosystems have often resulted in unpredicted consequences for both ecological systems and human welfare (HOLLING and MEFFE 1996).
Ironically, attempts to make eco- systems more predictable and reliable by reducing the range of natural varia- tion often reduce the resilience of eco- systems and lead to unforeseen and usually undesirable ecological surpri- ses. HOLLING and MEFFE (1996) con- clude that management should aim to assure natural ranges of variation in order to maintain ecosystem resilience.
An understanding of resilience is at the heart of maintaining a range of natural variability and ecosystem resili- ence. HOLLING and MEFFE (1996) recognize two definitions for resilience in the ecological literature.Equilibrium resilienceis the ability for an ecosystem to return to some steady-state equilib- rium condition after a disturbance. This concept of resilience is based on con- stancy and predictability, and thus at the core of a “pathology” of natural resource management (HOLLING and MEFFE 1996). In contrast to equilib- rium resilience, ecosystem resilience refers to the amount of instability an ecosystem can absorb before it changes to a new regime of relatively stable behavior (HOLLINGand MEFFE 1996).
This definition differs from the static concept of resilience in that it empha- sizes system dynamics that are in- herently unpredictable and may only become apparent for larger systems over longer time periods. As long as a range of variability in system behavior is retained, HOLLINGand MEFFE(1996) argue that ecosystem resiliency is maximized and ecological surprises or crises can be minimized.
Over the past several decades, scien-
tists and resource managers have re- cognized that ecosystems are dynamic and that change and instability are inherent parts of ecosystem function (PICKETT and WHITE 1985; BOTKIN 1990). Formerly, there was a widespread expectation of a “balance of nature”
and driving paradigms in ecology, such as the climax concept, stressed stability.
In contemporary ecological thought, ecosystem change is regarded as the norm, and periods change, both relatively rapid and gradual, should be expected and accommodated in management practices.
Understanding the causes and effects of natural variability in ecosystem patterns and processes provides oper- ational flexibility for management actions and protocols (LANDRES et al.
1999). Incorporating natural ecosystem patterns into management goals provides an initial strategy for dealing with sustainability of diverse and often unknown species requirements. Man- aging within boundaries of natural variability is probably also easier and less expensive to achieve than trying to manage outside of constraints imposed by driving factors of the system (ALLENand HOEKSTRA1992; LANDRES et al.1999). Historical patterns of eco- system conditions provide what may be the only viable model for how eco- systems have evolved and perpetuated themselves in the absence of significant human impacts.
3 The NRV of subalpine forests of Colorado
The high elevation (2600–3300 m) forests of the Colorado Rocky Mountains are primarily co-dominated by Engelmann spruce (Picea engel- mannii)and subalpine fir (Abies lasio- carpa). At lower elevations (< c. 3000 m), these two species sometimes successionally replace more shade- intolerant species such as lodgepole pine (Pinus contorta)or quaking aspen (Populus tremuloides). At higher elev- ations stand dynamics are almost ex- clusively dominated by Picea and Abies.
Stand-replacing fires are infrequent in Colorado subalpine forests and have return intervals of c. 300 to >600 years
(VEBLEN 2000). Despite their infre- quency, these fires exert important and lasting influences on forest structure and function. Dendroecological and paleoecological records of charcoal deposition suggest the historic fire regime of the subalpine zone in Colorado was dominated by large infrequent fires, suggesting that fires have been an important component of the ecology of these forests for millen- nia. Spruce and fir can regenerate fol- lowing stand-replacing disturbances such as fires and can co-dominate the site from the time of stand initiation (REBERTUS et al. 1992). Following initiation, stand development of spruce- fir stands varies due to variations in site conditions (REBERTUS et al. 1992).
However, following a stand-replacing disturbance, development has been generally described as occurring by several sequential stages (OLIVER 1981). Stand initiation occurs following stand-replacing disturbances such as fires or logging. This stage is character- ized by dense tree establishment until the available resources are utilized.
Once the stand reaches a high density, stem exclusion occurs. This stage is often characterized by a relatively dense canopy and little or no tree regeneration (Fig. 1). Competition among the initial cohort then becomes high and density-dependent mortality leads to an opening up of the canopy via self-thinning and thus to understory reinitiation. Eventually the stand reaches an old-growth stage that is characterized by canopy dominance of individuals that were not part of the initial post-disturbance cohort. In Colorado spruce-fir forests, this pro- cess can take >500 years.
Due to the long period of stand development before the initial cohort dies, there is a high likelihood that the stand will be affected by a natural dis- turbance such as a windstorm or spruce beetle outbreak. Exceptionally strong wind storms occasionally cause moder- ate (<100 ha ) to extensive (>10 000 ha) blowdowns in subalpine forests of the southern Rocky Mountains and are important determinants of stand devel- opment patterns (VEBLENet al. 1989;
FLAHERTY 2000; KULAKOWSKI and VEBLEN 2003). Small blowdowns of approximately a fifth to several hec- tares are also common in the subalpine
forests of Colorado (VEBLEN et al.
1991). While susceptibility to damage from severe wind storms largely depends on storm meteorology and topography, it also varies with forest type and stage of stand development.
As in studies in a number of other eco- systems (EVERHAM and BROKAW 1996), younger stands that are made up of younger trees and that are structur- ally more homogenous have been shown to be less susceptible to small and large blowdowns in Colorado sub- alpine forests (VEBLEN et al. 1991;
BAKER et al. 2002; KULAKOWSKI and VEBLEN2002). Forest development fol- lowing blowdown is highly contingent on the time since stand initiation, which largely determines the stage of development. Critical to the short-term response to the blowdown is the presence or absence of advanced re- generation. Later, once blown down logs become sufficiently decomposed, they serve as a substrate for the re- generation of both spruce and fir (KULAKOWSKIand VEBLEN2003). The increased regeneration on fallen logs and tip-up mounds can continue for over 65 years following blowdown.
Blowdowns or accumulation of log- ging debris sometimes trigger out- breaks of spruce beetle (Dendroctonus rufipennis) (SCHMID and FRYE 1977).
However, blowdowns do not always trigger outbreaks, even in highly sus- ceptible forest types (KULAKOWSKIand VEBLEN2003). This suggests that there may be regional variations in forest susceptibility to outbreaks associated with habitat conditions or that out- breaks are only triggered by blow–
downs if the following several years of weather are conducive to beetle out- breaks (HEBERTSON2004). The spruce beetle in the southern Rocky Moun- tains mainly infests canopy-size Engel- mann spruce (SCHMID and MATA 1996). Several major outbreaks have affected western Colorado in the past several centuries (USDA Forest Service 1994; EISENHART and VEBLEN 2000). Paleoecological data also suggest that outbreaks of spruce beetles may have been important in western Colorado since as long ago as the mid- Holocene (FEILER and ANDERSON 1993). Thus, there is abundant evidence that outbreaks of spruce beetles are a natural component of the ecology of these subalpine forests and are there- fore within the range of historical
variability. As with blowdowns, sus- ceptibility to beetle outbreak is lower for younger (< c. 70 years old) stands (VEBLEN et al. 1994; BEBI et al. 2003;
KULAKOWSKIet al. 2003). Forest devel- opment following outbreak is primarily by the release of the formerly sup- pressed advanced regeneration (Fig. 2).
Although it is widely believed that the mortality caused by an outbreak would increase subsequent fire risk, empirical data does not support this notion (BEBI et al. 2003; KULAKOWSKI et al. 2003).
Following a severe 1940s beetle out- break in western Colorado, there was no increase in fire occurrence in beetle affected stands during the 50 years fol- lowing the outbreak, nor were there extensive and severe fires (BEBIet al.
2003) until the extreme drought of Fig. 1. In the foreground is a young (c. 120
year-old) stand dominated by Engelmann spruce in Colorado. Such stands typically have homogenous, dense canopies and little regeneration.
Fig. 2. A stand c. 50 years after a beetle outbreak in Colorado. Such stands typically are multi-tiered and structurally heterogeneous.
2002, when there were extensive fires in much of Colorado. This suggest that the fire regime of subalpine spruce-fir forests is more influenced by extremely dry weather rather than differences in fuel conditions, such as those following beetle outbreak.
While disturbances to any given stand are infrequent in the subalpine spruce-fir forests of the Colorado Rockies, the occurrence of fires, blowdowns, and spruce beetle out- breaks are clearly within the natural range of variability of the past several centuries. The occurrence of disturb- ances affects forest structure and also affects processes, such as susceptibility to subsequent disturbances. Due to the benefit of a long reconstructed history of natural processes in the forests of the Rockies, researchers have learned that those forests have been largely shaped by large, infrequent disturb- ances (TURNERand DALE 1998). It is also clear that disturbances do not threaten the existence of these forests.
Forests that are dominated by long lived tree species and infrequent dis- turbances are likely to have a broad range of conditions that can be con- sidered within a range of natural variation (KULAKOWSKI et al. 2004).
Because of the dynamic nature of eco- systems, change is expected and should be prepared for in management scen- arios. Disturbances are a normal and natural part of many forest ecosystems and thus, forest health includes such processes.
4 The NRV of subalpine forests of the Alps
The subalpine forests of the Swiss Alps are primarily dominated by Norway spruce (Picea abies). Like P. engel- mannii, P. abies can live >300–500 years. In more continental climate conditions, European larch (Larix deci- dua)and different pine species (Pinus cembra, Pinus montana, Pinus sil- vestrys)may be co- or more dominant than Picea abies.
Major grazing and logging activities occurred in much of Switzerland until the end of 19th century, resulting in a very open forest structure. These prac- tices were largely abandoned after the
introduction of the first Swiss forest law in 1876, and subsequent refores- tation in subalpine forests of the Alps has resulted in a fairly homogenous forested landscape in terms of age and structure. The net effect has been a widespread homogenization of stages of development and a scarcity of old- growth forests in relation to what would probably exist naturally (OTTet al. 1997).
Because forests of the Swiss Alps have been greatly influenced by human activities for centuries, evidence of natural ecosystem development has been largely lost. Thus, assessing and understanding the range of natural processes and conditions in these forests is difficult. Several general questions are: 1) what is the natural range of critical patterns and processes of forests in the Alps? 2) are the present conditions outside of this range of natural variability? 3) if so, then what measures are required to return conditions to a more natural state? and 4) how long would a return to NRV conditions take? Answers to these questions cannot be answered based on the current body of research in the Alps. However, partial answers may be formulated based on the existing litera- ture in the Alps and other subalpine forests.
Much research in the Alps has been devoted to understanding forest dy- namics of the 20thcentury. While forest dynamics of the past c. 100 years are certainly important, Norway spruce has a longevity of >300–500 years and natural dynamics within this forest type are likely to fluctuate over many centuries. In this light, 100 to 150 year old forests are actually relatively young and a perspective of c. 100 years is re- latively short compared to the natural range of variability. Furthermore, it is probable that the full spectrum of system dynamics cannot be understood from studying a time span of only c.
100 years.
Although little information is avail- able on the long-term natural disturb- ance regime in the Alps, it is clear that natural disturbances, especially ava- lanches, beetle outbreaks, and wind blowdowns, are important in the devel- opment of these forests. It is also likely that, because these disturbances are fundamentally natural, in some ca-
pacity, the patterns and processes associated with these natural disturb- ances are within a range of natural variation for the forests of the Alps, as they are for the forests of the Rockies.
A number of natural disturbances affect forest development in the Alps.
Perhaps the most socially relevant natural disturbance in the subalpine forests of the Alps is caused by ava- lanches (see MARGRETH 2004 in this volume; LATERNSER und PFISTER 1997). In addition to avalanches, wind blowdowns also affect the forest of the Alps (see BRANG et al. 2004 in this volume). While the two largest blowdowns since 1879 occurred since 1990, historical records suggest that large blowdowns have affected forests in the Alps for centuries (PFISTER 1988). Norway spruce is also susceptible to attack by Ips typographus beetles and numerous outbreaks of this insect have been recorded in the Alps (ALBISETTI 2003). As with avalanches and blowdowns, unpublished docu- ments provide evidence that Ips has affected forests of the Alps for centuries (PFISTER 1988). While fires have not had a widespread influence during the 20th century, paleoecological work shows that fires have been a long- standing and important influence on forests in the Alps (CARCAILLET1998;
TINNERet al. 1999; GOBET et al. 2003;
STÄHLI 2004). For example, mean fire intervals in southeastern Switzerland prior to agricultural use of the area (beginning c. 3600 BC) were c. 300–800 years (STÄHLI2004). Since agricultural development, fire frequency has varied in relation to human land use.
The information that is available about natural processes in the Alps suggests that disturbances are likely to be an important component of NRV, as they are in the Rockies. The synchronous stand initiation in the Alps has lead to widespread homo- genization of stages of development and a scarcity of old-growth forests in relation to what would probably exist naturally. One current goal of man- agement in the Alps is to increase structural diversity (SCHÖNENBERGER
2001). Gradual stand development increases such characteristics, but natural disturbances such as insect out- breaks and blowdowns can also pro- mote and accelerate the creation of
structural heterogeneity, a high density of snags and logs, and other such characteristics (Fig. 2). Thus, disturb- ances may not only be within a range of natural variation, but may also serve important ecological functions as well as help achieve desired management goals. Salvage logging was common in Switzerland following blowdown and outbreak, but recently logs and snags are increasingly left following blow- down and outbreaks in the Alps (SCHÖNENBERGER2001). Such measures are likely to help restore structures that would be more characteristic of a natural range of variation.
5 Similarities between subalpine forests of the Rockies and the Alps
The subalpine forests of the Rocky Mountains and the Alps share import- ant similarities as well as important differences. Both ecosystems are domi- nated by relatively few coniferous tree species, with predominant dominance by spruce. The subalpine forests of the Southern Rocky Mountains are domi- nated by Engelmann spruce and sub- alpine fir. While the subalpine forests of the Swiss Alps are dominated by Norway spruce, it has been suggested that white fir (Abies alba) would be more prevalent in the montane zone of the Alps if not for the intense mana- gement of these forests (OTT et al.
1997). The fact that Abies lasiocarpa, which is present in the subalpine forests of the Rockies, has a higher shade tolerance than spruce is likely to make a difference in the development of forests in the Rockies versus those in the Alps. The structure of forests in the Rockies and the Alps is similar with generally high canopy closure and high stem density. The same types of natural disturbances are also important in both ecosystems. Although the re- lative importance of the disturbance types may vary between these eco- systems, fires, avalanches, insect out- breaks, and wind blowdowns all influence forest development in both regions. Another important similarity is that in both the Rockies and the Alps, people have settled in areas that are characterized by a risk of severe
natural disturbances. Thus, the hazards posed to settlements by fire in the Rockies and by avalanches in the Alps are somewhat analogous in terms of their human-nature interactions.
A number of differences also exists between the forests of the Rockies and the Alps (e.g. BUGMANN 2001). There are important differences in climate between the Rockies and the Alps, which result in substantially lower tree- line in the Alps as well as other dif- ferences. Another of the most important difference is the level of anthropogenic management and other alteration of the ecosystems. Modern human settle- ment of the Rockies has occurred only relatively recently, primarily in the mid to late 19th century. Although Native Americans have occupied the Rockies for c. 10 000 years, their ecological impact has been minimal and localized.
Furthermore, since settlement by modern Americans, subalpine forests have not been drastically altered in most areas, and, in fact, the structure and composition of these forests today is not fundamentally different than it has been for the hundreds of years prior to settlement (VEBLEN and DONNEGAN 2004; KULAKOWSKI and VEBLEN in review a, in review b). In contrast, the Alps have experienced considerably intense human influence for hundreds of years (SCHULER1996).
6 Convergence and diver- gence of NRV and hazard mitigation
To manage forests within a range of natural variation, it is important to understand how natural processes determine the composition, structure, and function of forested ecosystems, and how these processes and patterns may have been altered by humans.
However, we stress that ecological understanding is only one of many cri- teria in making resource management decisions which also must address eco- nomic and social issues (cf. BEBIet al.
2004 in this volume).
In some cases, there may be a con- vergence of the goal to manage forests in a natural condition and the goal to minimize hazards to human settle- ments. But, in other cases, these two
goals may be incompatible. In the case of the Rockies, much of the forested landscape has been significantly influ- enced by fires over the past several hundred years. The fire regime in the Rocky Mountains varies substantially with elevation and among forest types.
If settlements have been built in eco- systems that have developed with a regime of large and severe fires, then the goals of natural ecosystem function and hazard mitigation may be incom- patible. In these cases, managers may choose to reduce fire hazard immedi- ately surrounding houses, with a clear understanding that this management is resulting in forest structures that are unnatural, but may be necessary to protect human structures. However, if settlements are located in forests that were not primarily influenced by large and severe fires then there may be a convergence of the goals of natural ecosystem function and hazard miti- gation. In such cases management ac- tivities can maintain the natural forest structures and by doing so the hazard to settlements will also be reduced.
Similar questions of the convergence of maintaining natural forest structure and function and reducing hazards exist in the Alps, where avalanches are a significant component of the natural range of variability. If settlements or infrastructures are located below ava- lanche release areas without a suf- ficient forest cover to prevent the release of avalanches (MARGRETH 2004 in this volume) then it is unlikely that there will be a convergence between goals of natural ecosystem function and hazard mitigation. In such cases, natural structure and dynamics will need to be altered in order to reduce hazard (BRANG 2001). How- ever, if forests naturally exist in a certain area, then natural dynamics are likely to be able to continue to main- tain forest existence in that area, as long as the conditions for natural dynamics to occur have not been fundamentally altered. Of course, sus- ceptibility to avalanche and other disturbances is likely to be affected by forest structure, which may have been influenced by previous disturbance history.
Relatively young (10–150 years old) homogenous stands in the Alps, which are in an early stage of development,
do provide protection against natural hazards such as avalanches and rock fall (c.f. FOETZKIet al.2004; MARGRETH 2004 in this volume). Furthermore, forests with such structure do not appear to be more susceptible to wind throw than stands with more hetero- geneous structure (BRANGet al.2004 in this volume). While it is not known how long the stem-exclusion stage lasts in the Alps, as in the Rockies, it is likely to be a transient stage in stand devel- opment. With increasing self-thinning and stem regeneration, structural heterogeneity is likely to increase.
As in the Rockies, forests suscepti- bility to disturbances in the Alps is likely to be affected by past distur- bance history. Of particular interest in the Alps is the effect of blowdown and beetle outbreak on subsequent sus- ceptibility to avalanche. While further research is needed, initial findings suggests that susceptibility to ava- lanche does not increase following beetle outbreak in the Alps. In stands that were not managed following the disturbance, dead trees and root plates following bark beetle outbreak appear to provide sufficient protection from avalanches and rock fall (FREY and THEE 2002; ALBISETTI 2003). The natural regeneration that then follows outbreak, at least in some areas, also appears sufficient for the stand to con- tinue to serve a protective function once the snags and logs decays (ALBISETTI 2003). However, it is not known under which site conditions and scenarios the speed of forest regen- eration is sufficient to provide a pro- tection function before dead trees decompose.
7 Conclusions
The possible goals of ecosystem man- agement are manifold. Thus, the first question regarding an attempt to man- age an ecosystem within NRV is whether or not it is desirable to do so in a particular area. In some areas, the risk of natural hazards may be of such a paramount social or economic im- portance that management should not aim to maintain forests within be NRV (cf. BEBIet al. 2004 in this volume). But
in areas where a more natural state is desired we suggest that initial guiding principles can be deduced from research in similar forest types in other regions, such as in the Rocky Moun- tains or the Carpathians (see also KORPEL1995).
The present forest regeneration in individual stands in the Alps does not appear to be at exceptionally low in comparison to forests in the Rockies, which also experience stages of devel- opment with low regeneration. This stem-exclusion stage is a normal and temporary stage in forest development that is usually followed by natural self- thinning and stand re-initiation. The full spectrum of spruce forests in the Alps is likely to be evident only over centuries and the present stage of development is probably relatively early in this cycle. However, mana- gement and intensive use in the Alps are likely to have increased homo- geneity in many areas. This may have resulted in a scarcity of old stands and stands in latter stages of development in comparison to a natural range of conditions. Furthermore, due to the extensive and homogenous stands in the stem-exclusion phase, regeneration at a broad scale may be lower than under NRV. Both heterogeneity and regeneration are likely to increase as natural disturbances affect the current structure and as stands grow older.
Natural disturbances such as blow- downs, insect outbreaks, snow break- age, and avalanches are a natural part of forest development in the Alps.
Stands vary in their susceptibility to disturbances based on their location and stand characteristics. If forests are permitted to develop with such disturbances, the result will be a more heterogeneous landscape of stands that were and stands that were not affected by a given disturbance. This con- sequent mosaic of structure and com- position will likely affect susceptibility to subsequent disturbances, increasing the likelihood that disturbances will not affect entire vast areas of forests.
Where large areas are affected by a single disturbance, the rate of forest development is also likely to vary with pre-disturbance structure. For example, stands in latter stages of development are likely to have advanced regen- eration that will allow fairly rapid
recovery following blowdown by a reorganization response.
Forest development at high ele- vations is slow. But, if management intensity is reduced then forest patterns and processes will eventually return to a more natural state. These natural conditions are likely to include more heterogeneity at both stand and landscape scales, higher amounts of both standing and fallen dead trees in various stages of decay, and a higher density of advanced regeneration.
While it may not be socially desirable to allow natural processes to occur in all stands, initial research in the Alps suggests that the protective function of forests in the Alps is maintained fol- lowing natural disturbances, such as beetle outbreaks, when development following the disturbance occurs via natural reorganization and regener- ation.
Clearly, much research is needed to understand natural environmental variability and the natural patterns and processes in the subalpine forests of the Alps and on the protective function of unmanaged forests (KRÄUCHIet al.
2000; SCHÖNENBERGER 2001). How- ever, understanding natural forest dynamics in the Alps could serve as an important foundation for modern ecosystem management and hazard mitigation. Hypotheses for the sub- alpine forests of the Alps may include that 1) regeneration varies with stand age and structure so that stands go through temporal phases of relatively low and relatively high density of regeneration; 2) forests are able to persist in spite of natural disturbances and pre-disturbance structure affects susceptibility to and mode of reorgan- ization following disturbance and, 3) in some areas, stands will continue to serve a protective function even when natural forest development, including the effects on disturbances, is permitted.
Acknowledgements
We thank Peter Brang for helpful com- ments on an earlier version of this manuscript.
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