Convention on biological diversity

Elements of forest ecosystem functioning, keystones species, functional groups notion of resilience

In the context of massive biological extinctions driven by human impacts on land use, species invasions, atmospheric and climate change (WCMC, 1999), there is an urgent interest in understanding how loss of biodiversity affects ecological processes and, in turn, the functioning of the ecosystems. Many studies have focussed on the key ecological processes such as primary productivity, nutrient cycling and microbial activities. It appears that in most biomes, primary productivity seems to be weakly related to the number of plant species, although little to no information exists for forests (UNEP, 1995). However, diversity may play a role in the maintenance of productivity in the context of human induced and natural changes (UNEP, 1995; Chapin et al., 1998). Some results show that the form and cause of the relationship between species and diversity and productivity may be highly dynamic, changing over time and space (Cardinale et al., 2000).

Ecosystem functioning can be affected by the presence of keystone species and functional groups. Keystone species are defined as any which have an important effect on the community or ecosystem by virtue of unique traits or attributes (UNEP, 1995). The removal of keystone species can therefore result in dramatic changes in the functional properties of the ecological system. A functional group is a set of species that affect or control an ecosystem-level process in similar way (UNEP, 1995). A functional group can, for instance, be plants that have the same functions in the ecosystem. The concept of keystone species is an attractive one as it suggests potential indicators and perhaps targets for management effort. However it is still a relatively untested idea.

Resilience quantifies the speed to return to equilibrium after a perturbation and is closely connected to ecosystem functions (Mooney et al., 1996). More biologically diverse forests are generally thought to be more resilient and less liable to be affected by major outbreaks of pests and diseases.

2. Boreal Forest Biomes

Overview of the functioning of the ecosystem

Effect of species and genetic diversity on forest ecosystem processes

Functional mechanisms of biodiversity and functional groups

Plant tissue chemistry also represents a key functional attribute in boreal forest ecosystems, which integrates biodiversity with ecosystem properties. Tissue chemistry (particularly concentrations of nitrogen, resin, secondary compounds and lignin) controls decomposition and nutrient availability, palatability to herbivores and flammability and is also correlated with other functional plant traits such as life forms, growth rates and longevity (Pastor et al., 1996).

Most of the boreal birds are migratory. Some groups include several specialised feeders that can exert considerable influence on their insect prey, such as warbler species on spruce budworm outbreaks in North America (Pastor et al., 1996). The diversity of boreal mammalian fauna is intimately related to the history and diversity of trees. It is generally accepted that species richness decreases with increasing latitude, but recent studies have also shown a longitudinal gradient in the species richness of herbivores with the region near the Bering Sea being particularly species poor (Danell et al., 1994) The fact that this region supports the woody species most chemically defended against browsing suggests that such gradients of plant chemical defence in boreal forests may be partly responsible for gradients of mammalian species richness (Pastor et al., 1996).

Outbreaks of phytophagous insects can be significant regulators of forest primary production by releasing understory trees of species or age classes not susceptible to the outbreaks, and by returning nutrients to the forest floor (Mattson and Addy, 1975). Phytophagous insects such as spruce budworm (Choristoneura fumiferana), sawfly (Neodiprion sertifer), loopers (many genera), and bark beetles (e.g., Dendroctonus ponderousae, Ips typographus) are common major sources of tree mortality and morbidity in boreal regions. They typically show large oscillations in population levels, with spreading outbreaks occurring approximately ever few decades (Pastor et al., 1996). Because such outbreaks occur periodically, defoliators and bark beetles play a major role in maintaining variability and diversity at the landscape scale by preventing the attainment of stable equilibrium communities (Pastor et al., 1996).

Ants also represent an important component in forest ecosystems. The genera Formica and Campanotus form colonies that can have strong direct and indirect influences on nutrient cycling (Rosengren et al., 1979).

The extreme fluctuations of animal population are among the most striking features of the boreal forest. Such fluctuations represent a temporally dynamic aspect of biodiversity. The dominant cycle length for a wide variety of mammals and birds in North America appears to be about ten years, while in Fennoscandia its length is usually four years (Keith, 1963; Finerty, 1980; Erlien and Tester, 1984). Cycles of herbivores may result in differential survival of their preferred food species, such as balsam fir, aspen and birch, as well as their predators, such as warblers that prey on budworm, or the Canada lynx that preys on small mammals (Keith, 1963; Stenseth, 1977; Hansson, 1979; Haukioja et al., 1983; Bryant and Chapin, 1986; McInnes et al., 1992).

Disturbances and species richness, effect on biodiversity

By interacting with hydrological processes, the beaver (Castor canadensis) is an important agent of disturbance in boreal ecosystems and is considered to be a keystone species. Beaver ponds and wetlands can cover at least 13% of the land area in boreal landscapes (Johnston and Naiman, 1990). In North America, beavers are one of the major factors creating patches in the forested landscape (Hammerson, 1994).

Impact of human activities and their consequences on the delivering of goods and services

Global warming caused by anthropogenic emissions of greenhouse gases (such as carbon dioxide and methane) into the atmosphere, represents probably the greatest threat to the boreal region worldwide (Pastor et al., 1996). It is generally agreed that the greatest warming will take place in high-latitudes regions, and particularly in the boreal zone. Although the exact extent and speed of the warming is still to be determined, the boreal climate change will dramatically affect the ecosystem in various ways. Some reports suggest a general northward shift of forest ecosystems (Emanuel et al., 1985; Thompson et al., 1998). However, the warming might be more rapid than the migration rates of major tree species, which may result in massive regional extinctions (Davis and Zabinski, 1992). In general, boreal tree species in the south will be replaced by northern hardwoods or prairies and the probability of fire will be increased over much of the biome. Also the probability of outbreaks of insect pests may increase as trees become drought-stressed and more susceptible

Anthropogenic depositions (SO_x, NO_x etc.) have an impact on water quality, tree health and soil properties in some regions. Although the trends and the mechanisms are not entirely clear (Manion and Lachance, 1992), the changes in soil properties (pH, CEC) could have a negative impact on the functioning of forest ecosystems and thus on biodiversity.

Human activities that have an impact on boreal ecosystems at local and regional scales include resource management activities such as logging and forest management, fire management and hunting.

Intensive hunting of fur-bearing mammals since the 16^th and 17^th centuries has led to a permanent reduction in the populations of brown bears (Ursus arctos) and martens (Martes americana, Martes martes, Martes zibellina) in their southern range, with consequences on their prey and on the overall ecosystem (Pastor et al., 1996). The extermination of beavers from some areas of their previous range removed an important factor in landscape dynamics (Naiman et al., 1986). However, most boreal mammals species are not currently endangered except for furbearing species that have been over-exploited, predators considered to be pests, or animals requiring large undisturbed areas such as wolves (Canis spp.), brown bears (Ursus spp.) and woodland caribou (Rangifer tarandus). Indeed, because of the generally great resilience of boreal species (Holling, 1992), most reduced animal populations rebound rapidly once direct exploitation stops (Broschart et al., 1989).

Logging of natural primary forest may have major effects on the biodiversity of boreal ecosystems, unless old forests are permitted to redevelop (UNEP, 1995). Commercial logging currently dominates human-induced changes in boreal forests. As a result of logging, old growth stands have disappeared from large areas of boreal forest. Intensive forestry practices that consist of large-scale removal of natural mature stands and regeneration/replanting with an even-aged monoculture, often with soil preparation and artificial fertilisers, have had many negative ecological effects (Pastor et al., 1996). More sustainable forest management activities are being introduced in Scandinavia and Canada with the aim of mimicking natural fire and wind disturbance patterns. These practices use patch felling, or small clearcuts with the retention of some old and dead trees as a ‘biological legacy’ [reference needed?]. The effects on biodiversity and ecosystem function of these ‘sustainable forest management’ systems are not yet known, but they are being practised in an adaptive manner.

Changes in fire regimes through active suppression of wildfires, partly as a result of forest management, have altered landscape-scale patch structure and age class distributions, as well as composition and stand structure (Heinselman, 1973; Baker, 1989). These changes interact with ecosystem processes to alter future successional pathways and forest productivity further (Pastor et al., 1996). In turn, these changes in habitat distribution have an impact on biodiversity, particularly on bird populations (Pastor et al., 1996).

Containing 23% of the overall terrestrial carbon stock and 49% of the total carbon stock of the three main forest biomes, the boreal biome represents the primary terrestrial carbon pool, far ahead of tropical and temperate biomes (UNEP, 2000). More than 84% of the carbon contained in boreal forest is stored in soils (UNEP, 2000). In addition to being the largest terrestrial carbon pool, boreal and temperate forest biomes represent a net sink and compensate for the net emissions from tropical ecosystems (UNEP, 2000). In this respect, the boreal biome plays a key role in climate regulation and any major disturbance to the ecosystem through human activities that cause a major release of boreal carbon may have significant consequences for global climate. Over the past century, Canadian boreal forests show a sharp rise in growth rates consistent with rising temperatures. Change in age class structure suggests they have gone from being a sink (about 0.2Gt C per year) to being neutral or a slight source of carbon to the atmosphere (Kurz et al., 1995; Walker et al., 1999).