Boreal Forest Ecosystems

Rauno Väisänen

Abstract

The boreal zone is characterized by cold winters and extensive coniferous forests with considerable biological heterogeneity at landscape level. The ecosystems of the boreal zone show distinct latitudinal and longitudinal variation, with dynamic changes and resilience resulting from possible adaptations to the past environmental fluctuations including Pleistocene glaciations. The forest connection between Eurasia and North America was lost about six million years ago and subsequent glaciations produced vicariant species in southern refugia. Later expansion and colonization of boreal forests of the two continents had different patterns.

The dynamics of boreal forest ecosystems provides the basis for conservation and sustainable use of forest resources. Forest fires create the basic spatial mosaic of forest patches in different succession stages, though small-scale dynamics caused by storms, insects, pathogenic fungi, etc. may predominate on mesic sites. The outcome of these different processes is a complex and changing forest structure at landscape level.

The present scientific knowledge allows only preliminary generalizations regarding the minimum habitat requirements of forest species. Many mammals and birds require wide home ranges, while some insects and bryophytes may be preserved even in small patches. Among the diverse biotic interactions, functions of fungi and invertebrates in soil and decaying wood deserve more attention than have hitherto been given. There are indications that the importance of biodiversity among soil organisms in the boreal zone may have been underestimated.

Keywords: Biodiversity, boreal zone, landscape dynamics.

Introduction

The boreal zone of the northern hemisphere is often characterized as huge, homogeneous, species-poor and well-known area with coniferous forest, with relatively warm summers and cold winters. However, boreal ecosystems have two contrasting spatial elements: the enormous size and relative homogeneity of these predominantly forested areas; and heterogeneity of local habitats (Danks and Foottit 1989). Furthermore, different taxa perceive the patterns in the forest in different ways so that a stand what is uniform for birds may be heterogeneous for beetles (Haila et al. 1994).

In this context, the term biodiversity means all the biological heterogeneity including genetic, species and ecosystem variation. The information on the biodiversity of boreal forests is extensive but diffuse in biological and silvicultural literature. My approach is predominantly from a European perspective and ecological rather than environmental, silvicultural or socio-economic. Recent results on spatio-temporal heterogeneity in different scales, habitat continuity, disturbance dynamics and biotic interactions offer challenging prospectives for ecological research both at population and ecosystem level.

The Boreal Zone

The boreal vegetation can be divided into latitudinal subzones and regions (Larsen 1980, Hämet-Ahti 1981, Oechel and Lawrence 1985). They differ from each other by topography, edaphic factors, tree species composition, precipitation, and disturbance dynamics. The forests are ecologically connected with wetlands and aquatic habitats, which are common in many parts of the zone. For example, it is typical of northern boreal vegetation that peatland species occur on mineral soils, increasing the total number of species in forests (Tonteri 1994).

In several taxa, species diversity typically decreases towards the poles, with only a few exceptions, such as the species richness of peatland birds, aphids and sawflies (Kouki et al. 1994). In the boreal zone, the cold winter is the chief adversity for life, while the summer is usually warm. Adaptations of species reflect short growing seasons, cold winters, and a relatively low diversity of resources.

Habitat generalists tend to be more widespread than specialist species. For instance, among passerine birds in Finnish boreal forests, the habitat generalist species have the widest ranges (Virkkala 1993). However, southern species, which are habitat generalists in the core area of their distribution, may occur only in the most productive habitats at the northern margin of their ranges (Virkkala 1987). An investigation of the boreal insects in Canada (Danks and Foottit 1989) indicate that many boreal species are generalists, while only one-quarter of the boreal species is restricted to the boreal habitats. The few typically boreal groups include diprionid sawflies and scolytids living on trees.

History of Changes

The cool-temperate predominantly coniferous forest was established probably by the middle Miocene. Pleistocene glaciations and climate changes have influenced the more recent development of the coniferous forests (Danks and Foottit 1989). The species composition of the forests altered continuously with the advances and retreats of glacial ice. Results from biogeography and Quaternary ecology indicate that species equilibrium structure through co-evolved interrelationships within communities does not exist (Hengeveld 1994). In the boreal zone with distinct seasons, changes in temperature will have a different effect on species reproducing in the spring and fall, and perennial species will respond differently from annuals. The main scientific challenge is the relationship between natural and human-induced changes.

Recent studies stress the importance of systematics and historical biogeography for the management priorities for biodiversity (e.g. Vane-Wright et al. 1991). The forest connection between Eurasia and North America was severed about six million years ago. There were treeless land bridges later during the glaciations. Although northern refugia, e.g. Beringia, may have greatly influenced arctic biodiversity, southern refugia have been much more important for boreal species (Danks and Foottit 1989).

Danks and Foottit (1989) estimated that about 8% of the boreal and subarctic Canadian insect taxa are Holarctic (about 2% of sub-boreal and 49% of arctic, respectively). The proportion of Holarctic species of noctuid moths declines steeply from north to south (Mikkola et al. 1991). The effect of glaciations is seen as a lower number of species in Europe and in eastern North America. There are usually pairs of vicariant species in the two regions. This is consistent with the fact that almost all tree species are distinct in the two continents.

The glaciations had different impact on biodiversity in Eurasia and North America (Mönkkönen and Welsh 1994). The European species have experienced fragmentation of forests many times during the past two million years in contrast to the New World environments. Forests were largely eliminated from Europe during the glaciations. In North America, successive waves of glaciations resulted in a back-and-forth north to south movement of forest belts, but the forests remained extensive and in contact with the tropical forests. In the Nearctic, the current regional avifauna is richer and structurally more variable than in the Palaearctic region. The Nearctic forest contains a high proportion of migrant birds irrespective of successional stage, while the avifauna of mature forests in Europe are characterized by residents. In general, the European forest birds may be better adapted to fragmented landscapes than the Nearctic species due to their evolutionary origin and the longer history of human-caused changes in the European forests (Mönkkönen and Welsh 1994).

Compared to Europe, there are more tree genera and species in North America. On the other hand, shrews, red wood ants and earthworms are more abundant in the Palaearctic than in the Nearctic (Haila 1994). About 120 alien insects have invaded North American forests from western Palaearctic over the last 350 years, but only 17 Nearctic insects have colonized the European forests. This disparity can be explained by the higher probability of European insects surviving in North America than vice versa, owing to the greater abundance and diversity of woody plant hosts in North America, and maybe the superior competitive ability of European species (Niemelä and Mattson 1992).

Landscape Dynamics

The structure of boreal forests changesthrough time as a consequence of disturbances (Bonan and Shugart 1989, Hansson 1992, Mladenoff et al. 1993), such as fires, insect outbreaks, storms, and fluvial dynamics. Due to natural disturbances large continuous forests are structurally heterogeneous and diverse. The change in forest structure is non-linearin time, being rapid early after the disturbance but slowing down later. A mature coniferous forest is relatively stable at the stand level, and spatial heterogeneity is maintained by mosaic processes operating on a small scale within stands (Haila et al. 1994).

Species diversity of boreal forests is related to both the site type and the successional factors. The species richness of understorey vegetation in boreal forests is distinctly associated with site quality so that herb-rich sites are more species-rich than mesic or xeric sites. Changes in species richness during forest succession depend on site type. According to Tonteri (1994) the species richness of the understorey vegetation increases during succession until stands are 10-30 years old, and decreases thereafter (on southern boreal herb-rich sites it increases until stands reach an age of 80-100 years). The variation of species richness along the environmental gradients is small in the northern boreal zone. In the mycorrhizal and polyporous fungi, there is a tendency that species richness increases as the age of the stand increases (P. Paalamo and R. Penttilä, pers. comm.).

Forest fires are important to the dynamics of the boreal forest by maintaining a mosaic of forest types (Heinselman 1981, Dyrness et al. 1986, Engelmark 1984). In Sweden, approximately 5-10% of boreal forest area can be classified as fire refugia, which have virtually never burned. These include montane forests just below the tree line, wet forests, ravines, islands and spits in mires and lakes (Bernes 1994). The average cycle between successive forest fires varies in European and American forests between 50 and 200 years, and the extremes may reach 500-800 years (Wein and MacLean 1983, Engelmark 1984, Bonan and Shugart 1989, Clark 1990). In normal years fires may be few and affect only small areas, whereas destructive fires can affect large forest areas in certain years. Fire fields may range from a few to several thousands of hectares (Payette et al. 1989). The fires vary in frequency, intensity and severity in different boreal regions. Fire frequency may also be associated with insect outbreaks and fungal diseases.

The relative importance of large-scale and small-scale processes in different forest types is a key issue in ecosystem dynamics (Mladenoff et al. 1993, Kuuluvainen 1994, Syrjänen et al. 1994). While forest structure and dynamics in dry sites are dominated by repeated fires, disturbance by windstorms is an essential process in mesic sites. Also fungi and insects are major agents causing tree death. Recent studies in Komi, subcontinental Russian taiga, show that over half of the natural landscape can be covered by mesic spruce forest which represents the climatic climax (Syrjänen et al. 1994). Wind may cause disturbances in ecosystems varying from single treefalls to large-scale blowdowns over thousands of hectares (Schaetzl et al. 1989). Uprooting creates a dynamic system of patches in various stages of succession by increasing variation in soil properties and microtopography. It maintains colonist species and plant diversity on the relatively stable and species-poor forest floor (Jonsson and Esseen 1990). Thus, gap disturbance contributes to the structural, functional and species diversity both at local and regional levels.

The results from Komi (Syrjänen et al. 1994) reveal that pathogens and windfalls, together with waterlogging and uprooting of spruces, can provide disturbance gaps which allow the regeneration of aspens and birches. Succession will not necessarily lead to a total spruce dominance in boreal fire refugia. Even a small-scale disturbance may result in a long successional sequence in the bryophyte vegetation of forest floor (Jonsson and Esseen 1990). Disturbed patches are rapidly colonised by both colonists and late successional species, but convergence with undisturbed vegetation is slow. Bryophytes have an extensive and species-rich diaspore bank in forest soils (Jonsson 1993). The dispersal in time through a diaspore bank is a process by which plants can rapidly exploit habitats created by disturbances in forests. The early phase of the bryophyte succession is characterized by a largely random set of species. Later, the strongest competitors start to outcompete the other species (Jonsson and Esseen 1990).

Keddy and MacLellan (1990) suggest that competitive hierarchies structure forests, and that the patterns of species distribution are largely consistent with the centrifugal organization model in the Alaskan taiga and for northern transition forests. The model describes the patterns of species and vegetation along biomass gradients so that there is shared preference for a productive central habitat, but each species is specialized upon a different secondary habitat. Niche gradients radiate outward from the fertile mesic core habitat. At the periphery, extreme site conditions produce a range of vegetation types. Here, specialization upon different sites by species produces coexistence only at a landscape scale. Large scale disturbances such as fires or major floods may occur. In the intermediate region, species are organized along natural gradients by their decreasing levels of shade tolerance.

Minimum Habitat Requirements

There is a need to combine strategies to protect pattern of species richness with studies directed toward understanding fundamental ecological and evolutionary mechanisms. The outcome of the different processes in boreal forests is a complex and changing forest structure at landscape level. The population ecology and minimum habitat requirements of forest species are still rarely known. Many mammals and birds require wide home ranges, while some insects and bryophytes may be preserved even in small patches. Certain groups of organisms, often characteristic species of the taiga, have proved to be good indicators of biodiversity in old-growth forests. However, the underlying reasons for their vulnerability to forestry practices seem to be connected to various processes.

Avian communities of virgin boreal forests reflect general regional change in the abundance of bird populations. However, the size of primeval forest area and the proportion of virgin forests in a larger area affect the densities of species preferring old forests (Virkkala 1987). A patch size of 5-50 hectares may be sufficient to preserve suitable environments for a large proportion of forest birds and carabid beetles in southern Finland, where forests are heterogeneous (Haila 1994, Haila et al. 1994). In contrast, certain old-growth specialists and species requiring a particular type of habitat mosaic are more sensitive to changes in the landscape dynamics.

Although coniferous trees dominate the boreal zone, deciduous trees still support more herbivores. For example, the numbers of Finnish Macrolepidoptera associated with the Scots pine (Pinus sylvestris) and the Norway spruce (Picea abies) are only 18 and 19, while the numbers on deciduous trees are much higher (e.g. Salix 189, Betula 181, Populus 112, Alnus 94) (Seppänen 1970). Many other species are associated with deciduous trees, e.g. many rare lichens and bryophytes (Bernes 1994).

Temporal continuity of the habitat is critical to species requiring specific microhabitats produced by processes operating on a small scale (Haila et al. 1994). Okland (1994) observed that the number of fungus gnats (Diptera, Mycetophilidae) is correlated with variables related to dead wood in Norwegian spruce forests. Continuity may be a main factor for maintaining their diversity, since most species are not exclusively associated with dead wood and large amounts of dead wood were also available on clearcut areas. The mycetophilid fauna is influenced by the fungal diversity, since their larvae develop in fungal habitats. Tens of species of fungi are vulnerable to human-caused disturbance (Ohenoja 1988), and are mainly found in forests with continuity.

Decaying wood is a patchy and unevenly distributed habitat with a limited duration. Standing dead trees and fallen logs in various stages of decomposition may account for 40% of the total volume of timber present in certain virgin forests (Bernes 1994). Primeval forests which have large quantities of decaying wood are inhabited by a high number of saproxylic beetle species, many of which are now threatened (Väisänen et al. 1993). The proportion of beetle species associated with wood is high, e.g. 35% in northern Finland (Hanski and Hammond 1995). The bryophyte flora on decaying wood is species rich in natural forests due to the great frequency of habitats preferred by this flora such as logs of large diameter and deciduous tree logs (Andersson and Hytteborn 1991).

Several species of beetles are associated with burnt forest. They include woodborers, species developing under bark of damaged trees, species associated with fungi, and soil and litter-dwelling groups (Muona and Rutanen 1994). The fire specialists form a heterogeneous group, indicating that a substantial proportion of species have adapted to fire cycles.

The area of suitable habitat and the distances between habitat patches may be important factors for saproxylic species. If dead trees are not colonized at a sufficiently high rate, the entire metapopulation (Gilpin and Hanski 1991),
i.e. the group of interconnected small populations, is doomed (Hanski and Hammond 1995). There is some empirical support to this idea. Siitonen and Martikainen (1994) studied beetles associated with dying and dead aspens
(Populus tremula) in Finland and adjacent Russian Karelia. In Finland, such aspens are rare due to silvicultural practices, but they are common in Karelia. Altogether 12 beetle species red-listed in Finland were found in the Russian samples, whereas only one species of the lowest threat category was found in the Finnish samples. Similarly, there is a sharp contrastin the sub-cortical beetle fauna living in fallen tree trunks between protected areas and
surrounding managed forests in Finland (Väisänen et al. 1993).

Detailed studies on the ecology of target species are needed, and generalizations based on species-area relationship need to be tested. For example, the extinction of different species populations in scattered habitat fragments may or may not be unrelated (May 1994). Fragmentation of a habitat both reduces the total area and increases the ratio of edge-to-area among what is left. This favours organisms that like edges, but is bad for forest-interior species. The minimum viable population for rare or threatened species is hardly ever known. Although the taiga appears to be a relatively resilient ecological system (Haila 1994), more information is needed on the natural population fluctuations and ecological minimum requirements of species representing different evolutionary history, different grain size in both spatial and temporal scales, and various ecological roles in the ecosystem.

Novel Views to the Forest Ecosystems

The role and significance of biodiversity for the ecosystem function is still poorly understood. It may provide buffers against unexpected perturbations and disturbances, and raw material for evolution. Among the biotic interactions, especially biodiversity in boreal forest soil deserves more attention as do poorly-known groups of fungi and invertebrates, but even the role of large herbivores is often ignored. The following scattered examples illustrate the complexity of boreal forest ecosystems that largely waits to be revealed from the trees.

Much of the decomposition necessary for mineralization and recirculation of nutrients is performed by fungi. Conifers form ectomycorrhizae with fungi (Laiho 1990). Mycorrhizae enable trees to take up water and nutrients more efficiently than the roots themselves, and the fungus obtains carbohydrates from the tree. Mycorrhizal fungi are probably numbered in thousands but so far few are known. Gehring and Whitham (1991) report that insect herbivores affect the mutualism between mycorrhizal fungi and susceptible trees. Grazing by fungivores on mycorrhiza may influence the tree roots more than do the organisms that eat the roots (Persson et al. 1980).

The effects of ants extend over most trophic levels. Ants affect forest soils, nutrient cycles, plant dispersal, growth of trees, and distribution and abundance of other invertebrates. The multinest colonies of wood-ant species (Formica) may be comparable in age to forest trees. The territorial wood ants structure the ant community by suppressing other species (Punttila et al. 1994). Through nest-splitting the slowly growing colonies of the wood ants may monopolize large areas starting from fire refuges and spreading slowly through the forests. In such supercolonies, the individual nests seem to manage well even in very shady conditions, presumably due to the supporting nest network.

Microtine rodents in boreal forests disturb the bryophyte carpet through grazing and by making runways that create regeneration niches for bryophytes, lichens and vascular plants. This may result in cyclic changes in bryophyte community composition following vole population cycles in northern areas (Ericson 1977).

Herbivores influence ecosystems over different trophic, organizational, and spatial scales, and complex feedback loops in these interactions may produce surprising effects(Pastor et al. 1988). For example, the nitrogen, resin, and lignin content of plant tissues affect moose digestion, organic matter decomposition, and soil nitrogen availability, because all three processes are microbially mediated. Selective foraging by moose on hardwoods and avoidance of conifers alters community composition and structure (Pastor et al. 1993), which in turn can affect soil processes, nutrient cycles and productivity (McInnes et al. 1992). Grazing by reindeer increase the incidence of scleroderris canker on Scots pine, but is associated with a low incidence of snow blight (Helle and Moilanen 1993).

Beavers affect biogeochemical cycles and the accumulation and distribution of chemical elements over time and space by altering the hydrologic regime of riparian boreal forests (Naiman et al. 1994). The net effect of beaver activities is to translocate chemical elements from the originally inundated upland forest vegetation to downstream communities and to pond sediments.

The Biological Richness of the Boreal Forest

The boreal zone supports simple coniferous forests with some deciduous trees and an understory of a few common vascular plants, mosses and lichens (Danks and Foottit 1989). However, the monotonous appearance may be misleading and the number of species may be underestimated. For example, about 3,650 species of beetles are known from Finland; two thirds of them, about 2,500 species live in boreal forests; the species number may be 200-700 within a forest stand; and the number of the most abundant beetle species in the forest soil is 100,000-1,000,000 per hectare. Hanski & Hammond (1995) compared the numbers of beetle species in boreal (Oulanka, Finland; 20,000 ha) and tropical (Dumoga-Bone, Indonesia; 500 ha) forests, and estimated that the tropical beetle assemblage contained only 7-8 times more species than the boreal one.

Boreal forests appear to be richer in species below the surface of the soil than above it, thus clearly contrasting with tropical forests. The boreal forest is characteristically underlain by podzols. Certain invertebrates like oribatid and prostigmatid mites are better represented in coniferous than in deciduous or tropical forest soils (Petersen 1982). The litter of tree needles decomposes slowly and is not well mixed with lower layers. Fungi play a fundamental role in the boreal forest soils. According to Swedish estimates, different forests usually have 800-1,000 species of fungi (of 3,200 species assessed), and about 500 species in montane forests, compared to the 0-200 species usually found in other habitats. For example, about 100 species of mycorrhizal fungi can often be found per hectare (Bernes 1994).

References

Andersson, L.I. and Hytteborn, H., 1991: Bryophytes and decaying wood - a comparison between managed and natural forest. Holarctic Ecology, vol. 14, pp. 121-130.

Bernes, C. (ed.), 1994: Biological diversity in Sweden. A country study. Monitor (Stockholm), vol. 14, 280 p.

Bonan, G.B. and Shugart, H.H., 1989: Environmental factors and ecological processes in boreal forests. Annual Review of Ecology and Systematics, vol. 20, pp. 1-28.

Clark, J.S., 1990: Fire and climate change during the last 750 yr in northwestern Minnesota. Ecological Monographs, vol. 60, pp. 135-159.

Danks, H.V. and Foottit, R.G., 1989: Insects of the boreal zone of Canada. Canadian Entomologist, vol. 121, pp. 626-690.

Dyrness, C.T., Viereck, L.A., and Van Cleve, K., 1986: Fire in taiga communities of interior Alaska. In: Van Cleve, K., Chapin, F.S. III, Flanagan, P.W., Viereck, L.A. & Dyrness, C.T. (eds.), Forest ecosystems in the Alaskan taiga: a synthesis of structure and function. Ecological Studies: Analysis and Synthesis, Springer-Verlag, New York, vol. 57, pp. 74-86.

Engelmark, O., 1984: Forest fires in the Muddus National Park (northern Sweden) during the past 600 years. Canadian Journal of Botany, vol. 62, pp. 893-898.

Ericson, L., 1977: The influence of voles and lemmings on the vegetation in a coniferous forest during a 4-year period in northern Sweden. Wahlenbergia, vol. 4, pp. 1-114.

Gehring, C.A. and Whitham, T.G., 1991: Herbivore-driven mycorrhizal mutualism in insect-susceptible pinyon pine. Nature, vol. 353, pp. 556-557.

Gilpin, M. and Hanski, I. (eds.), 1991: Metapopulation dynamics: empirical and theoretical investigations. Academic Press, London, 336 p.

Haila, Y., 1994: Preserving ecological diversity in boreal forests: ecological background, research, and management. Annales Zoologici Fennici, vol. 31, pp. 203-217.

Haila, Y., Hanski, I.K., Niemelä, J., Punttila, P., Raivio, S. and Tukia, H., 1994: Forestry and the boreal fauna: matching management with the natural forest dynamics. Annales Zoologici Fennici, vol. 31, pp. 187-202.

Hämet-Ahti, L., 1981: The boreal zone and its biotic subdivision. Fennia, vol. 159, no. 1, pp. 69-75.

Hanski, I. and Hammond, P., 1995: Biodiversity in boreal forests. Trends in Ecology and Evolution, vol. 10, pp. 5-6.

Hansson, L., 1992: Landscape ecology of boreal forest. Trends in Ecology and Evolution, vol. 7, pp. 299-302.

Heinselman, M.L., 1981: Fire and succession in the conifer forests of northern North America. In: West, D.C., Shugart, H.H. and Botkin, D.B. (eds.), Forest succession: concepts and application. Springer-Verlag, New York, pp. 374-405.

Helle, T. and Moilanen, H., 1993: The effects of reindeer grazing on the natural regeneration of Pinus sylvestris. Scandinavian Journal of Forest Research, vol. 8, pp. 395-407.

Hengeveld, R., 1994: Biogeographical ecology. Journal of Biogeography, vol. 21, pp. 341-351.

Jonsson, B.G., 1993: Treefall disturbance, succession, and diversity in boreal forest floor vegetation. Dissertation, Department of Ecological Botany, University of Umeå, Sweden. 23 p.

Jonsson, B.G. and Esseen, P.-A., 1990: Treefall disturbance maintains high bryophyte diversity in a boreal spruce forest. Journal of Ecology, vol. 78, pp. 924-936.

Keddy, P.A. and MacLellan, P., 1990: Centrifugal organization in forests. Oikos, vol. 59, 75-84.

Kouki, J., Niemelä, P. and Viitasaari, M., 1994: Reversed latitudinal gradient in species richness of sawflies (Hymenoptera, Symphyta). Annales Zoologici Fennici, vol. 31, pp. 83-88.

Kuuluvainen, T., 1994: Gap disturbance, ground microtopography, and the regeneration dynamics of boreal coniferous forests in Finland: a review. Annales Zoologici Fennici, vol. 31, pp. 35-51.

Laiho, O., 1990: The significance of mycorrhizae to forest: a review. (In Finnish with English abstract). Silva Fennica, vol. 24, pp. 47-56.

Larsen, J.A., 1980: The boreal ecosystem. Academic Press, New York. 500 p.

May, R., 1994: Ecological science and the management of protected areas. Biodiversity and Conservation, vol. 3, pp. 437-448.

McInnes, P.F., Naiman, R.J., Pastor, J. and Cohen, Y. 1992: Effects of moose browsing on vegetation and litter of the boreal forest, Isle Royale, Michigan, USA. Ecology, vol. 73, pp. 2059-2075.

Mikkola, K., Lafontaine, J.D. and Kononenko, V.S., 1991: Zoogeography of the Holarctic species of the Noctuidae (Lepidoptera): importance of the Beringian refugia. Entomologica Fennica, vol. 2, pp. 157-173.

Mladenoff, D., White, M.A., Pastor, J. and Crow, T.R. 1993: Comparing spatial pattern in unaltered old-growth and disturbed forest landscapes. Ecological Applications, vol. 3, pp. 294-306.

Mönkkönen, M. and Welsh, D.A., 1994: A biogeographical hypothesis on the effects of human caused landscape changes on the forest bird communities of Europe and North America. Annales Zoologici Fennici, vol. 31, pp. 61-70.

Muona, J. and Rutanen, I., 1994: The short-term impact of fire on the beetle fauna in boreal coniferous forest. Annales Zoologici Fennici, vol. 31, pp. 109-121.

Naiman, R.J., Pinay, G., Johnston, C.A., and Pastor, J. 1994: Beaver influences on the long-term biogeochemical characteristics of boreal forerst drainage networks. Ecology, vol. 75, pp. 905-921.

Niemelä, P. and Mattson, W.J., 1992: Invasion of North American forests by European phytophagous insects: why so many? In: American forestry - an evolving tradition. Proceedings of the 1992 Society of American Foresters National Convention; Richmond, Virginia, Bethesda, Maryland, pp. 202-208.

Oechel, W.C. and Lawrence, W.T., 1985: Taiga. In: Chabot, B.F. and Mooney, H.A. (eds.), Physiological ecology of North American plant communities. Chapman & Hall, New York, London, pp. 66-94.

Ohenoja, E., 1988: Effect of forest management procedures on fungal fruit body production in Finland. Acta Botanica Fennica, vol. 136, pp. 81-84.

Okland, B., 1994: Mycetophilidae (Diptera), an insect group vulnerable to forestry practices? A comparison of clearcut, managed and semi-natural spruce forests in southern Norway. Biodiversity and Conservation, vol. 3, pp. 68-85.

Pastor, J., Dewey, B, Naiman, R.J., McInnes, P.F. and Cohen, Y., 1993: Moose browsing and soil fertility in the boreal forests of Isle Royale National Park. Ecology, vol. 74, pp. 467-480.

Pastor, J., Naiman, R.J., Dewey, B. and McInnes, P., 1988: Moose, microbes, and the boreal forest. BioScience, vol. 38, pp. 770-777.

Payette, S., Morneau, C., Sirois, L. and Desponts, M., 1989: Recent fire history of northern Québec biomes. Ecology, vol. 70, pp. 656-673.

Persson, T., Bååth, E., Clarholm, M., Lundkvist, H., Söderström, B.E. and Sohlenius, B., 1980: Trophic structure, biomass dynamics and carbon metabolism of soil organisms in a scots pine forest. In: Persson, T. (ed.), Structure and function of northern coniferous forests - an ecosystem study. Ecological Bulletin (Stockholm), vol. 32, pp. 419-459.

Petersen, H., 1982: The total soil fauna biomass and its composition. Oikos, vol. 39, pp. 330-339.

Punttila, P., Haila, Y., Niemelä, J. and Pajunen, T., 1994: Ant communities in fragments of old-growth taiga and managed surroundings. Annales Zoologici Fennici, vol. 31, pp. 131-144.

Schaetzl, R.J., Johnson, D.L., Burns, S.E. and Small, T.E., 1989: Tree uprooting: review of impacts on forest ecology. Vegetatio, vol. 79, pp. 165-176.

Seppänen, E.J., 1970: The food-plants of the larvae of the Macrolepidoptera of Finland. (In Finnish). Animalia Fennica, vol. 14, 179 p.

Siitonen, J. and Martikainen, P., 1994: Occurrence of rare and threatened insects living on decaying Populus tremula: a comparison between Finnish and Russian Karelia. Scandinavian Journal of Forest Research, vol 9, pp. 185-191.

Syrjänen, K., Kalliola, R., Puolasmaa, A. and Mattsson, J., 1994: Landscape structure and forest dynamics in subcontinental Russian European taiga. Annales Zoologici Fennici, vol. 31, pp. 19-34.

Tonteri, T., 1994: Species richness of boreal understorey forest vegetation in relation to site type and successional factors. Annales Zoologici Fennici, vol. 31, pp. 53-60.

Väisänen, R., Biström, O. and Heliövaara, K., 1993: Sub-cortical Coleoptera in dead pines and spruces: is primeval species composition maintained in managed forests? Biodiversity and Conservation, vol. 2, pp. 95-113.

Vane-Wright, R.I., Humphries, C.J. and Williams, P.H., 1991: What to protect? Systematics and the agony of choice. Biological Conservation, vol. 55, pp. 235-254.

Virkkala, R., 1987: Geographical variation in bird communities of old, intact forests in northern Finland. Ornis Fennica, vol. 64, pp. 107-118.

Virkkala, R., 1993: Ranges of northern forest passerines: a fractal analysis. Oikos, vol.67, pp. 218-226.

Wein, R.W. and MacLean, D.A. (eds.), 1983: The role of fire in northern circumpolar ecosystems. Based on conference held 22-24 October 1979, by Fire Science Centre, University of New Brunswick, Fredericton. Publications on behalf of SCOPE/ICSU. Wiley, New York. 322 p.