Introduction

The boreal regions are a circumpolar biome covering approximately 1.3 x 1Ü9 ha in upland forest and 0.26 x 109 ha in peatland in North America and Eurasia, or 20% of the world's forested regions (Olson et al. 1983; Shugart et al. Î992; Apps et al. {993}. They are second only to moist tropical forests in global extent (Olson et al. 1983). To the north, the boreal regions are bounded by arctic tundra. To the south, they are bounded by temperate dcciduous forests in areas where precipitation exceeds évapotranspiration, and by prairies and steppe where there is an annual water deficit (Shugart et al. 1992). The boreal regions are characterized by a coniferous forest cover generally succeeding shade-intolerant deciduous species after a disturbance. Tree species richness is low, generally less than six in any one stand, with large, monotypic stands being quite common and restricted mainly to the genera Pi nus. Picea, Larix, Abies, Betula and Populus. The number of understory species is much greater, and diversity in this stratum can be quite high at the southern boundary with deciduous forests (Maycock and Curtis 1960; Pastor and Mladenoff 1992). Ranges of tree genera are circumpolar, but this circumpolar distribution extends down to the species level for many herbaceous residents of this biome.

Boreal regions may seem unlikely candidates for investigations into the relationship between diversity and ecosystem processes because of the generally low species richness. Despite several recent provocative experiments (Naeem et al. ¡994; Tilman and Downing 1994), species richness per se has not been found to have any theoretical relationship to ecosystem processes such as productivity and nutrient cycling except insofar as species function differently with regard to those ecosystem properties, or until richness is reduced to such an extent that functional roles are lost from the community (Pastor 1995). Functional contrast between taxa or functional uniqueness of

Functional Roles of Biodiversity: A Global Perspective

Edited by H.A. Mooney, J.H.Cushman, E. Medina, O.E. Sala and E.-D. Schulze ffcdf) Cw © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd unlt particular taxa are the important aspects of diversity with respect to ecosystem functioning. By taxa, we mean any biological entity, from genotypes, to populations, species and homogeneous landscape units. Nonetheless, in communities with inherently low richness or experiencing decreases in richness upon human disturbance, there is at least the potential for a loss or impairment of ecosystem function; whether or not this potential is realized depends on the natural history of the taxa involved.

In the boreal forest, many species are functionally different becausc they respond to and affect resource availability, food supply for herbivores and disturbance regimes in very different ways (Bryant and Chapin 1986; Payette 1992; Pastor and Mladenoff 1992; Hobbie et at 1993). Not only is there strong functional contrast between species, there are strong feedbacks between many species and ecosystem properties. That is, the presence of a particular species may alter ecosystem properties to either reinforce or weaken its role in the community. These strong feedbacks between species, resources and disturbance regimes may in turn cause cyclic fluctuations in populations of animals (Hansson 1979; Haukioja et al. 1983); such cycles are a temporal aspect of biodiversity and may allow coexistence and higher diversity than might otherwise obtain in this severe environment of short growing seasons and low productivity.

The diversity of plant tissue chemistry among taxonomic units, from subspccies to genera, appears to be one mechanism integrating species diversity with ecosystem properties. Tissue chemistry - particularly concentrations of nitrogen, resin, secondary compounds and lignin - controls decomposition and nutrient availability, palatability and flammability (Bryant and Chapin 1986; Pastor and Mladenoff' 1992; Pastor and Naiman 1992). Tissue chemistry in turn is correlated with other functional plant traits such as life form, growth rates, longevity, etc. (Bryant and Chapin 1986; Chapin 1986).

Functional diversity (contrast between taxa) is therefore high, but each functional group is represented by only a few species. The loss of any one species could therefore affect ecosystem properties. This straightforward coupling of species traits with ecosystem properties is one of the advantages of boreal forests for testing hypotheses regarding species diversity and ecosystems (Hobbie et al. 1993).

Another advantage of the boreal region for ecosystem study is that the current assemblage of species is relatively new, generally 6000-8000 years old, and very well documented in the pollen record along with other indicators of past environmental conditions such as charcoal and isotope signatures (Ager 1984; Webb et al. 1984; Payette and Gagnon 1985; Ritchie 198?; Payette et al. 1989). Recent detailed, fine-scale analyses of the pollen record show that diversity and ecosystem processes in boreal regions change very rapidly in response to changes in exogenous forcing functions (MacDonald et al. 1993); such behaviors are predicted by simulation models of spccies interactions (Pastor and Post 1988; Bonan 1992; Leemans 1992). Thus, the boreal forest is one of the few biomes with a complete history of the development of community assemblages along with concurrent changes in disturbances and ecosystem processes; this historical record can be resolved at a fine enough scale to test long-term dynamics of simulation models.

A third advantage of boreal regions for investigations of diversity and ecosystem processes is that they remain largely in a wilderness condition. Timber harvesting, at least in North American and Siberia, has been mainly at the southern fringe, although it is increasing and extending northward. Trapping of fur-bearers has been extensive in the past, but species such as beaver (Castor canadensis) have recovered almost to levels prior to European settlement in the most heavily trapped areas along the southern margin (Broschart et al. 1989). Boreal forests and tundra may be among the few remaining large ecosystems in pristine condition with virtually intact diversity.

Finally, boreal regions are expected to experience the greatest changes in climate worldwide with increased atmospheric loading of radiativcly active gasses (Schlesinger and Mitchell 1985). Furthermore, these regions contain large pools of the world's carbon and, while currently a net carbon sink, can switch upon warming to becoming carbon sources (Post 1990; Apps et al. 1993). Thus, because of its circumpolar distribution and the tight coupling between functional diversity and ecosystem processes, the boreal regions may affect global climate in unexpected ways.

The purpose of this chapter is to expand upon these properties of boreal regions and to suggest long-term observations, experiments and models of this biome that can help elucidate the coupling between diversity and ecosystem processes. We will focus on the diversity of trees, mammals, birds and some major groups of insects, and their implications for ecosystem processes. The taxonomy, distribution and ecosystem properties of these taxa are better known than for other taxa in this biome, and they can be taken to represent broad-scale variation in patterns of diversity in boreal regions. However, much more work remains to be done on the diversity and ecosystem function of herbaceous plants, mosses, fungi and soil invertebrates, about which little is known, including any of their taxonomic affiliations. Wc will begin by reviewing natural biogeographic patterns of spccies richness and functional roles within trees, mammals, birds and insects, and then proceed to a discussion of landscape diversity and disturbance regimes. We will conclude this section with a synthesis of ecosystem consequences of natural patterns of diversity. We will then briefly discuss the consequences of human-induced changes in diversity of boreal regions and their ecosystem consequences. This chapter will conclude with suggestions of research needs and possible future research directions.

3.2 NATURAL DIVERSITY OF BOREAL REGIONS 3.2.1 Tree species

The current circumboreal forest region is a recently developed formation (<10 000 years) in a region of severe climate. Because of this, the boreal forest is characterized by low species richness when compared with other forest biomes, particularly low richness of tree species. However, this low species richness also suggests potentially important functional roles for individual tree species because of low redundancy and pronounced differences in ecosystem characteristics (Tabic 3.1) (Pastor and Mladenoif 1992). The North American and Eurasian boreal forests share major tree genera with similar characteristics (Table 3.1). These includc early successional broadleaved deciduous species of the genera Betula and Papains, and shade-tolerant conifcrs of the general Picea and Abies. Species of Pin us run the range from extremely shade-intolerant (P. bankslana) to moderately shade-tolerant (P. strobus). North America includes a greater diversity of specics of Pinus and other conifer genera (Pastor and Mladenoff 1992), but Eurasia contains three important species of Larix which arc shade-intolerant, deciduous conifers.

Latitudinal patterns of diversity are also similar in North America and Eurasia. On both continents there is greater diversity of broadleaved deciduous species and conifer genera, and more varied successional pathways where the boreal region is transitional with the temperature forest region (Pastor and Mladenoff 1992). Latitudinal transitions are more abrupt where the boreal forest meets steppe in the continental interiors. The transitional zone to temperate forests in Eurasia is geographically more limited than in North America, and also less diverse. The post-Pleistocene species assemblage was simpler in Eurasia, and the zone of maritime influence is narrower becausc of mountain ranges in Europe and Russia. Human land use has also displaced the transitional/temperate forest region with agriculture in Europe for centuries, which may be responsible for further reducing the gradient of latitudinal diversity in tree species.

Functional attributes of these species which are important for ecosystems and landscapes include tissue chemistry and its correlation with growth rate, growth form, decomposition and nutrient cycling, and herbivory, and reproduction mechanisms that determine large-scale migration and small-scale colonization of disturbed sites. The primary functional groupings are conifers vs. deciduous hardwoods, but even within these groups there are species differences that cause a wide range of functional diversity (Table 3.1). The correspondence between these traits and taxonomic categories, particularly in the species and generic taxonomic ranks, allows us to predict the ecosystem consequences of changes in biodiversity. Furthermore, the extreme contrast among many species in these traits with little redundancy within each functional group could cause chaotic or cyclic dynamics of ccosystem behavior with species replacement (Pastor et al. 1987; Pastor and Mladenoir 1992; Cohen and Pastor 1995).

Such interactions between species and ecosystem properties may have been key elements in the assembly of the boreal region after déglaciation. The current assemblage of tree species comprising the boreal forest (Table 3.1) arose after déglaciation only 6000-8000 years ago (Larsen 1980; Ager 1984; Webb et al. 1984; Ritchie 1987). There arc several key features of postglacial migrations of boreal species. First, during the full glacial extent, the ranges of current boreal species were compressed along the glacial margin and overlapped with many species, such as Ulmus, whose ranges are now disjunct from boreal regions (Webb et al. 1984). The species that currently comprise the boreal forest moved rapidly northward with the retreat of the ice sheets. Some such as Pice a glauca, moved as fast as 200 km per century, an order of magnitude faster than other tree species (Davis 1984), and interactions with fire and nutrient cycling as well as long-range seed dispersers such as crossbills (Loxia curvirostra. L. leucoptera) may have assisted in rapid migration (Payette 1992; MacDonald et al. 1993; Pastor 1993).

A second key feature of northward migrations of boreal species is the sequence of invasions. Globally, the order of invasions was Betula, followed by Alnus, the Picea, Abies, and finally Finns (Ritchie 1987). Given the divergence in traits that affect ecosystem properties and interactions with herbivores (Table 3.1), we might expect radical changes in ecosystem properties and food webs during this sequence of invasions. In fact, recent fine-resolution pollen analyses and isotope analyses suggest large changes in ecosystem properties over entire regions as such successive species invaded (MacDonald el al. 1993). In particular, the invasion of tundra by boreal forest in central Canada 400 years ago coincided with increased organic matter in lake sediments, higher pH and a higher ratio of lake inflow to évapotranspiration (MacDonald et al 1993). Therefore, the broad-scale replacement of one biome by another had important consequences for ecosystem functioning and land-water linkages.

Eastern North American and northeastern Asia east of the Yakutian Plateau (including Beringia) appear to be two epicenters of the spread of boreal species into northern regions of North America. There are taxonomic differences between subspccies of important trees in northeastern Asia-Beringia compared both with North America east of the Rockies and Russia and Fennoscandinavia west of the Urals (Bryant et al. 1989). These two epicenters harbored disjunct congeneric populations during the full glacial

Table 3.1 Major characteristics and ecosystems interactions of the major boreal tree spccics

Ecosystem properties

Interactions, with mammals

Table 3.1 Major characteristics and ecosystems interactions of the major boreal tree spccics

Interactions, with mammals

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