Ecological Sustainability of Birds in Boreal Forests. – Ecology

by G Niemi · Cited by 129 — Consequences of Not Managing for Sustainability of Bird Populations. ♢ Knowledge Gaps on Boreal Birds. ♢ Tools for Management of Sustainable Boreal Bird
25 pages

46 KB – 25 Pages

PAGE – 1 ============
Table of ContentsEcological Sustainability of Birds in Boreal Forests.0ABSTRACT0INTRODUCTION..1Boreal bird communities.1Status of programs on sustaining boreal bird populations.3DISCUSSION3Population variability3Ecological factors underlying sustainability: density dependence and independence.5Population trends6Continent-wide-scale disturbances in boreal forests..6Landscape-scale disturbances in boreal forests.7Patch- or stand-scale disturbances in boreal forests8Human impacts on boreal forests8Consequences of not managing for sustainability of bird populations.10Knowledge gaps on boreal birds..11Tools for management of sustainable boreal bird populations.12CONCLUSIONS13RESPONSES TO THIS ARTICLE.13ACKNOWLEDGMENTS:..13LITERATURE CITED..14

PAGE – 2 ============
Ecological Sustainability of Birds in Boreal ForestsGerald Niemi1, JoAnn Hanowski1, Pekka Helle2, Robert Howe3, Mikko Mönkkönen4, Lisa Venier5, and DanielWelsh51University of Minnesota; 2Finnish Game and Fisheries Research Institute; 3University of Wisconsin;4University of Oulu, Finland; 5Canadian Forest Service· Abstract· Introduction¨ Boreal Bird Communities¨ Status of Programs on Sustaining Boreal Bird Populations· Discussion¨ Population Variability¨ Ecological Factors Underlying Sustainability: Density Dependence and Independence¨ Population Trends¨ Continent-wide-Scale Disturbances in Boreal Forests¨ Landscape-Scale Disturbances in Boreal Forests¨ Patch- or Stand-Scale Disturbances in Boreal Forests¨ Human Impacts on Boreal Forests¨ Consequences of Not Managing for Sustainability of Bird Populations¨ Knowledge Gaps on Boreal Birds¨ Tools for Management of Sustainable Boreal Bird Populations· Conclusions· Responses to This Article· Acknowledgments· Literature CitedABSTRACTWe review characteristics of birds in boreal forests in the context of their ecological sustainability under bothnatural and anthropogenic disturbances. We identify the underlying ecological factors associated with boreal birdpopulations and their variability, review the interactions between boreal bird populations and disturbance, anddescribe some tools on how boreal bird populations may be conserved in the future. The boreal system hashistorically been an area with extensive disturbance such as fire, insect outbreaks, and wind. In addition, theboreal system is vulnerable to global climate change as well as increasing pressure on forest and water resources.Current knowledge indicates that birds play an important role in boreal forests, and sustaining these populationsaffords many benefits to the health of boreal forests. Many issues must be approached with caution, including thelack of knowledge on our ability to mimic natural disturbance regimes with management, our lack ofunderstanding on fragmentation due to logging activity, which is different from permanent conversion to otherland uses such as agriculture or residential area, and our lack of knowledge on what controls variability in borealbird populations or the linkage between bird population fluctuations and productivity. The essential role thatbirds can provide is to clarify important ecological concerns and variables that not only will help to sustain birdpopulations, but also will contribute to the long-term health of the boreal forest for all species, including humans.

PAGE – 3 ============
KEY WORDS:birds, boreal, conservation, disturbance, forests, forestry, natural resources, Nearctic, Palearctic, sustainability, trends.INTRODUCTIONThe northern boreal forest, or taiga, an area dominated by forests, peatlands, and water, is one of the largest andyoungest biomes in the world (Shugart et al. 1992, Helle and Niemi 1996). The current condition of the borealforest varies from areas that have received intensive management for wood production (such as in northernEurope and Canada) to large, pristine forest areas that have never been logged (such as in central Siberia and inAlaska). The dominant natural disturbance of the boreal forest is fire (Heinselman 1973, Zackrisson 1977, Weinand MacLean 1983), although insect outbreaks, wind, and other animals (e.g., beaver, Castor spp.) are alsosignificant disturbances in many areas (Pastor et al. 1996).Boreal regions are expected to experience the greatest change in climate worldwide, due to global warming(Pastor et al. 1996). Because boreal regions contain huge pools of the world’s carbon, climate warming in theseregions may change them from a carbon sink to a carbon source (Post 1990, Pastor et al. 1996). Climate changecan be viewed as a large, long-term stress on the boreal system, but logging is currently the primaryhuman-induced disturbance influencing change in boreal forests. Land cover in boreal regions such as in Canadais undergoing massive change as a result of forestry operations, primarily the removal of trees over vast areas.Vitousek (1994) suggests that land use/land cover change and loss probably represent the biggest component ofglobal change, with profound effects on biological diversity.Our focus here is to increase our understanding of “Sustainability in Boreal Regions,” the results of a workshopheld at Lake Itasca State Park, Minnesota, United States (this issue). Specifically, we focus on boreal birdpopulations as one aspect of the boreal system to be sustained, and also on the role birds play in sustaining theboreal system.The objective of the paper is to identify the important factors that influence and indicate sustainability of borealbird populations. Here, “sustaining” embodies the concept of “sustainable development,” in which developmentmeets present and future human needs without damaging the environment and biological diversity (Lubchenco etal. 1991). We are interested in presenting the underlying ecological factors that will maintain and conserve borealbird populations into the future or, theoretically, in “perpetuity,” in the face of both natural and anthropogenicdisturbances. Sustainability here refers to a variety of characteristics of bird populations, including theconservation of viable populations of all extant, indigenous species; the maintenance of inherent populationvariability; and, to the extent possible or realistic, the preservation of species composition and variability withinboreal forests. We will: (1) characterize the boreal bird community, (2) identify possible underlying ecologicalfactors associated with boreal bird populations, variability, and their trends, (3) describe what is known about theinteractions between boreal birds and disturbance, and (4) review some tools that would be useful for sustainingboreal bird populations in the future.Boreal bird communitiesA considerable body of literature exists on boreal bird communities, including studies of life history and ecologyfor many species (e.g., Godfrey 1966, Erskine 1977, Welsh 1987, Haila and Järvinen 1990, Helle andMönkkönen 1990, Helle and Niemi 1996, Kirk et al. 1996, Welsh and Lougheed 1996, Mönkkönen and Viro1997, Niemi and Hanowski 1997, and Birds of North America series, Poole, Stettenheim, and Gill, editors). Ingeneral, birds constitute the majority of the terrestrial vertebrate species in most boreal communities. Forinstance, birds represent about 70% of all terrestrial vertebrate species in the Superior National Forest of northernMinnesota, United States, where Green (1995) documents 155 bird species, 52 mammal species, and 18herpetofaunal species. In the western boreal forest of Canada (west of the Ontario-Manitoba border),

PAGE – 4 ============
approximately 81% of terrestrial vertebrates are birds (Smith 1993). In the boreal forest of northeastern Ontario,birds represent approximately 71% of the terrestrial vertebrate species (D’Eon and Watt 1994). In Finland, birdsmake up about 75% of the terrestrial vertebrates. Within the boreal zone, there are about three times as many birdspecies as mammal species over the Holarctic region (Mönkkönen and Viro 1997). In general, the number ofmammal, reptile, and amphibian species declines toward the north, so the proportion of the terrestrial vertebratecommunity consisting of bird species remains either relatively the same, or usually increases from south to northin the boreal forest.The breeding boreal bird community is made up of three different groups: permanent residents, short-distancemigrants (wintering in temperate areas), and long-distance migrants (wintering in the tropics). The proportion ofmigratory species in these communities varies considerably, depending on latitude and habitat type; however, thenumber of migratory species generally exceeds the number of permanent residents. For example, Helle andNiemi (1996) reported that permanent residents represent only 5-15% of the local breeding bird species. Thenumbers of long-distance migrant species tend to vary from about 30% (Haila and Järvinen 1990) to >50%(Helle and Niemi 1996) in many Nearctic boreal bird communities. More than 80% of all bird species breeding inthe boreal region of Canada winter farther to the south, and approximately 50% winter in tropical or subtropicalregions (Erskine 1977). In Fennoscandia, about 30% of the land bird species and 40% of individuals winter in thetropics, whereas 45% of land bird species and 48% of individuals, respectively, are classified as short-distancemigrants.Evidence for the importance of birds in a healthy forested ecosystem is increasing. In summer, most forest birdseat insects, especially phytophagous lepidopteran larvae. Several studies have shown that birds reduce insectdensities (Holmes et al. 1979, Atlegrim 1989), especially when the insect populations are at either low orendemic levels (Crawford and Jennings 1989, Holmes 1990, Torgerson et al. 1990). Holling (1988) reported thatbird predation of spruce budworm (Choristoneura fumiferana) could lengthen the time between budwormoutbreaks. Although not in a boreal setting, Marquis and Whelan (1994) conducted exclosure experiments onoaks in Missouri, United States, and showed that predation by birds had a significant and positive effect on treegrowth. They concluded that declines in North American populations of insectivorous birds may reduce forestproductivity, because they would result in higher numbers of leaf-chewing insects that have negative effects ontree growth. Folke et al. (1996) reported that a 50-70% decline in Neotropical migrants would probably result inmajor changes to the tree species composition of the boreal forest. In one of the few economic studies onmonetary benefits of bird predation, Takekawa and Garton (1984) estimated that bird predation on westernspruce budworm (Choristoneura occidentalis) resulted in $1,820 (U.S. dollars) of positive economic benefit peryear per square kilometer.In addition to impacts of insectivorous birds, predatory birds have major influences on small-mammalpopulations, with ultimate effects on forest regeneration and health. For example, Korpimäki (1993) andKorpimäki and Krebs (1996) present evidence on the controlling influence of predatory birds on vole populationcycles, especially in northern Fennoscandia. There are a variety of other ecosystem services that birds provide,including dissemination of seeds, nutrient and energy cycling, and predation of insects during nonbreedingseasons (Green 1995, Lanner 1996). Birds may also have some negative effects on forest regeneration in somesituations, due to their predation of seeds.This evidence indicates that there are strong ecological and economic reasons for sustaining healthy birdpopulations. If the value of $1,820 (U.S. dollars) is remotely accurate, then the pro bono contribution of birds tothe boreal forest is in the millions of U.S. dollars per year. This value does not include other ecosystem servicesthat birds perform, the increased value of the trees from higher growth rates, or other economic values such as therecreational and aesthetic value of birds.Another viewpoint is the essential role that a healthy, forested ecosystem has in maintaining healthy forest birdpopulations. For example, forest land management is focused on sustaining forest ecosystems, of which birds arebut one component. Even though we cannot yet identify all of the necessary components and links within theforest ecosystem, we have to rely on indirect measures of ecosystem health. One measure is the status of birdpopulations. Birds integrate many structural and functional aspects of the forest ecosystem, so understanding bird

PAGE – 5 ============
populations is a link to understanding forest ecosystems. This is also one reason that birds have often beensuggested as indicators or monitors of healthy forest ecosystems (Furness and Greenwood 1993).Status of programs on sustaining boreal bird populationsBroadly varied publications have addressed the issues of sustainable use of forest systems and of sustainingwildlife populations. For example, in Minnesota, a Generic Environmental Impact Statement (GEIS) on timberharvesting and forest management was conducted to address concerns about the increased logging of trees(Jaakko Poyry 1992). Minnesota has continued with this effort by funding a long-term program called theMinnesota Forest Bird Diversity Initiative. The program is now in its eighth year of a 12-yr effort to monitorforest bird populations, to conduct selected research, and to provide education to managers and the lay public onissues related to the sustainability of forest bird populations (Green 1995). In the western Great Lakes region,Howe et al. (1997) reported that habitat management is a first step toward bird conservation, but indicated thatspecies interactions and biogeographic factors can quickly complicate simple management prescriptions.There have been several Canadian initiatives to provide a framework for bird conservation. Partners InFlight-Canada (1996) developed a “Framework for Landbird Conservation in Canada,” which outlines thecomponents of a coordinated land bird conservation program and includes monitoring, research, and appliedconservation. On a smaller scale, the Federation of Ontario Naturalists, in conjunction with the Canadian WildlifeService and others, described a framework for achieving land bird conservation that includes identifying priorityspecies and habitats at multiple scales, developing habitat-based strategies to conserve birds, with identificationof core area networks, and developed a monitoring strategy based on current monitoring, research, and inventory(Chesky 1995). Key to both of these plans are substantial investments in monitoring and research.In northern Europe, many national programs and sets of management instructions have been developed forforestry to increase multiple-use and preservation of biodiversity in managed forests. National forest lawslaunched recently in many countries have placed a high priority on preserving biological sustainability,populations of less abundant species, special habitat types, and the total biodiversity of forested ecosystems. Animportant issue is that conservation should not be restricted to forest reserves (which are of crucial importancefor some species), but managed forests are important to preservation of the overall diversity of communities andecological processes at different scales (Tucker and Heath 1994).The Wild Birds Directive of the European Union is another major vehicle for the conservation of ecosystems innorthern Europe. For all species listed in the Bird Directive, every effort should be made to not decrease theirpopulation size or range due to human-related activities. The Nordic countries have the most responsibility forthe conservation of boreal forest species within Europe. For example, in Finland there are 22 forest bird speciesincluded in the list of the Wild Birds Directive, and 14 of these are the responsibility of Finland, because >10%of their European population breeds in Finland (Rajasärkkä 1997).DISCUSSIONPopulation variabilityWith the exception of economically important game mammals, birds probably have been studied moreintensively than any other group of organisms. Long-term monitoring schemes such as the North AmericanBreeding Bird Survey (Robbins et al. 1986), British Common Bird Census (Furness and Greenwood 1993), andFinnish land bird monitoring scheme (Koskimies and Väisänen 1991), provide comprehensive, ongoingdatabases covering several decades. Many other local studies provide databases covering 10 or more years

PAGE – 6 ============
(Hogstad 1993, Welsh 1995, Howe et al. 1997).A better understanding of the ecological factors that contribute to variability in boreal bird populations is critical,if society is to sustain populations over a variety of temporal and spatial scales. Holmes (1990) and otherecologists (e.g., Blake et al. 1992) argue that various factors influence variability in bird numbers, including: (1)successional changes in habitat structure; (2) variations in food abundance; (3) interactions with competitors andpredators; and (4) events on wintering grounds. Local disturbances such as logging inevitably modify thecharacter of these factors, thereby affecting population sizes. How these disturbances affect variability isunknown, but local populations of species that are adapted to disturbed habitats are likely to vary inversely withpopulations of late-successional species.Every long-term study has reported that bird abundance varies considerably. Patterns of variation are typicallycomplex and individualistic; as a result, information from a few local studies is difficult to apply to largergeographic scales (Taper et al. 1995). Conversely, large- scale and long-term trends are not always evident atlocal scales. Even for globally declining species, areas of population decline might be interspersed with areas oflittle change or even local population increase (Villard and Maurer 1996). An understanding of factorsresponsible for population variability, therefore, is an important and meaningful challenge (Mönkkönen and Aspi1998).Observed patterns of variability in bird numbers are consistent with a view that bird population dynamics operateon large spatial and temporal scales. Unlike many other groups of organisms (e.g., Thomas 1995), birds are ableto recover from local perturbations such as defoliation by insects (Bell and Whitmore 1997) and logging(Virkkala 1987), as long as adequate source populations are maintained nearby (Pullman 1988). On the otherhand, the negative effects of habitat degradation might take many years to be manifest. Site tenacity and survivalof local adult birds may delay the effects of changes in habitat suitability (Bengtsson et al. 1997).Even though population dynamics of birds appear to operate at large scales, local variability in bird abundance isof great interest because land management activities and conservation efforts typically occur at local (<1 km2)scales. In addition, little is known about dispersal in bird populations. Generalizations about local variability inbird abundances are also confounded by sampling effects. Helle and Mönkkönen (1986) found a strongcorrelation between population variability and average population density in boreal forests of Finland; rarespecies showed greater variability in numbers. Part of this variation undoubtedly can be attributed to samplingerrors and spatial variability within populations (Mönkkönen and Aspi 1998). Haila et al. (1996) documented ahigh degree of year-to-year stochasticity in the locations of bird territories in heterogeneous boreal forests. Theyattributed stochastic variation in many species to an overabundance of suitable sites. Individuals consistentlyavoided certain habitats, but within the remaining habitats, territory locations were highly variable and difficult topredict. Gaston and McArdle (1994), Link et al. (1994), and Mönkkönen and Aspi (1998) discuss methods forremoving the variance caused by incomplete sampling or spatial patchiness within bird populations. Unless theseeffects are taken into account, Mönkkönen and Aspi (1998) argue that we know far less about variability in birdpopulations than we think.Measuring density variability is important because it has been suggested that populations occupying sink habitatsshow higher temporal variability than do source habitat populations (Howe et al. 1991, Beshkarev et al. 1994).The identification of source populations and habitat, if they exist, is critical for sustaining populations, becauseregional existence of species is dependent on successful reproduction from population sources (Pulliam 1988,Pulliam and Danielson 1991). In Northern Europe, Järvinen (1980) suggested that some high-latitudepopulations are largely dependent on surplus individuals produced farther south in population source areas. InNorth America, it has been suggested that conservation of northern breeding areas is critical to populationsources and the continual supply of individuals to sink, temperate regions of the United States, where breedinghabitats have been fragmented and reproductive success is low (Terborgh 1992, Robinson et al. 1995). Thelarge-scale evidence for any of these patterns is minimal and speculative. PAGE - 8 ============ a positive relationship between abundance and breeding evidence for 14 of the 27 boreal passerine speciesexamined. The remaining 13 species showed no relationship between abundance and breeding evidence.Population trendsIn a relatively well-studied boreal area, Finland, more bird species have increased in abundance than havedecreased with the conversion of more natural forest landscapes into intensively managed ones. The general trendfrom the past to the present has been for many of the common species to become more numerous, while many ofthe rarer species have declined. Populations of species with high affinities to old forests, such as the Siberian Jay(Perisoreus infaustus), Siberian Tit (Parus cinctus), and Three-toed Woodpecker (Picoides tridactylus), havedecreased dramatically during the latter half of the 20th century, as a consequence of forest habitat loss andfragmentation due to intensive forestry (Helle 1985, Virkkala 1991). Although these patterns are well described,we do not know the precise mechanisms for these population declines. Among the few studied cases, Virkkala(1990) showed that in more pristine forests, Siberian Tit breeding success and the number of fledglings per nestwere higher than in heavily managed stands. This was possibly associated with the lower abundance ofinvertebrates in managed stands than in natural stands (Petterson et al. 1995). However, the populations ofSiberian Tits had declined more than expected by the loss of their preferred habitat, mature conifer forest. Thissuggests that other factors such as forest fragmentation, the deterioration in habitat quality, the selective removalof old trees, or climate change coupled with habitat changes are also involved in the population declines. Innorthern Finland, regional population changes (both decreases and increases) are more consistent than changes atthe scale of individual forest remnants (Väisänen et al. 1986). For example, Virkkala (1991) also found thatpopulation declines of birds in managed forests of northern Finland were not simultaneously observed in virginforest remnants. Capercaillie (Tetrao urogallus), Three-toed Woodpecker, Siberian Tit, Siberian Jay, and PineGrosbeak (Pinicola enucleator), all of which have declined in northern Finland, have remained relatively stablein large blocks of virgin forest. In contrast, Helle (1986) found that populations of many bird species variedsimilarly in virgin forest and nearby managed forests.In Canada, there are approximately 70 Breeding Bird Survey routes that fall within the boreal forest. Weexamined 30-yr trends (1966-1996) using these Breeding Bird Survey routes, which are available on a web site(Sauer et al. 1997). Of 62 species examined, there were insufficient data to examine trends for 11 species, sevenspecies had significant (P < 0.05) trends (five negative and two positive), and the remaining 44 species had nosignificant trends. Species with negative trends included Chestnut-sided Warbler (Dendroica pensylvanica),Yellow Warbler (Dendroica petechia), Common Yellowthroat (Geothlypis trichas), Yellow-bellied Sapsucker(Sphyrapicus varius), and Least Flycatcher (Empidonax minimus). The Forest Bird Monitoring Program (Welsh1995) has approximately 80 sites in the boreal forest of Ontario with three or more years of data for thecalculation of trends in the 1987-1997 time period. Trends were estimated for 54 species (occurring in at least 15sites). Eight species had significant (P < 0.01) trends, in which five were positive and three were negative.Species with negative trends included Eastern Wood-Pewee (Contopus pertinax), Winter Wren (Troglodytesaedon), and Golden-crowned Kinglet (Regulus satrapa) (Heather Dewar, Canadian Wildlife Service, personalcommunication). Long-term monitoring of birds in boreal Canada has not been completed on a scale that enablesus to assess the status of populations. This is a serious impediment to our ability to assess the sustainability ofboreal bird populations in North America.Continent-wide-scale disturbances in boreal forestsLong-term, large-scale changes resulting from climatic instability during the Pleistocene have impacted theranges of bird species and the composition of regional and continental species assemblages. The Quaternarypaleoenvironmental fluctuations have been more drastic in the western Palaearctic than in the Nearctic region.European forests were greatly diminished in extent, persisting as altitudinal belts on southern mountains thatwere fragmented, isolated from each other, and became floristically depauperate (Huntley 1993). Continuousforest did persist, however, in eastern and central Siberia and China (Kurtén 1972). In North America, temperate PAGE - 9 ============ forests were able to better maintain taxonomic diversity and a more extensive and continuous presence (Webb1988). Moreover, temperate forest zones of North America remained in close contact with the tropics during theglaciations (e.g., CLIMAP 1976), enabling a continuous interchange of tropical and temperate avifaunas. TheSaharan desert was, by contrast, more extensive during the glacial stages (CLIMAP 1976). As a result,forest-associated fauna of eastern North America, including those that use the boreal forest, probably had betteropportunities of finding refugia than did those of Europe. Huntley (1993) attributes the low taxonomic diversityin European forests principally to the limited area available to forest taxa. The Quaternary paleoenvironmentalconditions have obviously affected ecological attributes of extant species (e.g., behavioral plasticity, habitatpreferences, etc.) and, consequently, have had an influence on how species respond to contemporary,human-induced landscape changes (Mönkkönen and Welsh 1994). These large-scale changes have beenimplicated as critical events in the speciation of many North American species, such as the New World warblers(Mayr 1946, Bermingham et al. 1992). However, recent evidence using mitochondrial DNA questions theimportance of Late Pleistocene glaciation events in the speciation process (Klicka and Zink 1997). Regardless ofthe specific timing of speciation, many boreal species have been exposed to a variety of disturbances, which mayhave predisposed them to be relatively plastic to habitat and environmental changes.Landscape-scale disturbances in boreal forestsDisturbances such as forest fires, insect outbreaks, and large-scale wind and ice storms affect the structure offorest landscapes and forest stands at intermediate scales. This has resulted in heterogeneous landscapes with awide variety of different successional stages of forests at any given time (Haila 1994). These disturbances occurin the time scale of decades to a millennium, and may extend from hectares to millions of hectares in area.Secondary forest succession, initiated by a natural or human-caused catastrophe, is a directional change incommunities from a pioneer stage (more or less open ground) to a climax stage. The process lasts from one toseveral hundred years in the boreal setting (Heinselman 1996), and it is a useful framework for studying andunderstanding natural dynamics of the boreal forest.Landscape-level effects on birds have recently become better documented (Helle and Järvinen 1986, Rolstad andWegge 1987, Opdam 1991, Andren 1994, McGarigal and McComb 1995, Kurki et al. 1997), and indicate theeffects of regional forest area, patch size, and isolation on the number of species or densities of species. Theinfluence of landscape composition, configuration, and connectivity on population dynamics implies that theimpact of forest change (due to human or other disturbances) on the status of populations cannot be extrapolatedonly from local measures of forest character and change. Early documentation of landscape-scale effects waspredominantly for forest patches in agricultural landscapes (Opdam 1991). Examples of studies in forestedlandscapes are becoming more common, and several recent studies have been conducted in a boreal forestcontext. For example, Rolstad and Wegge (1987) presented a graphical model to predict the response ofCapercaillie males to fine-grained and coarse-grained forest landscape fragmentation. DesRochers and Hannon(1997) found that boreal forest songbirds were twice as likely to travel through 50 m of woodland than through50 m in open habitats when attracted by recordings of chickadee mobbing calls. Kurki et al. (1997) linkedlandscape change with the increase of fox (Vulpes vulpes) populations and the subsequent reduced breedingsuccess of grouse, particularly after peak vole years. Schmiegelow et al. (1997) studied the effect offragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old borealmixed forest. Richness of Neotropical migrants declined in both connected and isolated fragments, residentspecies declined in isolated fragments, and short-distance migrants, most of which are habitat generalists, did notchange. However, Schmiegelow et al. (1997) concluded that the magnitude of the fragmentation effects wassmall compared with those observed elsewhere, possibly because the fragments were embedded in a matrix ofuncut forest. If species have evolved where frequent disturbances create a naturally heterogeneous landscape,then they may better cope with human-induced landscape changes (Hansen et al. 1991, Hansson and Angelstam1991, Rudnicky and Hunter 1993, Schmiegelow et al. 1997).The surrounding landscape matrix around a forest stand has also been shown to influence the composition ofbreeding birds within a stand. For example, Pearson and Niemi (1998) found some breeding bird species to be PAGE - 10 ============ more common in less suitable habitat if this habitat was surrounded by favorable habitat. Similar patterns weresummarized by Andren (1994) in the Palearctic. Likewise, Väisänen et al. (1986) showed that the breeding birdcommunity of a forest stand changed along with changes in the surrounding landscape, although the forest standitself remained unchanged.Patch- or stand-scale disturbances in boreal forestsSmall-scale patch dynamics maintain the heterogeneity and diversity of forest stands. Here, the spatial scalevaries from gaps of individual trees to changes in entire stands. Small-scale patch dynamics (gap disturbance)have two important consequences for birds in boreal forests. First, they create openings in climax stands offorests, providing habitat for species that require young forests. In these cases, both forest-interior andearly-successional species are able to coexist in natural landscapes, even without large-scale disturbances (Lentand Capen 1995). Small regeneration patches increase the amount of habitat edge in a given area withoutfragmenting the landscape. Second, patch dynamics often maintain a considerable deciduous component inclimax coniferous forests (Syrjänen et al. 1994) because of the open canopy in many of these forests. Deciduoustrees are important for boreal birds such as woodpeckers and tits, and for foraging by many small passerines.Consequently, bird density and diversity in boreal bird communities have often been found to correlate positivelywith the proportion of deciduous trees (Haapanen 1965). These patterns match well with the intermediatedisturbance hypothesis (Connell 1978) that the greatest effective heterogeneity occurs in intermediately disturbedareas, where numerous species can coexist.Stand age is one of the most important variables affecting bird species density and community structure in theboreal forest (Helle 1985). Compared to temperate regions, relatively few studies have assessed successionalpatterns in boreal forest bird communities (Helle and Mönkkönen 1990). There is considerable variation in thepatterns of response by forest birds during succession. For instance, Helle and Mönkkönen (1990) and Helle andNiemi (1996) found that diversity of the breeding bird community generally increased with increasing stand agein both Eurasia and North America. However, a variety of studies contradict this general pattern (e.g., Morganand Freedman 1986, Westworth and Telfer 1993, and Kirk et al. 1996). Little consensus has emerged on howbird densities vary with successional age.Human impacts on boreal forestsFennoscandian forest landscapes have been influenced for centuries by slash-and-burn cultivation and charcoal,tar, and timber production (Esseen et al. 1997), but in many areas, forest harvesting started only a few decadesago or even more recently (see Mönkkönen and Welsh 1994, Syränen et al. 1994). Based on this historicalscenario, Mönkkönen and Welsh (1994) suggested that contemporary avifaunas are not likely to be equallyaffected by present human impacts. They distinguished three species groups in the Holarctic forest bird faunaaccording to their "sensitivity" to human impact, varying from less sensitive "European" fauna to particularlysensitive Nearctic species. Boreal species of Siberian-Canadian origin were supposedly of intermediatesensitivity.Modern human land use affects boreal ecosystems and their bird communities in many ways. In the boreal zone,forestry plays the main role and has replaced forest fire as the dominant disturbance (Brumelis and Carleton1988), but there is considerable variation in forestry practices throughout the boreal forest. Use of timber andother forest products has had a long tradition in northwestern Europe, especially since World War II (Mönkkönenand Welsh 1994). Because of the long history of forest exploitation in the Nordic countries, little if anyold-growth forest still exists. Old forests in natural condition are almost completely restricted to protected areas,and they compose <5% of all forest area in Fennoscandia. In northern Russia and especially in Siberia, on theother hand, the impact of forestry has been limited thus far, but this is rapidly changing (Myers 1997).In North America, logging is a relatively recent activity; in most areas of the boreal forest, it also has occurred 46 KB – 25 Pages