6 TAIGA L.H. Geiser and K. Nadelhoffer 6.1 Ecoregion Description 6.2 Ecosystem Responses Th e Taiga ecoregion (CEC 1997; Figure 2.1) includes Responses to increased nitrogen (N) deposition in most of interior Alaska and much of Canada’s northern boreal regions (which include Taiga and Northern boreal forest. Th e ecoregion description is adapted Forests ecoregions) include increased productivity, from CEC (1997). Th e Alaskan portion is underlain by foliar N concentration, and N leaching from soils, horizontal limestone, shale, and sandstones, creating plant community changes (including vascular plants, a fl at to gently rolling plain covered with organic bryophytes, lichens, and algae), and physiological changes. deposits, hummocky moraines, and lacustrine deposits; In boreal ecosystems and other largely oligotrophic the eastern portion is underlain by the Canadian environments, N is growth-limiting, and most plant Shield. Lowlands are mostly peatlands and permafrost species are adapted to low available N. As N availability is widespread. Nutrient-poor soils dominate southern increases, faster growing, but less N-use effi cient species portions and permafrost soils occur in the north. Th e (more N taken up per unit growth) typically out- climate is subarctic: the short summers have long compete slower growing species (Aerts and Chapin daylight and cool temperatures; winters are long and 2000). Increasing N deposition alters plant community very cold. Snow and freshwater ice persist for 6 to 8 structure, often leading to a short-term “positive” response months annually. Mean annual temperatures are -10 of increased productivity and vigor, followed by long-term to 0 oC; mean annual precipitation is 200 to 500 mm. changes in species composition and richness (Gough et Innumerable lakes, bogs, other wetlands, and forests al. 2000). Such community-level responses occur at low are interspersed with tundra-like shrublands and sedge deposition rates and have been used to mark the low meadows. In the north, forests transition to woodlands end of ecosystem N response (Bobbink et al. 2003, de with a lichen groundcover and fi nally merge into Vries et al. 2007). In boreal forests, overall ground fl ora tundra at the climatic limits of tree growth. Mid-zone, species number may not be aff ected by N enrichment, dwarf birch (Betula nana), Labrador tea (Ledum spp.), despite a drastic change in species composition, due to willow (Salix spp.), bearberry (Arctostaphylos alpina), reciprocal increases in nitrophilous species with declines mosses, and sedges dominate. In the south, open stands in typical species (Bobbink 2004). However, in peatland of stunted black spruce (Picea mariana) and jack pine bogs, where high water levels and low pH prevent (Pinus banksiana) are accompanied by alder (Alnus nitrophyte invasions (Allen 2004), decline in species spp.), willow, and tamarack (Larix larcinia) in fens richness is a more characteristic response. and bogs. White spruce (Picea glauca),black spruce, lodgepole pine (Pinus contorta), quaking aspen (Populus Increased N supply is often accompanied by increased tremuloides), balsam polar (Populus balsamifera), and foliar N concentration, which in turn may increase paper birch (Betula papyrifera) grow on well drained, susceptibility of vegetation to frost or diseases. Th is eff ect warm upland sites, rivers, and streams. Mat-forming is documented for boreal forest trees (e.g., Aronsson lichens can constitute in excess of 60 percent of the 1980, Balsberg-Påhlsson 1992, Kallio et al. 1985, winter food intake of caribou and reindeer (Longton Schaberg et al. 2002), for their common understory 1997). Th e abundant wetlands attract hundreds of ericaceous species (e.g., Strengbom et al. 2002, 2003), thousands of birds (e.g., ducks, geese, loons, and swans) and for bogs (e.g., Wiedermann et al. 2007). At higher which come to nest, or rest and feed on their way to deposition levels, as N is no longer completely taken up arctic breeding grounds. by vegetation or immobilized in soils, N saturation and leaching may occur (Lamontagne 1998; Lamontagne and Schiff 1999, 2000; Tamm et al. 1999). Chapter 6—Taiga GTR-NRS-80 49 Th e severity of N deposition impacts depends on: demonstrated increases in abundance of nitrophilous duration and amount of N deposition, the form(s) species with increased N deposition over time and of atmospheric N, the sensitivity of the ecosystem along N gradients. In Swedish boreal forests, changes components; other environmental conditions; and in ground vegetation, e.g., decreasing whortleberry management history (Bobbink et al. 2003). As a result, (Vaccinium myrtillus), occurred at deposition rates ecosystem response to N loading varies temporally, ≥ 6 kg N ha-1 yr-1 (Nordin et al. 2005, Strengbom spatially, and by habitat type. Change or loss of habitats et al. 2003) and increased growth of wavy hairgrass can have direct implications on the animals using these (Deschampsia fl exuosa) at ≥ 5 kg N ha-1 yr-1 (Kellner and habitats. Redbo-Torstensson 1995, Nordin et al. 2005). 6.3 Range of Responses Observed In an N fertilization experiment with 0, 12.5, and 50 -1 -1 Like the Tundra ecoregion, the paucity of North kg N ha yr (background atmospheric deposition 2 -1 -1 American studies and the many similarities in climate, to 3 kg N ha yr ), the abundance of wavy hairgrass -1 topography, and vegetation communities across the increased signifi cantly after 3 years with 12.5 kg N ha -1 circumboreal environment support consideration of yr while the abundance of whortleberry decreased European fi ndings to predict ecosystem N responses (Strengbom et al. 2002). Bryophyte species are also for North America. Th erefore, we include European responsive; the biomass of Schreber’s big red stem moss data in this discussion of the range of observed (Pleurozium schreberi) and a dicranum moss (Dicranum -1 -1 responses. European analyses considered Taiga and polysetum) fertilized with 25 and 30 kg N ha yr was Northern Forest ecoregions (CEC 1997) as a single reduced by 60 percent and 78 percent, respectively, after boreal biogeographic region under the European 4 years (Mäkipää 1998). University Information System (EUNIS) classifi cation system; hence the frequent use of the term ‘boreal’ Pest and disease resistance. In a large-scale fi eld study in the following sections. Responses to N inputs are of 557 coniferous stands in Sweden, the occurrences summarized in Table 6.1. of whortleberry, lingonberry (Vaccinium vitis-idaea) and wavy hairgrass, were investigated (Strengbom et -1 -1 6.3.1 Deposition al. 2003). Where N deposition was ≥ 6 kg ha yr , whortleberry was less frequent and more susceptible to Our knowledge of current N deposition and deposition the fungal leaf pathogen Valdensia heterodoxa. Frequency eff ects in the North American taiga is limited. Wet of lingonberry was also strongly negatively correlated deposition of inorganic N at the National Atmospheric with increasing N deposition (Strengbom et al. 2003). Deposition Program (NADP) monitors at Denali National Park and Preserve and Fairbanks, Alaska, has -1 -1 Whortleberry showed increased parasite burdens at been low and stable, averaging 0.23 kg ha yr (std. N fertilization ≥ 12.5 kg ha-1 yr-1 (Nordin et al. 1998, dev. = 0.129) from 1981 to 2007 (NADP 2008). Total - 2005; Strengbom et al. 2002). Disease incidence by nitrate (NO ) deposition in snow at two interior Alaska 3 the fungus Valdensia heterodoxa was more than twice sites north of Fairbanks in 1988 was consistent with this -1 -1 -1 as high in plots receiving 12.5 kg N ha yr and more range and averaged 0.32 kg N ha (Jaff e and Zukowski -1 than three times as high in plots receiving 50 kg N ha 1993). Recent measurements of N from ammonium -1 + - yr compared to controls. Th e abundance of the fungus (NH ) and NO ions in throughfall deposition at 4 3 V. heterodoxa on whortleberry was increased tenfold by remote sites in northeastern Alberta, Canada (Berryman -1 -1 -1 -1 -1 -1 25 kg N ha yr compared to 0.5 kg N ha yr . As a and Straker 2008) ranged from 0.6 to 2.0 kg N ha yr . consequence, whortleberry density decreased, and wavy 6.3.2 Forests and Woodlands hairgrass cover increased (Strengbom et al. 2002). In addition, shoots of whortleberry were signifi cantly more Plant community composition changes. A large number damaged by moth larvae such as the rusty tussock moth of studies, summarized by Bobbink et al. (2003), have (Orgyia antique) after addition of 12.5 kg N ha-1 yr-1 in 50 Chapter 6—Taiga GTR-NRS-80 the fi rst year of treatment (Nordin et al. 1998). Cover deposition in combination with leaching of mineralized of wavy hairgrass continued to increase over 5 years N from lichen and moss patches (Lamontagne 1998). of N additions between 8 and 12 kg ha-1 yr-1, induced Net nitrifi cation increased or remained similar to by increased light penetration resulting from disease reference sites in lichen patches, while N-amended forest - damage to whortleberry (Nordin et al. 2005). islands had a strong tendency to consume NO3 and + produce NH4 (Lamontagne and Schiff 2000). By the Cold tolerance. Frost hardiness and plant tissue cold second year, lichen-covered bedrock surfaces no longer hardiness can be reduced by N additions. For example, retained N additions. In contrast, N-amended and after 12 years of fertilization with ammonium chloride reference forest islands retained a similar proportion of -1 -1 (NH4Cl) at 15 kg N ha yr , fi rst-year foliage in red N inputs, indicating that forest islands did not become spruce (Picea rubens) at a high elevation forest in New N saturated in this time frame. However, because England (with ambient wet + dry deposition of ~10 kg the components of the boreal shield landscape are N ha-1 yr-1) showed diminished cold tolerance, greater hydrologically connected, there is concern that long- electrolyte leakage, and increased susceptibility to frost term N deposition to such a heterogeneous landscape damage (Schaberg et al.