Changes in climate are likely to be the greatest cause of changes in goods and services in the arctic (Walker et al., 2001). Projected climatic warming of 4-10°C by the end of the century probably would cause substantial increases in decomposition, nutrient release, and primary production. The net effect on the carbon balance will depend primarily on soil moisture (McKane et al., 1997; McGuire et al., 2000), which cannot be projected with confidence. In general, surface soils on slopes are expected to become drier as thaw depth increases. Lowlands may experience substantial thermokarst, impoundment of water, and reduced aeration.
Plant production frequently is limited in the arctic by excessive moisture and slow turnover of nutrients in soils, and warming and drying of soils is likely to enhance decomposition, nutrient mineralization, and productivity. Many of these changes in productivity may be mediated by changes in species composition and therefore are likely to lag changes in climate by years to decades (Chapin et al., 1995; Shaver et al., 2000). Threshold changes in productivity associated with poleward movement of the treeline is likely to experience time lags of decades to centuries because of limitations in dispersal and establishment of trees (Starfield and Chapin, 1996; Chapin and Starfield, 1997).
The net effect of warming on carbon stores in high-latitude ecosystems depends on changes in the balance between production and decomposition. Decomposition initially may respond more rapidly than production, causing trends toward net carbon efflux (Shaver et al., 1992; Smith and Shugart 1993).
Warming-induced thermokarst is likely to increase CH4 flux to the atmosphere in lowlands, particularly peatlands of northern Canada and western Siberia (Gorham, 1991; Roulet and Ash, 1992; see also Section 5.8) and the loess-dominated "yedoma" sediments of central and eastern Siberia (Zimov et al., 1997). Fires and other disturbances are likely to affect the thermokarsts; however, the role of these disturbanceswhich can be mediated with changes in regional climate in inducing thermokarstare poorly understood.
Changes in community composition associated with warming are likely to alter feedbacks to climate. Tundra has a three- to six-fold higher winter albedo than boreal forest, but summer albedo and energy partitioning differ more strongly among ecosystems within tundra or boreal forest than between these two biomes (Betts and Ball, 1997; Eugster et al., 2000). If regional surface warming continues, changes in albedo and energy absorption during winter are likely to act as positive feedbacks to regional warming as a result of earlier melting of snow and, over the long term, poleward movement of the treeline. Surface drying and a change in dominance from mosses to vascular plants also would enhance sensible heat flux and regional warming in tundra (Lynch et al., 1999; Chapin et al., 2000).
Poleward migration of taxa from boreal forest to the Arctic tundra will depend not only on warming climate but also on dispersal rates, colonization rates, and species interactions and therefore may exhibit substantial time lags. The arctic historically has experienced fewer invasions of weeds and other exotic taxa than other regions (Billings, 1973). Some of the most important changes in diversity in the arctic may be changes in the abundance of caribou, waterfowl, and other subsistence resources (see also Section 5.4). Changes in community composition and productivity may be particularly pronounced in the high arctic, where much of the surface currently is unvegetated and is prone to establishment and expansion of additional vegetation (Wookey et al., 1993; Callaghan and Jonasson, 1995).
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