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Models that predict changes in species distribution suggest reduced forest carbon storage as climate changes (King and Neilson, 1992; Smith and Shugart, 1993; Kirilenko and Solomon, 1998; Woodward et al., 1998), although the change in forest carbon stocks depends on species migration rates (Solomon and Kirilenko, 1997; see also Section 5.2). Where seed availability and dispersal are impeded (e.g., by fragmented landscapes), achieved/realized productivity may remain below the potential for some time (resulting in carbon losses) unless aided by human intervention (Iverson and Prasad, 1998; Sohngen et al., 1998; Iverson et al., 1999). Pitelka and Plant Migration Workshop Group (1997) point out, however, that increases in weed species that take advantage of human mobility may be an adverse effect.
Changes in forest distribution as a result of climate change are likey to occur subtly and nonlinearly (Davis and Botkin, 1985; Prentice et al., 1993; Neilson and Marks, 1994; Tchbekova et al., 1994; Bugmann et al., 1996; Neilson and Drapek, 1998). Prediction of changes in species distribution is complicated by the current lack of precise predictions of environmental changes themselves (especially precipitation) and responses of species to these changes. Price et al. (1999b) show that responses to precipitation are greatly dependent on assumptions made about species parameters and the temporal pattern of rainfall. The most rapid changes are expected where they are accelerated by changes in natural and anthropogenic disturbance patterns (Overpeck et al., 1991; Kurz et al., 1995; Flannigan and Bergeron, 1998).
At the stand level, climate-induced changes in competitive relationships are likely to lead to dieback and replacement of maladapted species, causing changes in stand population and productivity (e.g., Cumming and Burton, 1996; Rehfeldt et al., 1999b). In addition, increases in locally extreme events and disturbances (fires, insects, diseases and other pathogens) developing over different time scales (Wein et al., 1989; Campbell and McAndrews, 1993; Campbell and Flannigan, 2000) also lead to regionally specific increases in mortality and dieback. Resulting changes in age-class distributions and productivity are likely to have short- and long-term impacts on carbon stocks (Kurz et al., 1995; Fleming, 1996; Hogg, 1997, 1999; Fleming and Candau, 1998).
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