5.6.3.1.1. Forest and species distribution
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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