The Regional Impacts of Climate Change

	Other reports in this collection

(continued...)

All studies agree that where forests are limited by cold they will expand beyond current limits, especially to the far north. Whether forests will expand into the drier continental interior or contract away from it, however, depends on hydrological factors and remains uncertain. Vegetation distribution models that incorporate a direct physiological CO₂ effect indicate considerable expansion of all forest types into drier and colder areas and much enhanced growth over most areas-under the newer climate scenarios as well as some of the older studies. Under most of the FAR scenarios with a CO₂ effect and under the SAR scenarios without a CO₂ effect, however, forests would contract away from the continental interior because of increased drought stress.

Longer fire seasons and potentially more frequent and larger fires are likely.

Fire mediates rapid change and could increase in importance for vegetation change. Future climate scenarios could result in longer fire seasons and potentially more frequent and larger fires in all forest zones (even those that currently do not support much fire) because of more severe fire weather, changes in fire-management practices, and possible forest decline or dieback (Fosberg, 1990; Flannigan and Van Wagner, 1991; King and Neilson, 1992; Wotton and Flannigan, 1993; Price and Rind, 1994; Fosberg et al., 1996).

Fire suppression during much of the 20th century has allowed biomass in many interior forests to increase by considerable amounts over historic levels (Agee, 1990). With increased biomass, forests transpire almost all available soil water; they become very sensitive to even small variations in drought stress and are very susceptible to catastrophic fire, even without global warming (Neilson et al., 1992; Stocks, 1993; Stocks et al., 1996). Forests in the interior of North America are experiencing increased frequencies of drought stress; pest infestations; and catastrophic, stand-replacing fires (Agee, 1990). This sequence of events is a reasonable analog for what could happen to forests over much larger areas in the zones projected by biogeography models to undergo a loss of biomass or leaf area as a consequence of temperature-induced transpiration increases and drought stress (Annex C, Figures C-6 to C-9; Table 8-3) (Overpeck et al., 1990; King and Neilson, 1992).

Enhanced fire and drought stress will facilitate changes in species composition and may increase atmospheric carbon contributions from forests.

Given the ownership patterns and remote nature of much of the boreal forest lands, they are generally managed as natural systems. Even highly managed temperate forests are of such large extent that a rapid, large-scale management response would be logistically quite difficult and expensive.

On managed lands, harvesting of dead or dying trees, more rapid harvesting or thinning of drought-sensitive trees, and planting of new species could reduce or eliminate species loss or productivity declines. However, identification of which species to plant (and when) under a rapidly changing climate will be difficult management issues. The more rapid the rate of climate change, the more it may strain the ability to create infrastructure for seeding or planting of trees or support the supply of timber if there is a large amount of salvage. The fast rate of warming may limit some species that have slow dispersal rates or are constrained by human barriers, habitat fragmentation, or lack of suitable habitat-or already are stressed by pollution.

As fire-management agencies operate with increasingly constrained budgets, it is likely that any increases in fire frequency or severity will result in a disproportionately large increase in area burned (Stocks, 1993). More and larger boreal fires will result in a reevaluation of protection priorities, with likely increased protection of smaller, high-value areas and reduced protection over large expanses. If forests die back from drought, infestations, or fire in extensive, remote regions, the impacts could include large-scale changes in nutrient cycling and carbon sequestration, as well as loss of value for future timber harvests or as habitat for wildlife and biodiversity. Some adaptive practices-such as harvesting dead or dying trees or thinning-could impact biodiversity, soil erosion, stream quality, and nonmarket forest products, producing potentially conflicting management options.

Markham and Malcolm (1996) have concluded that ecological resiliency can be increased by conserving biological diversity, reducing fragmentation and degradation of habitat, increasing functional connectivity among habitat fragments, and reducing anthropogenic environmental stresses. They also indicate that adaptation strategies should include redundancy of ecological reserves, reserves with much structural heterogeneity, and the flexibility to spatially relocate habitat protection depending on shifts in future climate (Peters and Darling, 1985; McNeely, 1990). Current habitat fragmentation patterns and human barriers may hinder species migration. Thus, management of the "seminatural matrix" may play an increasing role in fostering species redistribution (Peters and Darling, 1985; Bennett et al., 1991; Franklin et al., 1991; Parsons, 1991; Simberloff et al., 1992).