(continued...)

Most of the early CO₂ research was done on juvenile trees in pots and growth chambers, which may limit the usefulness of some conclusions. New research is beginning to emerge that focuses on larger trees or intact forested ecosystems. Recent reviews of this newer literature (Curtis, 1996; Eamus, 1996a) indicate that acclimation may not be as prevalent when roots are unconstrained; that leaf conductance may not be reduced; and that both responses depend on the experimental conditions, the length of exposure, and the degree of nutrient or water stress. These results imply that forests could produce more leaf area under elevated CO₂ but may not gain a benefit from increased WUE. In fact, with increased leaf area, transpiration should increase on a per-tree basis, and the stand would use more water (Eamus, 1996a). Elevated temperatures would increase transpiration even further, perhaps drying the soil and inducing a drought effect on the ecosystem (Eamus, 1996a).

Nitrogen supply is prominent among the environmental influences that are thought to moderate long-term responses to elevated CO₂ (Kirschbaum et al., 1994; McGuire et al., 1995; Eamus, 1996b). Unless CO₂ stimulates an increase in nitrogen mineralization (Curtis et al., 1995; VEMAP Members, 1995), productivity gains in high CO₂ are likely to be constrained by the system's nitrogen budget (Körner, 1995). Increased leaf area production is a common CO₂ response; however, nitrogen limitations may confine carbon gains to structural tissue rather than leaves (Curtis et al., 1995). Thus, in areas receiving large amounts of nitrogen deposition, a direct CO₂ response could result in large increases in leaf area, increasing transpiration and possibly increasing sensitivity to drought via rapid soil-water depletion. Early growth increases may disappear as the system approaches carrying capacity as limited by water or nutrients (Körner, 1995). Shifts in species composition will likely result from different sensitivities to elevated CO₂ (Körner, 1995; Bazzaz et al., 1996).

North American forests also are being subjected to numerous other stresses, including deposition of nitrogen and sulfur compounds and tropospheric ozone, primarily in eastern North America (Lovett, 1994). The interactions of these multiple stresses with elevated CO₂ and climate change and with large pest infestations (of, for example, the balsam wooly adelgid, gypsy moth, spruce budworm, and others) are very difficult to predict; however, many efforts are under way to address these questions (Mattson and Haack, 1987; Loehle, 1988; Fajer et al., 1989; Taylor et al., 1994; Winner, 1994; Williams and Liebhold, 1995). Anthropogenic nitrogen fixation, for example, now far exceeds natural nitrogen fixation (Vitousek, 1994). Atmospheric nitrogen deposition has likely caused considerable accumulation of carbon in the biosphere since the last century (Vitousek, 1994; Townsend et al., 1996). However, nitrogen saturation in soils also can be deleterious, possibly causing forest dieback in some systems (Foster et al., 1997). Tropospheric ozone also can damage trees, causing improper stomatal function, root death, membrane leakage, and altered susceptibility to diseases (Manning and Tiedemann, 1995). Such ozone-induced changes can render trees more sensitive to warming-induced drought stress (McLaughlin and Downing, 1995). There are many other stress interactions, and researchers think that, in general, multiple stresses will act synergistically, accelerating change due to other stresses (Oppenheimer, 1989).

Assessments of possible consequences of climate change rely on linked atmospheric, ecological, and economic models. Significant uncertainties are associated with each model type, and these uncertainties may amplify as one moves down the line of linked models. The model capabilities of GCMs have improved significantly from the older (IPCC 1990, WG I, Chapter 3) to the newer (IPCC 1996, WG I, Chapter 6) scenarios, resulting in somewhat lower estimates of the potential 2xCO₂ climate sensitivity and shifting much of the burden of uncertainty to the ecological and economic models. Ecological models still carry large uncertainties in the simulation of site water balance (among many other issues), particularly with respect to the role of elevated CO₂ on plant responses to water stress, competition, and nutrient limitations. Economic models carry uncertainties with respect to future management and technology changes, future per-capita income and available capital, GDP, international trade, and how to couple land-use management with ecological model output, among others. Ecological and economic models are rapidly being enhanced to narrow these uncertainties; improving the linkages between the many different model types necessary to permit fully time-dependent simulations for integrated regional assessments is an ongoing research need.

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