Three broad forest types are recognized in this assessment of North American forests: boreal, temperate evergreen, and temperate mixed forests. The boreal forest (Annex C, Figure C-1) is constrained by cold temperatures to the north that limit tree height and reproduction (Lenihan and Neilson, 1993; Starfield and Chapin, 1996). The southern limits of the boreal forest generally are defined by their juxtaposition with temperate forests or with interior savanna-woodlands and grasslands. Boreal tree species generally are not limited from growing further south. Rather, temperate hardwoods and conifers are limited by cold temperatures from spreading further north; where temperate species can flourish, they outcompete boreal species. Fire and herbivore browsing also are important constraints on forest distribution and species composition (Bergeron and Dansereau, 1993; Landhauser and Wein, 1993; Suffling, 1995; Starfield and Chapin, 1996). Wildfire and insect outbreaks limit forest productivity and can produce considerable mortality: Annual tree mortality losses from insect outbreaks in Canada are about 1.5 times the losses from wildfire and amount to about one-third of the annual harvest volume (Fleming and Volney, 1995). Annual losses from insects and fire in the United States also are about one-third of the annual harvest (Powell et al., 1993). Warming-induced changes in the timing of spring frosts may be important in ending or prolonging outbreaks. Increased drought stress also may enhance insect outbreaks, and changes in climate could extend the ranges of some insects and diseases.
Temperate evergreen forests (Annex C), such as in the Pacific NorthWest, tend to occur in areas that are warm enough for photosynthesis during the cool parts of the year but often are too cold for deciduous species to fix sufficient carbon during the frost-free season (Woodward, 1987). Areas with dry summers also tend to favor conifers or hardwoods with water-conserving leaves (Waring and Schlesinger, 1985; Neilson, 1995). Summer drought and winter chilling for frost hardiness are critical climate factors, rendering these forests sensitive to global warming (Franklin et al., 1991). NorthWest conifers are long lived and need only successfully reproduce once during their life span for population sustainability (Stage and Ferguson, 1982; Parker, 1986; Savage et al., 1996). With global warming, however, establishment periods could become rare in some areas; after harvest, some forests may not be able to regenerate, even if mature trees could still survive the climate. Winter chilling may be required for adequate seed set or to confer frost-hardiness in some species (Burton and Cumming, 1995); because of the well-recognized spatial variation in the genetics of these species, however, such chilling requirements may not hold everywhere. Fire suppression in interior pine forests has left them in a sensitive condition with respect to drought, fire, and pests (Agee, 1990; Sedjo, 1991). Climate change could exacerbate all of these stressors (Williams and Liebhold, 1995). For example, increased drought stress could facilitate insect outbreaks; drought and infestation could lead to more fuel, increasing the risk of catastrophic fire.
Temperate mixed forests (mixed hardwood and conifer) are bound by cold temperatures to the north and subtropical dry regions to the south (the Caribbean coast in North America) and tend to occur in areas that are wet in both winter and summer. Temperate hardwood species also benefit from cold-hardening; with warmer winter conditions and less insulating snow cover, they can be sensitive to spring frost damage, which can kill roots and further sensitize the species to drought stress and widespread mortality (Auclair et al., 1996; Kramer et al., 1996). Southeastern U.S. pines within this type are among the most important commercial species on the continent. Natural southeastern pine stands historically relied on fire to maintain their composition (Komarek, 1974; Sedjo, 1991) but now are largely controlled by harvest. Compared to northWestern forests, southeastern conifers have a short rotation, which might confer more rapid adaptive capability through establishment of new genotypes.
Elevated CO2 affects the physiology of trees, possibly increasing productivity, nitrogen-use efficiency and WUE (reduced transpiration per carbon fixed, conferring some drought resistance), and other responses (Bazzaz et al., 1996; IPCC 1996, WG II, Section A.2.3). A review of 58 studies indicated an average 32% increase in plant dry mass under a doubling of CO2 concentration (Wullschleger et al., 1995). Norby (1996) documented an average 29% increase in annual growth per unit leaf area in seven broadleaf tree species under 2xCO2 scenarios over a wide range of conditions. WUE, examined in another review and indexed by reductions of leaf conductance to water vapor, increased about 30-40% (Eamus, 1991). If such responses were maintained in forests over many decades, they would imply a substantial potential for increased storage of atmospheric carbon, as well as conferring some increased tolerance to drought. However, some species or ecosystems exhibit acclimation to elevated CO2 by downregulating photosynthesis (Bazzaz, 1990; Grulke et al., 1990; Grulke et al., 1993); others do not exhibit acclimation (Bazzaz, 1990; Teskey, 1997). Understanding the sources of large uncertainties in the linkages between forest physiology and site water balance is a research need; no two models simulate these complex processes in the same way.
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