Forests have many pressures acting on them that result in changes to their structure and composition, as well as their function (see Figure 5-1). These structural changes, in turn, alter the function of forests in the physical climate system (Sellers et al., 1990; Apps, 1993).

5.6.2.1. Climate Variability and Climate Change

Changes in climatic conditions affect all productivity indicators of forests (NPP, NEP, and NBP; see Box 5-1) and their ability to supply goods and services to human economies. The effects on forested area and forest productivity, however, vary from location to location, with gains in some regions and losses in others. Furthermore, the impacts vary among different measures of ecosystem productivity. For example, in boreal and alpine forests—where short growing seasons and heat sums are limiting factors to growth—NPP of many forest stands may increase with increasing temperature (Bugmann, 1997; but see Barber et al., 2000), whereas NEP decreases as a result of increased decomposition (Schimel et al., 1994; Valentini et al., 2000; but see Giardina and Ryan, 2000). If higher temperatures lead to summer drought, even NPP may decrease as a result of lowered photosynthetic rates associated with reduced stomatal conductance (Sellers et al., 1997), exacerbating the decrease in NEP from decomposition. If drier conditions also result in increased fires, biomass and soil carbon losses may result in negative NBP (Wirth et al., 1999; Apps et al., 2000).

Projected changes in forest area, structure, NPP, and NEP as a result of climate change vary by forest type and biome (Neilson et al., 1998). Climate change also is likely to include changes in seasonality (Myneni et al., 1997), timing of freeze-thaw patterns (Goulden et al., 1998), the length of the growing season, nutrient feedbacks (Tian et al., 1998), disturbance regimes (Kurz et al., 1995), and diurnal temperature patterns (Clark and Clark, 1999). Changes in intra-annual variation that fall outside the historical norm for a particular region also may have catastrophic effects—for example, through local climatic extremes or through late and early frost (Repo et al., 1996; Ogren et al., 1997; Colombo, 1998). These factors are likely to influence the distributional range of some tree species (Macdonald et al., 1998; Rehfeldt et al., 1999a). Changes in precipitation may not have immediate effects on mature and old-growth forests, which have well-established root systems, but are likely to have pronounced effects on regeneration success for some species following disturbance, such as harvest or fire (Hogg and Schwarz, 1997; Price et al., 1999a,b).

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