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Some evidence suggests that insect populations already are responding to climate change (Fleming and Tatchell, 1995). In general, current forecasts of the response of forest insects and other pathogens to climate change are based on historical relationships between outbreak patterns and climate. These forecasts suggest more frequent or longer outbreaks (Thomson and Shrimpton, 1984; Thomson et al., 1984; Mattson and Haack, 1987; Volney and McCullough, 1994; Carroll et al., 1995; Cerezke and Volney, 1995; Brasier, 1996; Roland et al., 1998). Outbreaks also may involve range shifts northward, poleward, or to higher elevations (Williams and Liebhold, 1997). All of these responses will tend to reduce forest productivity and carbon stocks, although the quantitative extent of these changes is hard to predict (see Box 5-10).
At the time of the SAR, no experiments on intact forest ecosystems exposed to elevated CO2 had been performed. Since then, several FACE experiments have been implemented and are beginning to show interesting results. In a 13-year-old loblolly pine plantation (North Carolina), CO2 levels have been maintained at 200 ppm above ambient. After 2 years, the growth rate of the dominant trees increased by about 26% relative to trees under ambient conditions (DeLucia et al., 1999). Litterfall and fine root increment also increased under the CO2-enriched conditions. Total NPP increased by 25%. The study concludes, however, that stimulation is expected to saturate not only because each forest stand tends toward its maximum carrying capacity (limited by nutrient capital) but also because plants may become acclimated to increased CO2 levels.
Research on CO2 fertilization, however, has taken place only over a short fraction of the forest ecosystem's life cycle. Questions of saturation of response can be addressed through longer term experiments on tree species grown under elevated CO2 in open-top chambers under field conditions over several growing seasons (Norby et al., 1999). A review of such experiments by Norby et al. (1999) found that the evidence shows continued and consistent stimulation of photosynthesis, with little evidence of long-term loss of sensitivity to CO2; the relative effect on aboveground dry mass was highly variable but greater than indicated by seedling studies, and the annual increase in wood mass per unit of leaf area increased. Norby et al. (1999) also found that leaf nitrogen concentrations were lower in CO2-enriched trees, but not as low as seedling studies indicated, and the leaf litter C/N ratio did not increase. In the majority of CO2 chamber experiments, the decrease in the percentage of nitrogen in litter was matched by an increase in the percentage of lignin. Moore et al. (1999) have suggested, however, that lower litter quality caused by CO2 fertilization may have offset the expected temperature-induced increase in decomposition. A longer term perspective still is needed because long-term trends cannot be extrapolated directly from relatively short-term experiments on individual trees (Idso, 1999). Field experiments on elevated CO2 provide inconclusive evidence at this time to predict overall changes in carbon storage in forests.
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