At least early in stand development, loblolly pine and sweet-gum forests on nutrient-deficient soils and experiencing the full suite of biological interactions and variation in the environment have considerable capacity to respond to changes in atmospheric CO2 derived from the combustion of fossil fuels. The experimental simulation of plus 200 pl l-1 CO2 caused an additional 174 g C m-2 to be stored in the pine forest, representing a 41% stimulation of NEP, and an additional 111 g C m-2 to be stored in the sweetgum forest, representing a 28%
stimulation of NEP (Table 8.1). Although there is no evidence yet that the observed stimulation of forest productivity is systematically declining with time, there is considerable interannual variation in its absolute magnitude and enhancement. Net primary production in the pine forest was greater in years with more precipitation than in dry years, and this response to precipitation was not altered by elevated CO2. In contrast, elevated CO2 lessened somewhat the reduction in NPP observed at elevated temperature (Figure 8.4). Forest productivity is likely to be stimulated by increasing atmospheric CO2, at least early in stand development, and for pine forests this increase will be greater in warm than in cool years.
Respiratory fluxes return large quantities of C to the atmosphere and are important determinants of the total C sequestration in ecosystems, yet the magnitude and regulation of these fluxes remains poorly understood. In the pine and sweetgum stands, Ra alone returned 50% to 72% of GPP to the atmosphere and greater Ra in the sweetgum forest than in the pine forest may explain its lower NPP (Table 8.1). Further research on the different components of respiration, with an emphasis on understanding seasonal variation in its temperature dependence and the interaction of temperature dependence with the rate of substrate supply, will greatly enhance our understanding the forest C cycles.
Differences in how forests respond to elevated CO2 will alter their capacity to store additional C. In the pine forest exposed to elevated CO2 additional C was allocated to boles and branches, whereas the sweetgum forest responded with a disproportional increase in fine root production. These tissues have profoundly different mean residence times potentially altering the duration of C storage. The residence time of C is longer in wood than in fine roots suggesting that the pine forest offers a longer-term storage of atmospheric C than the sweetgum forest. This statement must be tempered by our lack of understanding of the fate of C derived from decomposition of sweetgum roots. Insofar as this C becomes incorporated in recalcitrant soil organic matter, its residence time in the soil can be greatly extended.
The imbalance between the rate of N supply and utilization in the pine forest (Finzi et al., 2002) suggests that the stimulation of productivity by elevated CO2 would be shortlived; 7 years or less, the duration of these experiments, admittedly is a small fraction of the "life" of these forests and an abatement in the growth response may appear in the future. The stimulation of productivity observed in these experiments may provide a short-term benefit to the forest products industry and slow the rate of increase of CO2 in the atmosphere; however, faced with such an enormous injection of C into the atmosphere, even if sustained, these CO2-induced stimulations in productivity are far from sufficient to reverse the accumulation of C in the atmosphere (DeLucia et al., 1999; Hamilton et al., 2002).
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