Geographic Variability of the Response of NPP to Environmental Variables

To assess global carbon exchange of the biosphere, one must assess net carbon exchange for each biome on earth. This task is nontrivial, because each of the world's major bio-

mes—tropical, temperate, and boreal forests, savannas, shrublands, grasslands, wetlands, tundra, and deserts—contribute differently to the global carbon budget. Factors to consider when constructing carbon budgets include their (1) respective land area; (2) physiological potential to assimilate and respire CO2; (3) the size and perturbation status of carbon pools; and (4) sensitivity of GPP and R to environmental drivers (e.g., light, temperature, and soil moisture).

Biogeochemical models provide one tool for assessing the net exchange of carbon between the terrestrial biosphere and atmosphere at continental and global scales (Melillo et al. 1993; Cramer et al. 1999). These models account for the diversity and complexity of the natural world by dividing the terrestrial biosphere into broad vegetation classes that are defined by their function, structure, and climate (Bonan et al. 2002). The type and amount of vegetation at a particular location is evaluated either diagnos-tically, using remote sensing information derived from satellites (Running et al. 1999), or prognostically, using dynamic vegetation models (Foley et al. 1998). For a given plant class, photosynthesis and respiration are computed using algorithms that are a function of environmental variables, such as light, temperature, and soil moisture (Cramer et al. 1999). The implementation of these algorithms, however, requires weather or climate data for each unit of the model grid. Furthermore, the models require many assumptions about each unit, its representativeness in space, and its uniformity in time.

Nevertheless, with a validated biogeochemical model, one can quantify how spatial gradients in available sunlight, leaf area index, nitrogen content of leaves, and rainfall impose spatial patterns on annual photosynthesis (Churkina and Running 1998; Law et al. 2002) and how respiration scales with temperature, body size, soil moisture, and net primary productivity (Amthor 2000; Enquist 2002; Reichstein et al. 2002a). On a regional basis, NPP is light and temperature limited in northern climates and biomes, has a low degree of light and water limitation in tropical and humid temperate regions, and is limited by water availability in semi-arid climates (Colorplate 10).

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