The changing role of forests in the global carbon cycle: Responding to elevated carbon dioxide in the atmosphere

Worldwide, forests have an enormous impact on the global C cycle. Of the 760 gigatons (1015 g, Gt) of C in the atmosphere, photosynthesis by terrestrial vegetation removes approximately 120 Gt, almost 16% of the atmospheric pool each year, and about half of this amount (56 Gt) is returned annually by plant respiration (Figure 8.1). The difference between gross canopy photosynthesis and plant respiration (see below) is defined as net primary production (NPP), and represents the annual production of organic matter that is available to consumers. Although estimates vary considerably, forests make up almost half of the global NPP, and approximately 80% of the terrestrial NPP (Figure 8.2). Thus, small changes in the capacity of forests to remove C from the atmosphere by photosynthesis, or return it to the atmosphere by respiration, or store it in wood and soils greatly affect the distribution of C between the terrestrial and atmospheric pool. Because trees use the C3 pathway of photosynthesis, they are very responsive to increases in atmospheric CO2, and it has been hypothesized that a stimulation of photosynthesis and growth of trees may reduce the rate of accumulation of C in the atmosphere derived from fossil fuels. Mounting evidence suggests that a significant portion of the imbalance in the global C cycle, the 2.8 Gt year-1 that is unaccounted for when all known sinks are subtracted from known sources (Figure 8.1), may be explained by additional C uptake in temperate forests (Fan et al., 1998; Pacala et al., 2001; Janssens et al., 2003). How much of this sink is derived from land use change vs. growth enhancement of trees by elevated CO2, nitrogen deposition, and changes in climate remains uncertain.