Introduction

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.

The combustion of fossil fuels and other human activities, including deforestation and other changes in land use, is driving an imbalance in the global C cycle. Prior to the Industrial Revolution, the concentration of CO2 in the atmosphere was approximately 280 pl l-1, and it was at this level for at least the previous 1000 years (Houghton et al., 1996). The injection of CO2 into the atmosphere by the widespread combustion of fossil fuels currently adds approximately 6.4 Gt C to the

Fossil fuels 6.4 Gt C y-1

Photosynthesis 120 Gt C y-1

Plant respiration 64 Gt C y-1

Net sources of CO2 in the atmosphere (Gt C y-1)

Fossil fuel combustion 6.4

Deforestation 1.6

Total 8.0

Atmospheric increase 3.2

Oceanic increase 2.0

Total 5.2

Defroestation

16 GtC y1 Physiochemical diffusion

Fossil fuels 6.4 Gt C y-1

Plant respiration 64 Gt C y-1

Atmospheric increase 3.2

Oceanic increase 2.0

Total 5.2

Defroestation

16 GtC y1 Physiochemical diffusion

Figure 8.1 The global carbon cycle. Anthropogenic emissions are causing the amount of C in the atmosphere to increase by approximately 3.2 Gt year-1. The cement plant and truck represent anthropogenic fluxes of C to the atmosphere caused by the combustion of fossil fuels during cement production, and the tree stump represents the contribution of changes in land use, primarily deforestation. The mass balance indicates a missing C sink of approximately 2.8 Gt. (Values compiled by K.L. Griffin from Field [2001], Prentice et al. [2001], and Schimel et al. [2001]. Drawing courtesy of K.L. Griffin, Lamont-Doherty Earth Observatory of Columbia University. With permission.)

Figure 8.1 The global carbon cycle. Anthropogenic emissions are causing the amount of C in the atmosphere to increase by approximately 3.2 Gt year-1. The cement plant and truck represent anthropogenic fluxes of C to the atmosphere caused by the combustion of fossil fuels during cement production, and the tree stump represents the contribution of changes in land use, primarily deforestation. The mass balance indicates a missing C sink of approximately 2.8 Gt. (Values compiled by K.L. Griffin from Field [2001], Prentice et al. [2001], and Schimel et al. [2001]. Drawing courtesy of K.L. Griffin, Lamont-Doherty Earth Observatory of Columbia University. With permission.)

atmosphere each year, and deforestation contributes another 1.6 Gt (Figure 8.1). About half of this anthropogenic CO2 remains in the atmosphere. Carbon dioxide is a potent greenhouse gas; along with water vapor, methane and other gases, it maintains the habitable temperatures on Earth, but its further accumulation in the atmosphere is the primary driver of global warming. During 2002, the CO2 concentration in the atmosphere was ~373 pl l1, and it is expected to double from

Figure 8.2 Two estimates of the contribution of forest ecosystems to global net primary production (NPP); black represents forest NPP, fine cross-hatch represents NPP from other terrestrial ecosystems and diagonal hatch represents ocean NPP. Field et al. (1998) combined estimates of radiation-use efficiency with satellite estimates of foliage cover to calculate NPP (global value: 105 Pg C year-1). Whittaker (1975) derived NPP by census and biometric methods (Lieth and Whittaker, 1975) (global value: 85 Pg C year-1, assuming C = 0.5 dry biomass). Tropical savannas were included as forest in both estimates.

Figure 8.2 Two estimates of the contribution of forest ecosystems to global net primary production (NPP); black represents forest NPP, fine cross-hatch represents NPP from other terrestrial ecosystems and diagonal hatch represents ocean NPP. Field et al. (1998) combined estimates of radiation-use efficiency with satellite estimates of foliage cover to calculate NPP (global value: 105 Pg C year-1). Whittaker (1975) derived NPP by census and biometric methods (Lieth and Whittaker, 1975) (global value: 85 Pg C year-1, assuming C = 0.5 dry biomass). Tropical savannas were included as forest in both estimates.

its pre-Industrial level to ~560 pl l1 during the 21st century. Recent estimates suggest that a doubling of atmospheric CO2 will force a 1.4°C to 5.8°C increase in global mean temperature (Houghton et al., 2001); this magnitude of warming is similar to the increase that occurred from the peak of the last ice age, approximately 15,000 years ago. But current climate change is happening over a much shorter time scale, a mere 50 to 100 years, and is far too rapid for many biological and ecological systems to adapt to the change.

The objectives of this chapter are to describe the major components of the terrestrial C cycle with an emphasis on current uncertainties in estimating these components, particularly for forest ecosystems; and, to compare the responses of two contrasting forest ecosystems to an experimental increase in atmospheric CO2.

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