Autumn Rains GPP(+), R ,co (+)
Figure 15.6. Seasonal variation of NEP of Mediterranean/subtropical woodland ecosystems and the environmental drivers that perturb the carbon fluxes. Data sources: Leuning and Attiwill (1978); Valentini et al. (1996); Eamus et al. (2001); Reichstein et al. (2002a); unpublished data from Joao Periera (Fluxnet data archive, http://www.daac.ornl .gov/FLUXNET/) and from Baldocchi and Xu (own data and data from AmeriFlux archive).
For Mediterranean-type grasslands, frost limits GPP during the winter growing season (Figure 15.7). Rapid growth occurs after the last frosts and ceases when the soil water profile is depleted and the plants die. On a year-to-year basis, timing of cessation of winter rains sets the end of the growing season. Rain pulses during the summer stimulate huge, short-term rates of respiration, as the plants are dead and GPP is zero (Xu and Baldocchi 2003). Seed germination and a new growing season commence with the arrival of the autumnal rains. In ecosystems with low NEE, such as Mediterranean grasslands, a few large pulses of soil respiration have the potential to change the ecosystem from being a sink of carbon to a source (Xu and Baldocchi
Perennial temperate grasslands are dormant during the winter, when they are often snow covered. During this period, material in the grass canopy is dead and the system
• Mediterranean Grassland O Temperate C4 Grassland
Figure 15.7. Seasonal variation in net CO2 exchange of a Mediterranean annual grassland and a temperate continental C4 grassland. Also shown are the environmental and biological factors that affect the seasonal dynamics of CO2 exchange. Data sources: Valentini et al. (1996); Kim et al. (1992); Saigusa et al. (1998); Meyers (2001); Sukyer and Verma (2001); Flanagan et al. (2002); Xu and Baldocchi (2003).
is respiring. During spring, the grass begins to grow again. Initially, rates of photosynthesis are low, because photons are intercepted by living and dead material. Consequently, the slope of the light response curve changes markedly with leaf area index (LAI) (Suyker and Verma 2001; Flanagan et al. 2002; Xu and Baldocchi 2003). NEE saturates with high sunlight when LAI is low and becomes a quasi-linear function of sunlight as LAI increases and the canopy closes. Summer drought also has a marked effect on NEE, limiting photosynthesis and respiration (Kim et al. 1992).
In general, peak rates of CO2 assimilation are greater for C4 grasslands than for C3 grasslands, and they are higher for grasslands with a summer growing season than those with a winter/spring growing season (Figure 15.7).
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