The current distributions of C4 plants within grassland ecosystems at an atmospheric C02 level of 350 ppmV are well predicted by the quantum-yield model (Fig. 1). Across the Great Plains of North America, the crossover between C3- and C4-dominated grasslands is predicted to occur at a latitude of approximately 45°N (Figure 7). Both long-term aboveground harvest studies (Epstein et al, 1997) and belowground soil organic carbon studies (Tieszen et al, 1997) independently indicate a C3/C4 transition near 45°N. In the case of C3/C4 grasses from the Great Plains as well as all other monocot studies, the relationships between C3 and C4 grass abundances were all very highly correlated with temperature (Ehleringer et al, 1997). In most of these studies, >90% of the variance in C3 /C4 abundance in today's ecosystems is explained by temperature alone.
Collatz et al (1998) extended predictions of the quantum-yield model to the global scale (Fig. 8). Their model predicted that C4 abundances are expected in all geographical regions where the monthly mean temperature exceeds 22°C (the crossover temperature) and where precipitation exceeds 25 mm (i.e., the soil must be wet for plants to grow). This model predicts a much broader distributional range for C4 taxa than is observed for undisturbed ecosystems, with C4 taxa extending into currently forested regions of tropical and subtropical latitudes. However, when the competitive advantage of tree height is factored in, the Collatz et al extrapolation correctly predicts the observed C3/C4-grass abundances on a global basis (Fig. 8).
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