The implications of a reduced quantum yield (initial slope of the photosynthetic light response curve) in C4 plants and temperature dependence of quantum yield in C3 plants on total canopy primary production were investigated using computer simulations. Since reduced quantum yield represents the only known disadvantage of the C4 photosynthetic pathway, simulations were conducted with grass canopies (high LAI and hence photosynthesis in most leaves will be light-limited) to see if quantum yield is a significant factor in limiting the primary production and thus distributions of C4 grasses. Simulations were performed for three biogeographical or environmental conditions: the Great Plains region of North America, the Sonoran Desert of North America, and shade habitats. For all three cases, the simulations predicted either spatial or temporal gradients in the abundances of C4 grasses identical to the abundance patterns of C4 grasses observed in the field. It is thus concluded that while the C4 photosynthetic mechanism may be highly advantageous in specific environments, it may be disadvantageous in others.