Grasses are grouped into "cool season" (C3) and "warm season" (C4) types based on their leaf anatomies and enzymes used to carry out photosynthesis. These differences are important with respect to their optimal growing conditions, N and water-use efficiency, forage quality, and seasonal production profile. C3 species have temperature optima of 15-30 C whereas C4 species have temperature optima of 25-40 C. Although most of the North American grassland biome is dominated by C4 species, C3 species become more important with increasing latitude and elevation because of the cooler climate (Kephart et al. 1995).