Climatic patterns and the distribution of C₄ grasses in North America

A stepwise multiple regression analysis was used in an attempt to correlate statistically the geographic patterns in the abundance of C₄ grasses with patterns in climatic variables. The percent of grasses having the C₄ pathway was computed for the total grass flora in twenty-seven widely spaced regions of North America. From long-term climatic records seasonal and annual values for solar irradiance, water supply, heat availability, and combinations of these variables were assigned to each of the twentyseven regions. The results of the analysis suggest that high minimum temperatures during the growing season have the strongest correlation with the relative abundance of C₄ grass species in a regional flora. It appears that the deleterious effects of low temperatures during growth negate the advantages of possessing the C₄ pathway in cooler habitats.