CLIMATIC PATTERNS AND DISTRIBUTION OF C4 GRASSES IN NORTH-AMERICA

A stepwise multiple regression analysis was used to correlate statistically the geographic patterns in the abundance of C4 grasses with patterns in climatic variables. The precent of grasses having the C4 pathway was computed for the total grass flora in 27 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 27 regions. High minimum temperatures during the growing season probably have the strongest correlation with the relative abundance of C4 grass species in a regional flora. The deleterious effects of low temperatures during growth apparently negate the advantages of possessing the C4 pathway in cooler habitats.