Climate and the distribution of C-4 grasses along the Atlantic and Pacificcoasts of North America

In this study, relationships between temperature, precipitation, and the pe rcentage of C-4 grasses in local grass floras from the Atlantic and Pacific coastal regions of North America were examined. The proportion of C-4 spec ies in a local grass flora increased as latitude decreased on both coasts. At a given latitude, the C-4 percentage on the Atlantic coast was higher th an the Pacific coast. This difference was related to the Atlantic coast hav ing greater July minimum temperatures than Pacific coast locations of simil ar latitude. Linear regression analysis showed that the proportion of C-4 s pecies in a local flora was positively associated with July minimum tempera ture on both coasts. The regression line between July minimum temperature a nd C-4 representation was similar for each coast, indicating growth-season temperature has a similar control over C-4 presence on the two coasts. Prop ortionally more of the annual precipitation fell in midsummer on the Atlant ic than the Pacific coast, but this difference in the seasonal occurrence o f precipitation did not alter the relationship between July minimum tempera ture and the contribution of C-4 grass species to local floras. The Atlanti c coast locations with the most precipitation had the highest C-4 grass occ urrence, indicating aridity alone did not increase the C-4 representation i n a grass flora. On both coasts, the proportion of NADP-malic enzyme C-4 sp ecies in local C-4 grass floras was positively correlated with mean annual precipitation; however, at equivalent percentages of NADP-malic enzyme subt ype occurrence, precipitation levels were substantially lower on the Pacifi c than Atlantic coast. The trend between latitude and the percentage of C-4 species in exotic grass floras was similar to the trend between latitude a nd the percentage of all C-4 grasses in an entire grass flora. Thus, the C- 4 pathway appears to play no obvious role in enhancing the invasibility of exotic grasses in North America.