Climate and the distribution of C4 grasses along the Atlantic and Pacific coasts of North America

Abstract

In this study, relationships between temperature, precipitation, and the percentage of C4 grasses in local grass floras from the Atlantic and Pacific coastal regions of North America were examined. The proportion of C4 species in a local grass flora increased as latitude decreased on both coasts. At a given latitude, the C4 percentage on the Atlantic coast was higher than the Pacific coast. This difference was related to the Atlantic coast having greater July minimum temperatures than Pacific coast locations of similar latitude. Linear regression analysis showed that the proportion of C4 species in a local flora was positively associated with July minimum temperature on both coasts. The regression line between July minimum temperature and C4 representation was similar for each coast, indicating growth-season temperature has a similar control over C4 presence on the two coasts. Proportionally more of the annual precipitation fell in midsummer on the Atlantic than the Pacific coast, but this difference in the seasonal occurrence of precipitation did not alter the relationship between July minimum temperature and the contribution of C4 grass species to local floras. The Atlantic coast locations with the most precipitation had the highest C4 grass occurrence, indicating aridity alone did not increase the C4 representation in a grass flora. On both coasts, the proportion of NADP-malic enzyme C4 species in local C4 grass floras was positively correlated with mean annual precipitation; however, at equivalent percentages of NADP-malic enzyme subtype occurrence, precipitation levels were substantially lower on the Pacific than Atlantic coast. The trend between latitude and the percentage of C4 species in exotic grass floras was similar to the trend between latitude and the percentage of all C4 grasses in an entire grass flora. Thus, the C4 pathway appears to play no obvious role in enhancing the invasibility of exotic grasses in North America.Key words: bioinvasions, biogeography, C4 photosynthesis, C4 subtype, Poaceae, photosynthesis.