Relative abundance of plant functional types in grasslands and shrublands of north America

We analyzed the geographic distribution and climatic controls of the distribution of plant functional types (PFT) in temperate grasslands and shrublands of North America. It has been widely accepted that temperature is the principal control on the distribution of C-3 and C-4 species. Our results show that precipitation and its seasonal distribution are also important. C-4 grass distribution was positively related to three climatic variables: mean annual precipitation. mean annual temperature, and the proportion of the precipitation falling in summer. These variables accounted for 66% of the total variability of this functional type. C-3 grass abundance decreased with mean annual temperature and increased with the proportion of the precipitation falling during winter (r(2) = 0.37). Sixty-two percent of the variability in the relative abundance of shrubs was explained by mean annual precipitation and the proportion of winter precipitation. Latitude and longitude explained a substantial portion of the variability of the distribution of the relative abundance of shrubs, C-3 grasses, and C-4 grasses (53, 46, and 61%, respectively). Along a given longitude, C-3 grasses increased with latitude. As one moves westward, C-4 grasses are replaced by shrubs. The relative abundance of C-4 grasses reached a maximum at southern latitudes and eastern longitudes. Succulents showed a marginal decreasing trend with latitude. No relationship with geographic variables was detected for forbs.