Climate and the US distribution of C4 grass subfamilies and decarboxylation variants of C4 photosynthesis

I compared the C-4 Eras flora and climatic records for 32 sires in the United States. Consistent with previous studies. I found that the proportion of the grass flora that uses the NADP malic enzyme (NADP-ME) variant of C-4 photosynthesis greatly increases with increasing annual precipitation, while the proportion using the NAD malic enzyme (NAD-ME) variant (and also the less common phosphoenolpyruvate carboxykinase [PCK] variant) decreases. However the association of grass subfamilies with annual precipitation was even stronger than fur the C-4 decarboxylation variants. Analysis of the patterns of distribution by partial correlation analysis showed that the correlations between the frequency of various C-4 types and rainfall were solely due to the association of the C-4 types with particular grass subfamilies. In contrast, there was a strong correlation of the frequency of the different subfamilies with annual precipitation that was independent of the influence of the different C-4 variants. It therefore appears that other, as yet unidentified, characteristics that differ among grass subfamilies may be responsible for their differences in distribution across natural precipitation gradients.