The Geographic Distribution of C4Species of the Dicotyledonae in Relation to Climate

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Abstract
The geographic distribution of C4 species in the Dicotyledonae of North America was examined in relation to climatic parameters in an attempt to identify environmental conditions which favor the presence of C4 species. The percentage of C3 dicot species in the spermatophyte flora in a geographic region is best predicted by a combination of summer pan evaporation and dryness ratio. Both of these variables are closely related to plant water balance, supporting previous contentions that the C4 pathway arose in the Dicotyledonae as an adaptation to arid conditions. The distribution of the C3 species in the C4 dicot families is also strongly correlated with summer pan evaporation. There appear to exist adaptive properties of these families, in addition to the type of photosynthetic pathway, which favor their presence in arid regions. The climatic correlations of C4 dicot species differ from those of the C4 species of the Gramineae, in which minimum daily temperatures are most important. Evolution of the C4 photosynthetic pathway may confer different ecological attributes to plant taxa in different phylogenetic groups.