Water–energy dynamics are often correlated with geographical patterns of terrestrial plant richness. However, the relative importance of water and energy on species richness is still being debated. Some studies suggest a transition in the relative importance of water and energy along a latitudinal gradient, i.e. that water is the most important factor at low latitudes, whereas energy is the leading factor at high latitudes. The generality of this transition is yet to be established and is the focus of the current study. In particular, we examine whether differences in the evolutionary histories of two subgenera of Asian oaks influence the derived models of water–energy dynamics.
Asia (70–140° E; 10–60° N).
We performed geographically weighted regressions to quantify geographical variation in the relative importance of water and energy on the species richness of Asian oaks (Quercus L.). We also evaluated how evolutionary history influences the relative importance of water and energy in determining species richness, by comparing patterns in the two oak subgenera: Quercus subg. Quercus and Quercus subg. Cyclobalanopsis. In particular, we assessed how the ancestral climatic niche of these two subgenera (i.e. subgenus Quercus of temperate origin versus subgenus Cyclobalanopsis of tropical origin) relates to the relative importance of water and energy on contemporary diversity patterns.
We found no geographical transition line in the relative importance of water and energy along the latitudinal gradient studied. Instead, we found that the importance of energy relative to water on the species richness of subgenus Quercus increased from mid-latitudes (warm temperate regions) towards both tropical and boreal regions: this pattern might reflect that this subgenus originated in the temperate region. In contrast, the importance of both water and energy on the species richness of the tropical subgenus Cyclobalanopsis increased from tropical to boreal regions, probably due to a lack of adaptation to winter coldness.
Our results suggest that differences in the evolutionary history – specifically the ancestral climatic niche – of the two subgenera of oaks influence the effects of water–energy dynamics on species richness along a latitudinal gradient through niche conservatism.|