Effects of geographical extent on the determinants of woody plant diversity

Despite a long history of study, the mechanisms underlying the geographical patterns of species richness are still controversial. Patterns and determinants of species richness are well-known to vary with spatial scale. However, most studies on the effects of scale have focused on grain size whereas the quantitative effects of geographical extent are rarely tested. Here, using distribution maps of 11 405 woody species found in China and associated environmental data to the domain, we investigated the influence of geographical extent on the determinants of species richness patterns. Our results revealed consistent extent dependence of all species, narrow- and wide-ranged species: with the expansion of geographical extents, the explanatory power of climate (i.e. environmental energy, water availability and climatic seasonality) increased, while the explanatory power of habitat heterogeneity and human activities decreased. Although the primary determinant of species richness patterns varied significantly at small to meso-geographical extent, we showed that species richness was predominantly determined by environmental energy at large extent. Our findings indicate that differences in geographical extent may have led to the controversies regarding the primary determinants of richness patterns in previous studies, and that a multi-scale perspective not only with regard to grain-size but also extent is likely to shed new light on this old debate of what determines richness patterns.