What causes geographical variation in the species-area relationships? A test from forests in China

The increase of species richness with sampling area and the decrease with latitude and altitude are two of the most frequently studied patterns in biogeography. However, few studies have simultaneously examined these two patterns to investigate how speciesarea relationships (SAR) vary with latitude and altitude. In this study, we explore the spatial patterns of SAR in forests in China by investigating numbers of species by life form group (trees, shrubs and herbs) in 32 nested plots from 12 mountains ranging from 18.7 degrees N to 51.9 degrees N in latitude and from 300 to 3150 m in altitude. The slopes of the power law SAR (z-values) decreased with increasing latitude for all life forms except herbaceous plants, and also decreased with increasing altitude for all life forms but not for shrubs. Latitude and altitude, as well as their interactions, together explained 65.4, 61.8, 48.9 and 45.3% of the variation in z-values for overall species, trees, shrubs and herbaceous plants, respectively. In addition, actual evapotranspiration affected SAR significantly, but this effect varied significantly among life forms. We concluded that there are significant geographical patterns of SAR for China's forests, which is primarily controlled by energy availability.