Spatial patterns of correlation between conspecific species and size diversity in forest ecosystems

Recently correlations between spatial species and size diversity have been found in many forest ecosystems around the world. They are likely to play a prominent role in nature's mechanisms of maintaining species and size diversity. In this study, we analysed the species population means of spatial species-mingling and size-inequality indices in 36 large forest monitoring plots from the temperate and subtropical zones in China. Based on the literature we included eleven diversity-index combinations and considered their correlations for increasing numbers of nearest neighbours. Generally, positive correlations are related to between-species population size differences whilst negative correlations reflect within-species population size differences. Our results showed that the selected species-mingling and size-inequality indices produced different correlation patterns in one and the same monitoring site. We therefore defined a species-mingling size-inequality correlation space by computing the 0.025 and the 0.975 quantiles from the correlation data of the eleven index combinations. We noticed that each observed correlation space included 1–3 combinations of five basic geometric types and can be interpreted as the unique signature of a forest ecosystem in time. The correlation space allowed us to understand more clearly at which spatial scale within-species correlation was more influential than between-species inequality and vice versa. The shape of the correlation space is interpretable and gives important clues about the forest development stage of a forest ecosystem.