Spatial size diversity in natural and planted forest ecosystems: Revisiting and extending the concept of spatial size inequality

Abstract Plant size diversity or plant size inequality is a much neglected aspect of plant diversity and biodiversity. And yet size diversity plays an important role in understanding the dynamics of plant diversity and natural mechanisms of maintaining plant diversity, which is of great significance in a time of ongoing global changes. In this study, we introduced and investigated a new measure of spatial size inequality, namely the size segregation function. The new characteristic depends on distance and turned out to be a meaningful and more general extension of the traditional size differentiation index, because it describes local size inequality more extensively. To test the size segregation function we analysed and modelled thirteen large, fully mapped plots from temperate and subtropical forest ecosystems in China. We tested and validated the new function through cluster analysis and a special variant of partial spatial reconstruction. Our results showed that the size segregation function discriminated well between different patterns of spatial size diversity in the complex, mostly natural forests of China. Two random-labelling tests used in the study also highlighted how closely linked spatial species and size diversity are in such species-rich forest ecosystems: Often each species represents a distinctive size range and then spatial size inequality can be interpreted as being a consequence of spatial species mingling. This close relationship between species and size should also be taken into account in analyses of species-rich plant populations. For practical conservation this implies that high size inequality can often be found where there is high species mingling and vice versa.