Linking Tree and Shrub Biomass to Plant Diversity During Early-Stage Alpine Afforestation.

Afforestation has considerable potential to restore and maintain plant diversity, which is closely associated with ecosystem functions and services. However, there remain numerous uncertainties regarding alpine afforestation performance. Hence, it is necessary to determine the factors contributing to plant diversity during the early stages of afforestation in alpine regions. Our main objective of this study was to examine the effects of tree and shrub biomass on plant diversity, namely, species (SD) and phylogenetic (PD) diversity, during the early period of afforestation. We used a general linear mixed model (GLMM) to determine the associations between tree and shrub biomass and multiple indices (Shannon, Simpson's diversity, species richness, Pielou's evenness, Faith's phylogenetic diversity, and net relatedness) at two spatial scales (1 × 1 m and 10 × 10 m), based on 78 10 × 10 m sites, each containing five 1 × 1 m plots. On the basis of the GLMM results, we established that both tree and shrub biomass had significant effects on plant diversity at the site and plot scales, and found that the responses of SD and PD to tree and shrub biomass were non-linear. Overall, whereas at the site scale, SD (i.e., Pielou's evenness index) was highest at a median level of tree biomass, there were negative relationships between shrub biomass and the Shannon index, species richness, and Faith's PD at the plot scale. The effects of biomass on SD and PD were found to be dependent on different spatial scales (i.e., plot and site) and life form (i.e., tree or shrub), thereby providing further evidence regarding the efficacy of ecological restoration subsequent to alpine afforestation. The findings of this study will provide a novel empirical reference for optimal tree species selection and planning for planting practices in alpine regions.