Abstract: Periodic climatic oscillations and species dispersal during the post-glacial period are two important causes of plant assemblage and distribution on the Qinghai-Tibet Plateau (QTP). To improve our understanding of the bio-geological histories of shrub communities on the QTP, we tested two hypotheses. First, the intensity of climatic oscillations played a filtering role during community structuring. Second, species dispersal during the post-glacial period contributed to the recovery of species and phylogenetic diversity and the emergence of phylogenetic overdispersion. To test these hypotheses, we investigated and compared the shrub communities in the alpine and desert habitats of the northeastern QTP. Notably, we observed higher levels of species and phylogenetic diversity in the alpine habitat than in the desert habitat, leading to phylogenetic overdispersion in the alpine shrub communities versus phylogenetic clustering in the desert shrub communities. This phylogenetic overdispersion increased with greater climate anomalies. These results suggest that 1) although climate anomalies strongly affect shrub communities, these phenomena do not act as a filter for shrub community structuring, and 2) species dispersal increases phylogenetic diversity and overdispersion in a community. Moreover, our investigation of the phylogenetic community composition revealed a larger number of plant clades in the alpine shrub communities than in the desert shrub communities, which provided insights into plant clade-level differences in the phylogenetic structures of alpine and desert shrub communities in the northeastern QTP.

We surveyed 20 sampling sites from the desert habitat and 41 from the alpine habitat with little or no disturbance (total = 61; Figure 1). Three 10-m × 10-m subplots were established at each sampling site to investigate the species composition of the community, identify species and collect plant specimens. Eventually, a combination of the species found in the three subplots was used to represent the species composition of the community at each site.

We constructed a phylogenetic tree to present the evolutionary relationships between the 285 species found at our sampling sites. As in many previous studies, we used the megatree containing 32,223 plant species published by Zanne et al. (2014) as a backbone, and used the Phylomatic platform to prune this megatree to include only the species found at our sampling sites.

To identify differences in phylogenetic diversity, we used the PD index. We assessed differences in the phylogenetic community structures of shrub communities between the two habitats using the net relatedness index (NRI) and nearest taxon index (NTI) based on this dataset. Specific method seen in picante package.