Abstract: Shrublands are one of the most important ecosystems in Taihang Mountain region of China. Shrub communities plan an important role in maintaining biodiversity, providing ecosystem services, and promoting ecological security in the Beijing-Tianjin-Hebei region. We explored differences in community assembly and associated environmental factors for two of the most representative shrub communities in the region, Vitex negundo var. heterophylla and Spiraea trilobata, by assessing their net relatedness index (NRI) and using multiple regression. We further used principal coordinates of phylogenetic structure (PCPS) to determine the key phylogenetic nodes giving rise to the community phylogenetic structure. We found that the phylogenetic structures of these two shrub communities were significantly different despite no detectable difference in their species diversity. The S. trilobata community showed significant phylogenetic overdispersion, whereas the V. negundo var. heterophylla community showed no significant phylogenetic structure, despite exhibiting a greater degree of clustering degree than S. trilobata community. Community assembly of S. trilobata shrubs was driven by niche differentiation. By contrast, habitat filtering together with niche differentiation drove community assembly of V. negundo var. heterophylla shrubs. Habitat filtering related to drought stress was the main reason for the increase in phylogenetic clustering of V. negundo var. heterophylla shrubs. The PCPS two-dimensional ranking results showed that the phylogenetic clustering trend of the V. negundo var. heterophylla shrub community was mainly attributed to the lack of distantly related groups, such as Rosales and Fagales, while the species comprising the S. trilobata shrub community contained more phylogenetic clades. Our results suggest that habitat filtering was not a major driver of shrub community assembly in the Taihang Mountain region and that water limitation was an important factor in determining regional community phylogenetic structure of shrubland ecosystems.

Key words: Community construction, niche differentiation, habitat filtration, principal coordinates analysis, water limitation