Understanding community restoration state and the corresponding assembly mechanism is helpful to assess the restoration measures and predict community dynamics. We collected plots by shrub cover (low, medium and high) in three Caragana mircophylla shrub-encroached grasslands which were fenced since 1979, 1983, and 2003 (fencing duration) in the northern China, to explore the effect of fencing duration and shrub cover on the community restoration by vegetation investigation and phylogenetic approach. There were significant differences in community composition among different fencing duration or shrub cover treatments. Species richness in the site of fencing since 1979 or in the plots of high shrub cover was relatively higher than that in any other sites or plots. By phylogenetic analysis, functional traits were phylogenetically convergent. Based on the standardized effect sizes of mean pairwise distance (SESMPD) ranged from -1.96 to 1.96 in six out of nine plots, which suggested that stochastic processes dominated community assembly. SESMPD were lower than -1.96 in the rest three plots which indicated that competitive exclusion drove community assembly. These results indicated that the increase of fencing duration or shrub cover could enhance competitive exclusion. The present findings highlighted the importance of shrub in influencing the community composition and community assembly, supporting that shrub-encroached grassland is another stable state in the semi-arid northern China. Therefore, it is essential to distinct shrub-encroached grassland from degraded grasslands when formulating relevant conservation and management measures in the semi-arid regions.

Understanding community assembly mechanism is essential for us to predict community dynamics and restoration process in the context of intensive human activities. In this study, we aimed to explore how the removal of shrub canopy influenced the community assembly in shrub-encroached grassland by both trait-based and phylogenetic approaches. We carried out an experiment of shrub canopy removal with three frequencies (no removal, removal once and twice a year) in a shrub-encroached grassland in China. We investigated community species composition under each treatment and collected plant functional traits for each species. We calculated the standard effect size (SES) of phylogenetic and trait-based functional diversity based on null model analysis to determine the dominant community assembly process. By phylogenetic analysis, the random process drove the community assembly, being independent of the removal frequencies and locations; while by trait-based approach, random process dominated the assembly in communities under ‘no removal’ treatment beneath shrub and under ‘removal twice a year’ treatment in shrub-interspace when using some certain single traits, and the limiting similarity process dominated the assembly in the rest communities. Removal of shrub canopy twice a year released more niches which promoted the performance of herbaceous species beneath shrub and promoted the settlement of annuals in shrub-interspace. Compared with phylogenetic analysis, the results of trait-based approach were more sensitive to the human activities. All these findings indicated that removal of shrub canopy twice a year would be an effective way for the restoration of shrub-encroached grassland.