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.
Plant negative density dependence is the result of interactions between plants themselves and between plants and the environment. We selected a succession series comprising early successional, mid-successional and late successional stages (Artemisia ordosica, Sophora alopecuroides and Stipa bungeana communities, respectively) in a semi-arid area. We investigated plant density and biomass and determined the nitrogen (N) and phosphorus (P) contents of each plant species in each of 225 quadrats, and we calculated the N and P contents of vegetation using biomass as a weighted coefficient. We found that the total plant density of the A. ordosica community increased with the increase in vegetation N:P ratio while the total plant density of the S. bungeana community decreased with the increase in vegetation N:P ratio, with the latter (late successional stage) community exhibiting negative density dependence. In the communities representing the early successional and mid-successional stages, the vertex coordinate of the quadratic function relationship between plant total density and vegetation N/P ratio was (16.6, 353.3); that is, if the N:P ratio of the vegetation was greater than 16.6, the community was characterized by negative density dependence. The analysis showed that the negative density dependence was due to P limitation. These findings reveal that the vegetation N:P ratio in a semi-arid region is the driver of negative density dependence.