Understanding community assembly mechanisms is helpful to predict community dynamics. To explore which community assembly mechanism(s) drive(s) the grassland restoration in semi-arid region, we investigated the relationships between plant trait and species relative abundance (SRA), and estimated community functional diversity indices for each community under different treatments (enclosure, grazing and mowing treatment) in a restoration region of Stipa grandis – Leymus chinensis communities in the northern China from 2010 to 2012. There was a high fraction of significant relationships between trait value and SRA, suggesting that niche theory structured the grassland restoration in this region. The functional richness was higher and the functional divergence was lower in the enclosure community than that in the grazing or mowing community, and significantly positive plant height - SRA relationship was found in the enclosure community. These findings demonstrated that limiting similarity based on niche theory was more important in structuring the enclosure community and that environmental filtering based on niche theory played a more important role in driving the grazing or mowing community. Only the factor of year significantly affected the functional evenness (FEve), and the lowest FEve in 2011 implied that the relatively lower precipitation could enhance the effect of limiting similarity on community assembly in the semi-arid grassland.

Understanding community restoration state and assembly mechanisms is helpful to assess restoration measures and predict community dynamics. In order to explore the effects of fencing duration and shrub cover on community stability and assembly, we investigated the community information and assessed the assembly mechanisms in plots using shrub cover of Caragana microphylla within each of the three sites fenced since 1979, 1983, and 2003 in a semiarid steppe region of China. Community composition was different among various fencing duration or shrub cover treatments. Shrub cover had a positive effect, and fencing duration had a negative effect on community stability, and both had indirect effects via regulating vegetation cover. Both shrub cover and fencing duration influenced phylogenetic structure directly and negatively, and indirectly via regulating Simpson's diversity and vegetation cover. Considering that the functional traits were phylogenetically convergent, community assembly mechanisms assessed by the values of phylogenetic structure shifted from stochasticity to competitive exclusion with the increase of shrub cover and fencing duration, and competitive exclusion dominated community assembly in the plots of low or high shrub cover in the site of fencing since 1979. The responses of community stability and community assembly to the changes of shrub cover and fencing duration suggest that the shrub‐encroached grassland is an alternative stable community state in semiarid steppe regions. Therefore, it is essential to distinguish shrub‐encroached grasslands from non‐encroached degraded grasslands when formulating relevant conservation and management measures in similar regions.

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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.