Both fencing duration and shrub cover facilitate the restoration of shrub-encroached grasslands

Abstract

Fully understanding community assembly mechanisms is crucial to better predict grassland community dynamic under restoration. The community assembly over time is more complicated in shrub-encroached than in non-encroached degraded grasslands because the presence and size of shrub could regulate community processes and diminish effects of restoration time. However, how shrub size influences community assembly under restoration in shrub-encroached communities remains poorly understood. Therefore, in Caragana microphylla shrub-encroached grasslands in the Inner Mongolia typical steppe region, three plots with different shrub cover (low, medium and high) were set up in each of three sites with different fencing duration (15-, 35- and 39-year fencing). We assessed the assembly mechanisms of shrub-interspace herbaceous communities by estimating standardized effect size (SES) of functional dispersion (FDis) based on the first two principal components (PC1 and PC2) of traits characterizing resource acquisition strategies. According to the SES of FDisPC1, the assembly mechanisms shifted from stochastic processes to competitive exclusion over time of fencing in the plots of low and medium shrub cover and across a shrub cover gradient in the sites of 15-year and 35-year fencing, with the increase of soil moisture, species richness, Shannon-Wiener diversity and relative density of perennial grasses. Competitive exclusion dominated the communities in the site of 39-year fencing and in the plots of high shrub cover. According to the SES of FDisPC2, the competitive exclusion dominated all of the 9 communities investigated in this study. By variance component analysis, shrub cover showed the highest contribution in explaining the variance in community-weighted mean (CWM), and shrub cover and fencing duration showed the first two highest contributions in explaining the variance in SES of FDis. The present findings highlight the significance of shrub cover in influencing the community assembly in shrub-encroached grasslands under restoration in the European-Asian typical steppe region.