Shrub‐encroached grassland as an alternative stable state in semiarid steppe regions: Evidence from community stability and assembly

Abstract

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.

Soil nutrients mediate the indirect effects of shrub canopy removal: How distance from shrubs affects the herbs and grasses community in a shrub‐encroached grassland

“…In this study, the finding that most of the absolute values of SESs were lower than 1.96 indicated that random processes dominated the community assembly in the study site or perhaps that there was a balance between environmental filtering and limiting similarity. In the Inner Mongolia steppe, random processes usually dominate community assembly in the early stage of restoration succession or in harsh local environmental conditions with low soil nutrient status (Hao et al, 2021; Koyama et al, 2015). However, there was significant difference between the responses of SESs assessed by both approaches.…”

“…The different letters show significant differences (p < 0.05) among different shrub canopy removal treatments under a fixed distance from shrubs. The "*" in the bracket show significant (p < 0.05) differences between two sampling distance treatments under a fixed removal frequency [Colour figure can be viewed at wileyonlinelibrary.com] assembly in the early stage of restoration succession or in harsh local environmental conditions with low soil nutrient status (Hao et al, 2021;Koyama et al, 2015). However, there was significant difference between the responses of SESs assessed by both approaches.…”

Strong homogenization effects of shrubs on nematode communities across large spatial scales on the Qinghai-Tibet Plateau

“…Beta diversity can be partitioned into two different components: turnover component (some species disappear and replaced by others) and nestedness component (the assemblages of sites with smaller numbers of species are subsets of the assemblages at richer sites), which can provide insight into the processes that maintain biodiversity (Baselga, 2010;Legendre, 2014). Shrub encroachment is an irreversible process with potential effects on the soil environmental heterogeneity and plant communities, thus affecting the beta diversity of soil communities (Hao et al , 2021). On the one hand, shrubs, which function as habitat for soil communities, could limit the range of potential colonizers by favoring or excluding species with particular functional attributes, which would directly decrease beta diversity of soil communities (Siegwart Collier & Mallik, 2010;Wardhaugh et al , 2012;Lanta et al , 2018;Xiang et al , 2019).…”

“…On the other hand, shrub triggers large shifts in plant communities and soil environmental heterogeneity, indirectly affecting beta diversity of soil communities. For example, plant communities could affect niche space and niche differentiation for soil communities; environmental heterogeneity may increase the variation in abiotic factors that limit local species coexistence (Chesson, 2000;Schöb et al , 2013;Trentanoviet al , 2013;Losapio et al , 2018;Albrecht et al , 2021;Hao et al , 2021). These processes would alter the turnover and nestedness components between habitats, resulting in changes in beta diversity of soil communities.…”

“…In order to take large-scale processes into account, we examined nematode communities, plant communities, and soil properties across a 1200 km x 1200 km area with widespread shrub on the Qinghai-Tibet Plateau. We tested the following hypotheses: 1) shrubs decrease the nematode beta diversity and the nestedness component (Trentanoviet al , 2013;Dar & Reshi, 2014;Zhang et al , 2019); 2) leguminous shrub plays a stronger role in decreasing the nematode beta diversity compared with non-leguminous shrubs due to their nitrogen fixation capacity (Zhou et al , 2020); 3) shrub effects on nematode beta diversity increases with MAT and MAP, because shrubs are more stable in response to climate change than open space (Hao et al , 2021); 4) shrub affects beta diversity of nematode communities directly and indirectly through changes in beta diversity of plant communities and soil heterogeneity (Zhang et al , 2019).…”