Woody species have stronger facilitative effects on soil biota than on plants along an aridity gradient

Questions Woody-shrub encroachment affects community structure and composition. However, most studies focus on their effects on understorey plant communities, and the relative importance of shrubs in affecting plants vs soil biota communities is poorly known. Location Inner Mongolian Steppe, China. Methods We examined the effect of shrubs on multiple community attributes, including plants, soil biota (bacteria, fungi and nematodes), and soil fertility, and quantified how these effects changed from semi-arid to hyperarid conditions (from 281 mm to 110 mm of mean annual precipitation). In addition, we assessed whether the effects of shrubs on plant communities were directly mediated by biotic filtering in seed germination and establishment, or indirectly mediated by plant biomass or soil fertility in the case of soil organisms. Results The effect of shrubs on soil biota was generally more positive than on plants, and it increased with aridity. We found that a larger proportion of below-ground taxa depended on shrub presence (36%) than plants (20%). Soil nematodes and soil bacteria were directly influenced by shrub presence whereas soil fungi were indirectly influenced by enhanced soil fertility. Shrubs also increased plant biomass under all conditions but only increased plant species richness in the most arid conditions. Despite the generally positive effect of shrubs, and the fact that they weakened the filtering effects of aridity on seed germination, aridity was a stronger predictor of changes in species composition than shrub presence was, particularly for plants. Conclusions Our results illustrate the variety of positive effects of shrubs and show that they are particularly important in supporting biodiversity in the most arid conditions. These strong and positive effects could partially buffer the impacts of increasing aridity on dryland soil biodiversity, but our study suggests that facilitative interactions may not be able to completely mitigate the impacts of increasing aridity on drylands