The importance of facilitation on community assembly disappears under severe drought stress

Despite the recognition of positive interactions as an important driver of species coexistence and community structure, the underlying mechanism of how facilitation affects assembly processes along stress gradients is poorly explored. Understanding the responses of functional diversity to benefactor species at the extreme end of the stress gradient could provide valuable insight about facilitation-involved assembly mechanisms and contribute to the predictions of species coexistence under climate change. In the drought-stressed community in the Badain-Jaran Desert, the responses of the local community to the nurse shrub species Calligonum mongolicum Turcz. were evaluated using hierarchical Bayesian models. For the 3-year experiment, summer rainfall in each year formed a natural gradient of drought stress. To evaluate the shrub's effects on the assembly process along that gradient, individual samples were collected in pairwise under-shrub and open habitats, and four traits related to stress tolerance and resource acquisition were measured simultaneously. Under moderate drought stress, we observed shifting community-weighted means, broadening ranges and reducing overlaps of functional traits under shrubs. These effects were partly driven by a distinct microenvironment created by shrub plants, in particular the improvement and heterogeneity of soil moisture conditions. However, this influence on trait distributions was strongly dependent on the environmental context, and generally disappeared as drought stress shifted towards its driest end, almost in line with the decreased positive interaction assessed by plant density and species diversity. This study focused on water-limited community that lies at the driest end of drought gradient and confirmed that facilitation can drive the assembly process through both environmental filtering and niche differentiation. More importantly, these assembly mechanisms are proven to become less efficient under extreme drought stress, which may suggest the occasionally disappearing role of benefactor plants on community assembly and an increasing risk of biodiversity loss in the context of climate change. Read the free Plain Language Summary for this article on the Journal blog.