Data from: Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub-canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or
intensity of competition and facilitation will change inversely along
abiotic stress gradients. It was originally postulated that increasing
environmental stress can induce a monotonic increase in facilitation.
However, more recent models predicted that the relationship between
severity and interaction exhibits a hump-shaped pattern, in which positive
interactions prevail under moderate stress but decline at the extreme ends
of stress gradients. In the present study, we conducted a field experiment
along a temporal rainfall gradient for five consecutive years, in order to
investigate interactions in a shrub-herbaceous plant community at the
southern edge of the Badain Jaran Desert, and, more specifically,
investigated the effects of Calligonum mongolicum, a dominant shrub
species, on both abiotic environmental variables and the performance of
sub-canopy plant species. We found that shrubs can improve sub-canopy
water regimes, soil properties, plant biomass, density, cover, and
richness and, more importantly, that the positive effect of shrubs on
sub-canopy soil moisture during the summer diminishes as rainfall
decreases, a pattern that partly explains the collapse of the positive
interaction between shrubs and their understory plants. These results
provide empirical evidence that the positive effect of shrubs on
understory plant communities in extreme arid environments may decline and
become neutral with increasing drought stress.