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.

胁迫梯度假说（stress gradient hypothesis, SGH）预测，竞争与促进作用的重要性或强度会沿非生物胁迫梯度呈反向变化。该假说最初提出，环境胁迫加剧会使促进作用呈单调递增趋势。然而，近期更多模型预测，胁迫严重程度与物种互作的关系呈现驼峰型模式：即中等胁迫下正互作占据主导，而在胁迫梯度的极端两端，正互作强度会出现下降。本研究针对巴丹吉林沙漠（Badain Jaran Desert）南缘的灌草植物群落，沿时间降雨梯度开展了为期五年的连续野外试验，以探究该群落内的物种互作关系；并重点研究了优势灌木种沙拐枣（Calligonum mongolicum）对非生物环境变量以及冠层下植物物种生长表现的影响。研究结果显示，灌木可改善冠层下的水分状况、土壤性状、植物生物量、植株密度、植被盖度及物种丰富度；更为关键的是，夏季灌木对冠层下土壤含水量的正效应会随降雨量减少而减弱，这一模式在一定程度上解释了灌木与林下植物间的正互作为何会衰退。本研究结果提供了实证证据，表明极端干旱环境中灌木对林下植物群落的正效应会随干旱胁迫加剧而减弱，最终转为中性效应。