The Tibetan Plateau is among the most ecologically important and sensitive regions worldwide, which has undergone severe desertification in recent decades, particularly in the Zoige region. In an effort to stem the progression of desertification, shrubs such as Salix cupularis have been planted in some localities in this region. However, the mechanisms whereby the interspecific relationships between these shrubs and the undergrowth vegetation change in response to an increase in shrub canopy area remain unclear. In this study, we examined the traits of plant communities and analysed soil samples under S. cupularis in a degraded alpine meadow along a gradient of four different canopy areas (1.3–1.8, 3.8–5.5, 12.6–14.5, and 27.3–28.3 m²). Our results revealed that plant community characteristics shifted from facilitation to competition at a canopy area of 3.8–5.5 m². However, changes in soil nutrients and the activities of sucrase and catalase were not detected until the canopy area of S. cupularis had reached 2.6–14.5 m². Furthermore, we found that microbial communities, particularly bacteria, showed trends similar to those shown by soil properties. In line with expectations, we found that the soil fungal community showed trends opposite to those shown by the bacterial community. Our findings highlight that competition rather than facilitation tends to be the predominant interspecific relationship that develops in response to the continued growth of nurse plants. Consequently, these results provide evidence in support of the stress-gradient hypothesis, which states that interspecific competition is particularly prevalent in fertile environments, particularly when there are changes in facilitation during periods of increasing environmental stress. Importantly, with respect to the shift in interspecific relationships from facilitative to competitive, we demonstrate that the response of the belowground interactions was slower than that of interactions above ground. Collectively, our observations indicate that the nurse effect of S. cupularis on ecosystems could have significant implications for the restoration of desertified grasslands.

青藏高原是全球最重要的生态和敏感地区之一，最近几十年来，该地区经历了严重的荒漠化，特别是在佐伊格地区。为了阻止荒漠化的发展，在该地区的某些地方种植了灌木如铜柳（Salix cupularis）。但是，这些灌木与灌木丛之间的种间关系响应灌木冠层面积的增加而变化的机制仍不清楚。在这项研究中，我们研究了植物群落的特点并分析土壤样品下S. cupularis沿四个不同的树冠区域（1.3-1.8，3.8-5.5，12.6-14.5梯度的退化草甸，和27.3-28.3米²）。我们的结果表明，在3.8–5.5 m ²的树冠面积上，植物群落特征从促进转变为竞争。但是，直到S. cupularis的冠层面积达到2.6–14.5 m ^2时，才检测到土壤养分的变化以及蔗糖酶和过氧化氢酶的活性。。此外，我们发现微生物群落，特别是细菌，显示出与土壤特性相似的趋势。与预期相符，我们发现土壤真菌群落显示出与细菌群落相反的趋势。我们的研究结果突出表明，竞争而非促进趋向于是随着苗木植物的持续生长而发展的主要种间关系。因此，这些结果提供了支持应力梯度假说的证据，该假说指出种间竞争在肥沃的环境中特别普遍，尤其是在环境压力增加的过程中便利化发生变化时。重要的是，关于种间关系从促进到竞争的转变，我们证明了地下相互作用的响应要慢于地面相互作用。从总体上看，我们的观察表明生态系统上的S. cupularis对荒漠化草地的恢复可能具有重大影响。