Shift in nurse effect from facilitation to competition with increasing size of Salix cupularis canopy in a desertified alpine meadow on the Tibetan Plateau

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.