The joint effects of resource stress and grazing on plant-plant interactions in alpine meadows on the eastern Qinghai-Tibetan Plateau

The stress gradient hypothesis (SGH) proposes that the frequency of positive interactions among plants is increased with stress. However, in alpine meadow ecosystems, the joint effects of herbivore grazing and resource stress on plant-plant interactions remain poorly understood. By investigating the spatial associations of two dominant, widely distributed species-Carex praeclara (unpalatable) and Carum carvi (palatable)-we explored changes in plant-plant interactions at three desertification levels (light, moderate and severe) under grazed and ungrazed conditions to test the generality of the SGH in the desertified alpine meadows on the eastern Qinghai-Tibetan Plateau. Without grazing disturbance, intraspecific interactions of C. praeclara and C. carvi changed from positive to negative, while their interspecific interactions shifted from negative to positive along the desertification gradient from light to severe level. However, intraspecific interactions of C. carvi under grazing remained neutral at all desertification levels, while the net interspecific interactions between these species changed from competitive to facilitative under moderate stress. But, the positive interactions collapsed under severe stress, which is inconsistent with the SGH prediction. Our results reinforced the evidence that livestock grazing strongly influences the balance of intra- and interspecific interactions along the stress gradients in the alpine ecosystems.