How beneficial are nurse plants? A meta-analysis of the effects of cushion plants on high-Andean plant communities

Facilitation by nurse plants is one of the most commonly reported interactions between plants and is regarded as an important factor in structuring plant communities. We used a meta-analysis to examine the generality of these effects, focusing on cushion plants, a common life-form occurring in high-Andean ecosystems. We targeted the following questions: (1) is there a generalized positive effect of cushions on other vascular plant species along the Andes? (2) do different species groups (i.e., annuals and perennials, natives and exotics) display different association responses to cushions? (3) does the nurse effect of cushions increase with environmental severity? Results indicated that the overall effect of cushions is positive, however these positive effects were more significant amongst exotic plants than in native plants; effects were only positive for perennial plants, and were notably negative for annuals. The positive effects of cushions also increased with physical stress, but only for perennial plants. These results allow us to suggest that as a whole cushions may be acting as keystone species that maintain the structure and diversity of high-Andean plant communities. Nevertheless, since cushions also positively affect the performance of exotic plants, we should be aware of their potential role in promoting biological invasions.