A systematic review and conceptual framework for the mechanistic pathways of nurse plants

Aim To conceptualize the mechanistic pathways of the nurse-plant syndrome by life-form and to identify the implications of positive plant–plant interactions for landscape and evolutionary ecology. Location Global. Methods We conducted a quantitative review examining 298 articles to categorize the literature on nurse-plant interactions based on geographic region, mechanism of facilitation, ecological hypothesis and nurse life-form. Results A total of nine different nurse mechanisms were identified and two were classified as meta-mechanisms. We found that shrubs were the dominant nurse life-form (46% of total studies) and that studies of positive plant interactions were most frequent in areas of high abiotic stress. Nurse-plant studies were also distributed unevenly around the globe with nearly a quarter in the South American Andes and Spain. Studies testing the direct nurse–protégé interactions were the most frequently performed, including the ecophysiological responses of protégé species (32.2%). Research gaps identified in the nurse-plant literature included indirect interactions and seed trapping as well as the large-scale implications for landscape ecology and evolution. Main conclusions Nurse plants are often considered keystone species because they commonly structure plant communities. This is an important confirmatory finding in many respects, but it is also novel in that it challenges traditional plant ecology theory and has important implications for landscape-level dynamics over time. The categorization of mechanisms proposed provides a conceptual framework useful for organizing the research to date and can accelerate linkages with theory and application by identifying important connections. It is becoming increasingly apparent that future studies of the nurse-plant syndrome must decouple and consider multiple mechanisms of interaction to explain the processes that influence community structure, particularly in high-stress conditions, given a changing climate and potential shifts in biodiversity.