Plant interactions govern population dynamics in a semi-arid plant community

1 The structure and composition of plant communities are influenced by positive and negative interactions between plants, the balance of which may change in intensity and sign through time and space, depending on availability of resources and on plant life history. 2 Over a 2-year period we analysed the balance of interactions between different life stages of a perennial grass, Stipa tenacissima, and a shrub, Cistus clusii, the dominant species in a semi-arid community in south-east Spain. 3 Cistus shrubs acted as nurses for juvenile Stipa plants, improving their water status, nutrient content, carbon assimilation rates and growth. The mechanisms underlying this facilitation effect were mainly the improvement of microclimatic conditions and soil physical and chemical properties under shrub canopies. By contrast, juvenile Stipa plants had an overall neutral effect on Cistus shrubs, although Cistus suffered some competitive effects during periods of water shortage. At this life stage, the short-term outcome of the interaction for both species was dependent on resource availability. 4 Close spatial association between adult plants had no negative effects for the interacting species, although positive effects most likely counterbalanced negative effects. 5 The long-term outcome of the interaction is reflected in the spatial distribution of both species, and determines population dynamics in this semi-arid plant community. 6 Our data show that the short-term balance of plant interactions may easily shift in response to environmental variability, which in turn may have important consequences for plant community structure.