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Plant-plant interactions are largely influenced by both environmental stress and ontogeny. Despite the effects of each of these factors on the overall outcome of these interactions has received considerable attention during the last years, the joint effects of both factors as drivers of such outcome are poorly understood. We used the combination of spatial pattern analysis, fruit production surveys, carbohydrate assays, sowing experiments and dendrochronological techniques to explore the interaction between Stipa tenacissima (nurse) and Lepidium subulatum (protegee) in two different slope aspects. This battery of techniques allows us to study the effects of the nurse plant during the whole life cycle of the protegee, and to assess the role of spatio-temporal variability in abiotic stress as a modulator of ontogenetic shifts in plant-plant interactions. Spatial pattern analyses suggested a net facilitative effect of S. tenacissima on L subulatum. This effect was particularly important during the germination, shifting to competition (growth reduction) early after establishment. Competition was gradually reduced as the shrub aged, suggesting niche differentiation. The magnitude of competition was reduced under low rainfall levels in south-facing slopes, whereas this response was observed due to other abiotic factors in north-facing slopes. Our results highlight the crucial effect that positive interactions at early life-stages have to determine the long-term outcome of a given plant-plant interaction, and the existence of multiple shifts between facilitation and competition along different life-stages of the protegee. They also show how these ontogenetic shifts are modulated by abiotic factors, which differ among slope aspects. These findings may help to refine conceptual and theoretical models about shifts between facilitation and ontogeny under current climate change scenarios. (C) 2010 Rubel Foundation, ETH Zurich. Published by Elsevier GmbH. All rights reserved.