An evolutionary approach to studying the relative importance of plant-plant interactions along environmental gradients

1. Abiotic stress and interactions with neighbours are major selective forces but their relative importance for local adaptation has rarely been separated. Plant community theory predicts increasing importance of competition with decreasing stress. We experimentally separated the role of neighbours and drought stress for local adaptation of two annual plant species. We predicted that neighbours amplify patterns of local adaptation with adaptation to competition prevailing in benign habitats. 2. During one growing season, we combined reciprocal transplants with neighbour removals along a gradient of more than an eightfold increase in annual rainfall using populations from sites representative of Arid, Semi-arid, Mediterranean and Mesic Mediterranean ecosystems. We evaluated statistical interactions between origin and planting site with and without neighbours and quantified plant-plant interaction importance. 3. An extreme drought (<63% of the average annual rainfall at the arid sites) reduced the probability of detecting neighbour effects. There was weak but non-significant evidence for local adaptation and for the amplification of adaptation patterns with neighbours for populations from wetter sites of the two species. Our results together with previous findings in similar gradients suggest that detecting local adaptation to neighbours in unpredictable environments depends on the climatic conditions of the study season. 4. We coupled a central concept from plant community ecology with an evolutionary approach to separate the role of abiotic vs. biotic factors for local adaptation. This permits quantifying the importance of interactions with neighbours and we advocate its use in future studies of local adaptation, which should be conducted over several years.