Abstract

A standard belief is that species can be organized in competitive hierarchies, leading to concepts of plant strategies. Based on species' average competition values, this approach may fail to predict specific outcomes and overlook common facilitative interactions, biasing understandings of species interactions. Using a mesocosm experiment, we examined interactions of 13 native grassland species grown alone and in 168 pairwise combinations under two nutrient conditions. We measured the positive and negative effects of plants on neighbours and their responses to neighbours. All species engaged in both facilitative and competitive interactions, and neighbour effects and responses were positively related, suggesting that species specialize at either end of the interaction gradient. Nutrient addition reduced the strength of both competitive and facilitative plant-plant interactions, thereby reducing variation in plant growth. These results suggest the existence of a competition-facilitation continuum, integrating both effects on and from neighbours, along which plant strategies may fall.