Facilitation from an intraspecific perspective - stress tolerance determines facilitative effect and response in plants.

Plant-plant interactions are reciprocal and include effects on and response to neighbours. Distinct traits confer competitive effect and response ability, but how specific traits determine effect and response in facilitative interactions has not been studied experimentally. We utilized the model species Arabidopsis thaliana to test for trait dependence of facilitative interactions. Salt-sensitive (sos) mutants or salt-tolerant wild-types were exposed to an experimental salinity gradient with and without intraspecific neighbours and the intensity of plant-plant interactions was measured for three performance variables. We tested whether salt tolerance can predict facilitative effect and response and whether a tradeoff exists between competitive ability and tolerance to stress. Interactions shifted very clearly from negative to positive with increasing stress. Salt-sensitive genotypes were less negatively affected by competition but more dependent on facilitation than were wild-types, indicating a tradeoff between competitive ability and stress tolerance. Surprisingly, sensitive genotypes imposed stronger facilitative effects, despite being much smaller under stress, probably because they retrieved more salt from the soil. Stress tolerance defined facilitative effect and response via distinct mechanisms. We advocate more controlled experiments with model species to advance our understanding of the trait dependence of biotic interactions and their consequences for community organization.