Mazal, L.; Malagoli, P.; Bourceret, A.; Selosse, M.-A.; Corenblit, D.; Till-bottraud, I.; Fumanal, B.

Numerous studies on plant-plant interactions aim to distinguish positive from negative effects, especially among related individuals. Most studies interpret a reduction in biomass when grown with related individuals as kin recognition. However, these interactions can shift from positive to negative depending on environmental or biotic factors. We investigated these interactions in Black poplar (Populus nigra L.), a riparian tree species growing in high densities and stressful habitats, by examining the effects of relatedness and water deficit during early developmental stages. We set up two experiments where seedlings from four kin groups were competing with related or unrelated individuals, in well-watered versus water deficit environment, and studied their response through growth, root nitrogen uptake and mycorrhizal diversity. The four kin groups exhibit similar intrinsic growth capacities. However, for two kin groups, we found differences in competitive ability. Thus, we found that individuals responded differently depending on whether they were grown with related or unrelated individuals. Water deficit did not change the outcomes of interactions. Differences in competitive ability between kin groups was the most parsimonious interpretation, thus eliminating kin recognition as an explanation. Our study suggests that results obtained in general in kin studies must be carefully analyzed and that alternative interpretations, such as differences in competitive ability, must be tested before conclude wrongly to kin recognition. Our study also suggests that mycorrhizal communities develop differently according to the kin groups considered and can potentially have impacts on the growth of individuals.