Interspecific Competition and Intraspecific Facilitation Shape Coastal Dune Shrub Responses to Experimental Drought

We investigated how water restriction affects plant-plant interactions in two Mediter-ranean shrubs, Thymus carnosus and Retama monosperma. The aim was to test whether intra- and interspecific interactions between these species respond differently to drought. A greenhouse experiment was conducted with 5-month-old seedlings from seeds col-lected in El Rompido, (Huelva, Spain), grown in a factorial design under three interaction types (interspecific, isolation, and intraspecific). After a 7-month growth phase, a wa-ter-restriction treatment was applied. Growth and branching were measured before drought treatment, while shoot water potential and photochemical efficiency of chlo-rophyll were monitored weekly during the treatment. At the end of the experiment, biomass allocation was analysed, and leaf samples were collected for stable isotope analysis. Results showed a negative effect of Retama on Thymus regarding access to be-lowground resources and drought response, while Thymus exhibited an intraspecific facilitation effect, improving growth and reducing water stress. Despite producing less below- and above-ground biomass, Thymus showed a better physiological response to drought than Retama. These results highlight how drought can alter the balance between competition and facilitation in plant interactions. Given the predicted increase in drought frequency and severity, understanding these effects is relevant to predicting vegetation dynamics under global climate change.