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

We investigated how water restriction affects plant-plant interactions in two Mediterranean shrubs, <i>Thymus carnosus</i> Boiss and <i>Retama monosperma</i> (L.) Boiss, to test whether intra- and interspecific interactions between these species respond differently to drought. A greenhouse experiment was conducted with 5-month-old seedlings under three interaction types (interspecific, isolation, and intraspecific). After a 7-month growth phase, a water restriction treatment was imposed, and shoot water potential and photochemical efficiency of chlorophyll were monitored weekly. Biomass allocation and stable isotope composition were analysed at the end of the experiment. <i>Retama</i> plants growing alone exhibited the highest relative elongation rate (9.6 cm day^-1 per plant), whereas for the combinations involving <i>Thymus</i>, the highest relative elongation rate occurred under intraspecific competition (3.63 cm day^-1 per plant). Results showed a negative effect of <i>Retama</i> on <i>Thymus</i> regarding drought response, while <i>Thymus</i> exhibited an intraspecific facilitation effect, improving growth and reducing water stress. Although <i>Thymus</i> produced less biomass than <i>Retama</i>-with <i>Retama</i> producing over 2 g root biomass per plant compared to 0.25 g in <i>Thymus</i> and >7 g aboveground biomass versus 2.7 g in <i>Thymus</i>-it maintained better physiological response to drought than <i>Retama</i>, where all combinations involving <i>Retama</i> showed water potential below -2.3 MPa in both species. These findings reveal contrasting strategies: <i>Retama</i> prioritises rapid growth but is drought-sensitive, whereas <i>Thymus</i> benefits from intraspecific facilitation that enhances drought tolerance. Our results highlight how drought can alter the balance between competition and facilitation in plant interactions, with implications for Mediterranean plant communities' dynamics under climate change.