Species traits interact with stress level to determine intraspecific facilitation and competition

Questions Flooding and drought stress are expected to increase significantly across the world and plant responses to these abiotic changes may be mediated by plant-plant interactions. Stress tolerance and recovery often require a biomass investment that may have consequences for these plant-plant interactions. Therefore, we questioned whether phenotypic plasticity in response to flooding and drought affected the balance between competition and facilitation for species with specific adaptations to drought or flooding. Location Utrecht University. Methods Stem elongation, root porosity, root:shoot ratio and biomass production were measured for six species during drought, well-drained and submerged conditions when grown alone or together with conspecifics. We quantified competition and facilitation as the 'neighbour intensity effect' directly after the 10-day treatment and again after a seven-day recovery period in well-drained conditions. Results Water stress, planting density and species identity interactively affected standardized stem elongation in a way that could lead to facilitation during submergence for species that preferably grow in wet soils. Root porosity was affected by the interaction between neighbour presence and time-step. Plant traits were only slightly affected during drought. The calculated neighbour interaction effect indicated facilitation for wetland species during submerged conditions and, after a period to recover from flooding, for species that prefer dry habitats. Conclusions Our results imply that changing plant-plant interactions in response to submergence and to a lesser extent to drought should be considered when predicting vegetation dynamics due to changing hydroclimatic regimes. Moreover, facilitation during a recovery period may enable species maladapted to flooding to persist.