Rosa multiflora’s performance under water stress: the role of positive and negative density-dependent intraspecific interactions

It is well known that biotic interactions may significantly alter plant population and community dynamics in natural ecosystems. Multiple studies have reported that density-dependent positive interactions can modify the effect of extreme stress on plant performance (in line with predictions by the stress gradient hypothesis, SGH). However, the performance of invasive species and the role of intraspecific density dependence, either negative or positive, in successful invasions are not well understood. In the present study, we tested if monocultures of Rosa multiflora , a highly invasive shrub, experience a group advantage under extremely low water availability. Using a manipulative greenhouse study, we tested the growth response of R. multiflora seedlings to a 4-level conspecific density treatment and a 4-level water availability gradient. Overall, our results provide preliminary evidence that biotic interactions under stressful conditions between R. multiflora individuals do not experience a group advantage, as predicted. Instead, our study indicates that seedling performance was strongly driven by negative density dependence, in which individuals grown at higher conspecific densities displayed lower plant performance, regardless of water availability. This demonstrates that R. multiflora experiences intense intraspecific competition even under a stressful, low resource environment. Thus, management efforts of R. multiflora need to ensure that re-sprouting of individuals after treatment is eliminated, as release from negative density dependence could lead the invader population to rebound.