Tolerance of intraspecific competition contributes to invasion success, despite drought and inbreeding

Inbreeding, intraspecific competition, and drought stress interact to shape plant fitness, species distributions, and ecosystem structure. However, it is uncommon for studies to examine more than one or two of these factors simultaneously. We conducted a common garden experiment to assess the combined effects of induced inbreeding, intraspecific competition, and drought on the fitness of the highly invasive annual forb Senna obtusifolia (Fabaceae). We conducted a fully factorial experiment in which we grew seeds derived from self or outcross pollen, either alone or in competition with a conspecific, under drought or well‐watered conditions. We measured plant performance across multiple life stages, including seed size and germination, biomass allocation, growth rate, reproductive output, and defense response. We found some evidence that a single generation of induced inbreeding reduced plant performance. Low inbreeding depression (δ cumulative = 0.2) suggested that S. obtusifolia can uniparentally reproduce with little fitness impact, at least in the short term. This may enhance its invasion success by ensuring that progeny are strong competitors when pollinators and mates are scarce, common conditions during early colonization stages. Competition did reduce individual plant fitness for every response variable, and this effect was exacerbated under drought stress for both relative growth rate and root:shoot ratio, which highlighted that S. obtusifolia is tolerant to single stressors but combined abiotic and biotic stress shifted resource allocation and slowed growth rates, impeding plant growth and fitness. Critically, collective reproductive output outweighed the individual reproductive output per unit area demonstrating a positive density dependent response, even under combined stressors. Overall, tolerance to inbreeding ensures that species can reproduce uniparentally in the short term and remain competitive, tolerance of intraspecific competition enhances seed abundance and propagule pressure, and tolerance to drought stress demonstrates ability to remain competitive in changing environments.