Genetic variation within a dominant shrub species determines plant species colonization in a coastal dune ecosystem

The diversity and structure of plant communities is often determined by the presence and identity of competitively dominant species. Recent studies suggest that intraspecific variation within dominants may also have important community‐level consequences. In a coastal dunes ecosystem of northern California, we use a decade‐old common garden experiment to test the effects of a genetically based architectural dimorphism within a dominant native shrub, Baccharis pilularis , on plant colonization success and understory plant diversity. We found that erect Baccharis morphs had higher richness and cover of colonizing plant species (both native and exotic species) compared to prostrate morphs, as well as higher biomass of a dominant exotic dune grass ( Ammophila arenaria ). Trait differences between architectural morphs influenced the abiotic understory environment (light availability, soil surface temperature, and litter depth) and were associated with species colonization success. Taken together, our results demonstrate that incorporating within‐species variation, particularly within dominant species, into community ecological research can increase the ability to predict patterns of species diversity and assembly within communities.