Direct and indirect interactions between shrubs and the flowering annual community in an arid ecosystem

Shrubs commonly shape the plant community in arid ecosystems such as deserts. Plant interactions between shrubs and the associated species can be both negative and positive, and direct and indirect. The importance of shrub effects on reproduction is also a key component of facilitation studies in deserts. Here, we tested the hypothesis that shrubs facilitate understory plant community composition and reproduction through both direct and indirect interactions mediated by the associated plant neighbourhood. Using the shrub species Larrea tri-dentata, we tested the effects of facilitation on understory plant density, floral density, and species richness in the Mojave Desert, California. We found that shrubs decreased the net community-wide plant assembly measures relative to open microsites including total density and species richness at the site tested. There was also evidence that shrubs directly decreased plant fitness by reducing floral density, but this effect depended on the species. The net density and diversity of the annual plant community also influenced plant and floral density under shrubs suggesting significant indirect effects between shrubs and all abundant annual plant species. While both direct and indirect interactions between shrubs and the annual plant community were species-specific, the effects of these interactions had greater impact on plant density than floral density measures. Annual plant density and to a lesser extent floral density are thus driven by both direct and indirect interactions with shrubs.