Non-native weed reaches community dominance under the canopy of dominant native tree

Abstract

Whether facilitation from native plants is strong enough to trigger community dominance by non-natives remains unclear. We explored the possibility that facilitation from Prosopis caldenia, the dominant native tree in the semiarid open forest of central Argentina, drives local community dominance by Chenopodium album, an annual herb native to Europe. We assessed this hypothesis by conducting extensive field sampling in which we recorded the relative abundance of species growing under the canopy of P. caldenia (caldén microsites) and in adjacent locations free of this tree (open microsites). If our hypothesis is correct, then the relative abundance of C. album will be greater than that of the rest of the species only when growing under P. caldenia. Also, we measured C. album performance, estimated its soil seed bank, and characterized growing conditions in caldén and open microsites. We found that the relative abundance of C. album was over seven times greater than that of any other species in communities occurring in caldén microsites; by contrast, C. album co-dominated communities with several other species in the open. Chenopodium album density, cover, biomass, and fecundity were all several times greater in caldén than open microsites. Similarly, C. album seed bank displayed an eight-fold increase in caldén as compared to open microsites. Growing conditions were markedly different between microsites, which could explain positive responses from C. album. Our results suggest that facilitation from natives is indeed strong enough to trigger local community dominance by non-natives, advancing the understanding of community-level consequences of this interaction.