Does a history of population co-occurrence predict plant performance, community productivity, or invasion resistance?

A history of species co-occurrence in plant communities is hypothesized to lead to greater niche differentiation, more efficient resource partitioning, and more productive, resistant communities as a result of evolution in response to biotic interactions. A similar question can be asked of co-occurring populations: do individual species or community responses differ when communities are founded with plants sharing a history of population co-occurrence (sympatric) or originating from different locations (allopatric)? Using shrub, grass, and forb species from six locations in the western Great Basin, North America, we compared establishment, productivity, reproduction, phenology, and resistance to invaders for experimental communities with either sympatric or allopatric population associations. Each community type was planted with six taxa in outdoor mesocosms, measured over three growing seasons, and invaded with the annual grass Bromus tectorum in the final season. For most populations, the allopatric or sympatric status of neighbors was not important. However, in some cases, it was beneficial for some species from some locations to be planted with allopatric neighbors, while others benefited from sympatric neighbors, and some of these responses had large effects. For instance, the Elymus population that benefited the most from allopatry grew 50% larger with allopatric neighbors than in single origin mesocosms. This response affected invasion resistance, as B. tectorum biomass was strongly affected by productivity and phenology of Elymus spp., as well as Poa secunda. Our results demonstrate that, while community composition can affect plant performance in semi-arid plant communities, assembling communities from sympatric populations is not sufficient to ensure high productivity and invasion resistance. Instead, we observed an idiosyncratic interaction between sampling effects and evolutionary history, with the potential for seed source of individual populations to have community-level effects.