Natural selection favors root investment in native grasses during restoration of invaded fields

Abstract

Competitive interactions with invasive species can cause evolutionary changes in native populations. In the Great Basin, USA, the invasive annual grass Bromus tectorum pre-emptively accesses limiting soil resources through extensive root production. Here, we determined whether evolutionary changes in root traits occurred during re-seeding of Elymus elymoides, a native bunchgrass. We grew plants from seeds collected from survivors in two restoration sites and compared them with plants grown from the original seed, with and without competition from B. tectorum. We quantified competition, root allocation, and morphology over time. Relative to the original seed source, E. elymoides plants that established in restoration sites showed small but significant increases in root biomass fraction, and plants were smaller, both aboveground and belowground, with parallel results seen in both field sites. In contrast, E. elymoides seed sources were not significantly different in competitive response to or in competitive effect on B. tectorum. Thus, results are consistent with natural selection favoring traits that allowed plants to establish in B. tectorum invaded areas but not traits that increased competitive ability. Selecting or breeding restoration plants for smaller size but increased relative root investment could improve establishment under reduced soil resource conditions and hence native plant restoration.