Competitive outcomes between two exotic invaders are modified by direct and indirect effects of a native conifer

When organisms interact in multi-species groups, the direct effects of facilitation and competition can be modified by indirect interactions. We explored multispecies interactions among the native Pinus ponderosa, the invasive annual grass Bromus tectorum, and the invasive forb Centaurea stoebe in intermountain prairie of the northern Rocky Mountains. Centaurea is much less abundant under Pinus than in surrounding open grassland and Bromus is more abundant under Pinus. We found that the more fertile soil associated with Pinus facilitated both invasive species and did not alter competitive outcomes. Pinus litter and litter leachate inhibited both species, but litter also shifted competitive outcomes in favor of Bromus and against Centaurea. The effects of Pinus litter leachate were also strong and leachate eliminated the competitive effect of Centaurea on Bromus while not changing the competitive effect of Bromus on Centaurea. There are many other ways that Pinus may affect understory composition, but by altering the competitive playing field through leaf litter Pinus appears to indirectly facilitate Bromus by more strongly inhibiting Centaurea chemically, an unusual case of a native inhibiting an invader through allelopathy. Our results also provide an unexpected and novel perspective on indirect interactions among competitors, but not through intransitive competitive relationships. Instead, one species (Pinus) strongly ‘modified’ interactions between two other species in addition to disproportionately affecting one species more than another.