Competitive outcomes may change predictably with variation in the abiotic environment, or shift in complex and unpredictable ways.  We compared direct responses and competitive outcomes of plant species to environmental variation associated with patchy exotic plant invasions of ponderosa pine savannas in western Montana. 

Results/Conclusions

We found that nutrient-rich soil from under Pinus ponderosa dramatically increased the growth of the invaders Centaurea maculosa and Bromus tectorum, and caused moderate but variable increases in the growth of native grasses, relative to nutrient-poor soil from prairie.  In a greenhouse experiment with conifer and prairie soil we found weak competitive interactions among Bromus, Centaurea, and the native grass Festuca idahoensis in prairie soil.  Conversely, in conifer soil Bromus reduced Festuca and Centaurea biomass by 50% and 45% respectively.  Festuca reduced Bromus biomass by 38% in conifer soil, but the greatest competitive effect was a 66% biomass reduction of Bromus when competing with Centaurea.  Despite similar growth responses to conifer soil when grown alone, Centaurea showed stronger competitive effects than Bromus when grown together.  This contrasts with field patterns where Centaurea invades open grassland aggressively while Bromus is far more abundant under the canopies of Pinus.  Interestingly however, the addition of Pinus litter eliminated the competitive effect of Centaurea on both grass species.  By ameliorating the intensity of competition via litterfall, savanna pines may allow less competitive native species to persist during invasion by Centaurea but at the same time enhance conditions for Bromus invasion.  The complexity of these interactions suggests that community organization cannot be easily predicted from the behavior of isolated populations.