Matthew J. Schultz, Laura G. Perry, and Mark W. Paschke. Colorado State University
Invasive exotic species have severe ecological and economic impacts. Some exotic plant species can invade relatively undisturbed native plant communities, which suggests that they are competitively superior to native species even as seedlings competing with established adults. Other exotic species appear to require disturbance to initiate invasion, but once established can exclude native species. We examined the effects of establishment order on competitive interactions between native plant species and the invasive plant Centaurea stoebe (spotted knapweed) in a greenhouse experiment. Native Gaillardia aristata (common blanketflower) and Festuca idahoensis var. Winchester (Idaho fescue) individuals were separately established before (12 weeks), simultaneously, and after (12 weeks) Centaurea individuals. The species were also grown in monoculture for each time period as controls. Activated carbon, which adsorbs organic compounds, was used to separate effects of chemical interference competition (i.e., allelopathy) from resource competition. To minimize light competition, plastic dividers were installed to separate individuals aboveground. Aboveground biomass was harvested, separated, dried, and weighed after 27 weeks. Early establishment conferred a strong competitive advantage regardless of species identity. The presence of activated carbon did not alter this competitive outcome for any of the species, which suggests that resource competition was more important than chemical interference competition when the species were established at different times. When the competitors were established simultaneously, Festuca appeared to be a stronger competitor with Centaurea than Gaillardia both in terms of responses to Centaurea competition and effects on Centaurea biomass. These experimental results support the hypothesis that Centaurea may require disturbance for invasion, and suggest that rapid colonization, not superior competitive ability, may contribute to Centaurea invasiveness.
