Native North American pine attenuates the competitive effects of a European invader on native grasses

Strong direct competitive interactions between two species can be attenuated or exacerbated by the asymmetric effects of other species. In this context we explored a natural pattern in which an exotic invader, Centaurea stoebe, was much less abundant under ponderosa pines than in open grassland. In a field experiment Centaurea had strong competitive effects on the native Pseudoroegneria spicata in open grassland. Pines had no effects on either Centaurea or Pseudoroegneria when they were grown alone, but the strong competitive effects of Centaurea on Pseudoroegneria found in open grassland did not occur under pine canopies. In common garden experiments we found that shade mimicking one effect of pine canopies inhibited Centaurea but did not affect the native Festucaidahoensis. Shade did not alter competitive outcomes between the native and the invader. In the same experiment, soil from under pines increased Centaurea size and its competitive effects on Festuca, the opposite of what would be predicted from natural patterns in the field. However, pine litter strongly suppressed Centaurea in field and greenhouse experiments, inhibited growth of Festuca when grown alone, and importantly also eliminated competition between Festuca and Centaurea. Overall, our results suggest that ponderosa pine may resist Centaurea invasion both directly and by attenuation of the invader’s competitive effects. This biotic resistance appears to occur in part through asymmetric direct effects of pine litter, and in part from ways in which the litter, canopy effects, and soil under pines modify interactions between Centaurea and natives.