Biotic resistance to invasion: Indirect interactions in ponderosa pine savannas

Plant invasions can have powerful negative effects on indigenous communities.  Despite the tendency of ecologists to focus on these unusually strong effects, invasions can be highly patchy.  In some cases this patchiness is considered to be evidence for local “biotic resistance”.  Spotted knapweed (Centaurea maculosa) invasions in ponderosa pine (Pinus ponderosa) savannas are highly patchy with much lower knapweed densities under the canopies of solitary pines than in the open grassland.  We investigated three plant-plant interactive processes that might contribute to greater native plant resistance to invasion under pines: 1) direct inhibition of knapweed by ponderosa, 2) direct facilitation of native species by ponderosa, or 3) a subtle tree-mediated shift in the competitive balance that reduces the competitive effect of knapweed on the natives.  In field and common garden experiments knapweed grew 30-50% larger in full sun than in shade, but surprisingly grew 35-76% larger in conifer soil than in prairie soil.  These results suggest that the fundamental way that pines inhibit knapweed is competition for light.  Manipulative experiments with the two most abundant native grasses grown alone suggest that they are not significantly facilitated by conifer shade or soil.  Current experiments are investigating shifts in the competitive balance between spotted knapweed and native grassland species in shade and soil conditions created by conifers.  Regardless of the specific mechanism, our results so far suggest that indirect interactions among natives and spotted knapweed play an important role in structuring invaded ponderosa pine savanna communities.