Invasion success is strongly influenced by the competitive environment current residents create. With increasing frequency, those residents happen to be other species of previously established exotics. Established exotics may either improve biotic resistance to new invasions or promote invasional meltdown by facilitating subsequent exotics. Ponderosa pine savannas of western Montana have been heavily invaded by Centaurea maculosa for decades but are currently experiencing a rapidly advancing invasion by Bromus tectorum. However, these two invasions exhibit markedly different spatial patterns with C. maculosa dominating native vegetation in the open prairie and dense patches of B. tectorum often constrained to beneath the dripline of isolated ponderosa pine.  Thus, it appears that large pines may have strong direct effects on these invaders and may also drive shifts in competitive interactions among the invaders and among natives and invaders.

Results/Conclusions

We have conducted common garden experiments and found that both invasive exotic species grew larger in conifer soil than in prairie soil, corresponding with high phosphorus availability in soils under tree canopies. Festuca idahoensis and Pseudoroegneria spicata, two native grasses, did not respond positively to conifer soil, but their competitive effects and responses with C. maculosa were improved in conifer soil. In contrast to the soil effects, conifer litter mixed with prairie soil dramatically reduced germination and growth of C. maculosa, moderately reduced the performance of B. tectorum, but had no effect on F. idahoensis. These strikingly different germination responses to the chemicals found in conifer litter may help to explain why C. maculosa invades native plant communities poorly under a canopy, but are insufficient to explain the understory success of cheatgrass. Our results demonstrate substantial conditionality in the invasion processes of two important exotics, and suggest that ponderosa pine may improve native community resistance to C. maculosa through chemical means, but facilitate B. tectorum by increasing soil nutrient availability.