Investigating competitive inhibition for Canada thistle (Cirsium arvense) and cheatgrass (Bromus tectorum) by comparing their response in relation to functionality and nativeness

Background/Question/Methods

Community composition is determined to a large extent by the interactions among plants.  Large scale disturbances on a local scale decrease the diversity of plant communities and cause a shift from native plants to ruderal invasive plants.  One goal of restoration is to develop an ecosystem functioning similar to that prior to the disturbance.  This study investigates the possibility of utilizing native functionally equivalent plants as a means to control the spread and dominance of noxious weeds.  Greenhouse and field studies were conducted to investigate the impact plant functionality and nativeness have on competitive ability of the ubiquitous noxious species Bromus tectorum and Cirsium arvense. The following hypotheses were investigated: non-native species Bromus tectorum and Cirsium arvense will grow better with native species Achillea lanulosa and Vulpia octoflora than with non-native species and functionally equivalent natives will competitively inhibit functionally equivalent noxious weeds more than functionally different natives (annual graminoid vs. annual graminoid and perennial clonal forb vs. perennial clonal forb).  Vulpia and Bromus were considered functionally equivalent with both being annual graminoids and Achillea and Cirsium considered functionally equivalent with both being perennial clonal forbs.   A two-way ANOVA with nativeness and functional similarity was conducted for species grown in the greenhouse. A forward stepwise regression was employed to determine if the percentage of forbs or the percentage of invasive species in the field could predict success of Cirsium arvense.  

Results/Conclusions

Results of the two-way ANOVA indicate that the native species Vulpia octoflora and Achillea lanulosa inhibit the growth of Bromus tectorum but have little impact on Cirsium arvense.     Aboveground biomass and number of leaves were significantly less (p = 0.001 and p = 0.005 respectively) for Bromus when grown with natives. For Vulpia, this supports the hypothesis of functionally similar inhibition, but not necessarily better than any functional native. In the field, increased graminoid cover predicted decreased performance of Cirsium arvense (p = 0.0201).  Growth patterns (perennial-clonal and annual) may be a better indicator of functional similarity than form (forb and graminoid).  Our results indicated that the current practice of planting native perennial grasses in reclamation sites on the shortgrass steppe is an effective control measure for containing Cirsium arvense because of similarity of growth patterns and that planting the functionally equivalent Vulpia octoflora and native Achillea lanulosa may be useful in controlling Bromus tectorum.