Experimental effects of cover crops on non-native weeds and prairie grass restoration

We hypothesized that cover crops could be used in restoration to both decrease non-native weed invasion and increase the establishment of C₄ prairie grasses.  Specifically, we predicted that a C₃ cover species, western wheatgrass (Pascopyrum smithii), would indirectly increase establishment of C₄ prairie grasses by inhibiting non-native weeds and would directly increase establishment of C₄ prairie grasses by acting as a nurse plant.  We compared the effects of western wheatgrass to another common cover crop, smooth brome (Bromus inermis).  To test these predictions we factorially manipulated both the presence of cover species (no cover species; western wheatgrass; smooth brome) and the abundance of non-native weedy species (no non-native species sown; low density of non-native species sown; 10x the low density of non-native species sown).  We sowed four species of C₄ prairie grass into each treatment (Andropogon gerardii, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans).  The experiment was begun in September 2005 in a greenhouse using field soil collected in Kansas.  Our results show that western wheatgrass and smooth brome did not significantly affect the percent groundcover of non-native weeds or C₄ prairie grasses.  However, successively higher sowing densities of non-native weedy species did decrease the percent groundcover of C₄ prairie grasses (P < 0.05).  The effects of non-native weedy species may have been due to quick growth and resource pre-emption since higher weed sowing densities caused both soil moisture and light levels to decrease significantly.  There were no significant interactive effects of weed sowing density and C₃ cover crop identity.  Overall, our results suggest that non-native weeds suppress the establishment of C₄ prairie grasses.  We will also discuss soil nutrient and individual species abundance results as well as results from an ongoing cover crop field experiment.