The Effects of Cover Crops and Invasive Species on Native Plant Establishment across Several Experiments.

Cover crops have been proposed to control invasive species and increase the establishment of native target plants.  Cover crops could therefore act as an ecological bridge to native plant dominance.  However, few experiments have explicitly examined the effects of cover crops in restoration.  Across two experiments at Fort Riley, Kansas and one experiment in Iowa, we added nine different species of cover crops to different treatments to test the effectiveness of many species of cover crops, and we added yellow sweetclover (Melilotus officinalis) in combination with several cover crops to examine the effectiveness of cover crops in weed control.  In the Iowa experiment, we also examined the direct, facilitative effects of cover crops on target species by removing all weeds in subplots and leaving only cover crops and target species.  We found that cover crops did not significantly increase native target species establishment in any experiment, nor did they decrease yellow sweetclover establishment.  Cover crops did occasionally decrease invasive plant abundance, but this did not lead to increased native target species establishment, likely because cover crops also competed with target species.  Cover crops did not directly facilitate target species since target species grew as well or better in the absence of cover crops as in their presence when weeds had been removed.  Different cover crop species did lead to different plant community composition in both Kansas and Iowa.  Therefore, cover crops do decrease the growth of some species and favor the growth of others.  However, we have not found any cover crop species that decreases the growth of only invasive species while favoring the growth of only target species.  It is unlikely that such species will exist except in rare instances.  Therefore, cover crops may be of generally limited use in subhumid grassland restoration.