| Current methods of controlling weeds in non-cropland often preclude the management of such areas for conservation of biological diversity. It may, however, be possible to control weeds, which are adapted to high resource availability, by restoring native late-successional plant communities, which tend to reduce resource availability. To test this hypothesis, I examined weed invasion in randomized plots of restored prairie and unrestored old-field vegetation. Over 7 years, restoration reduced the total biomass of extant weed species by 94%, and significantly reduced stem number of five extant weed species. In order to control for differences in propagule pressure and examine mechanism(s) underlying weed responses, I added weed species to subplots within restoration treatments, and treated subplots with all combinations of burning and N addition. Restoration reduced total added weed biomass by 93% in year one and 76% in year two, and reduced biomass of 6 individual weed species. Several lines of evidence suggest that prairie reduced weed invasion through competition and/or recruitment limitation, including characteristics of restored prairie (high biomass, high litter mass, low light availability), decreases in numbers and size of weeds with restoration, and amelioration of negative effects of restoration on weed species with N addition and burning. These results suggest that well established restored prairie can control a broad array of weed species. Shortly after restoration, however, resource levels are higher, and fast growing weed species often impede the establishment of prairie species. A potential solution to this problem is to approximate a late successional environment by adding C to the soil, thereby inducing soil microorganisms to immobilize available N. I tested this hypothesis by comparing the productivity of 10 weed species and 10 tallgrass prairie species under 14 levels of C addition. Relative to untreated plots, the highest level of C addition resulted in an 86% decrease in available NO3-N, a 14-fold increase in early-season light availability, a 54% decrease in weed biomass, and a 7-fold increase in prairie biomass. Nitrogen addition significantly reduced or reversed all of these effects. Results indicate that C addition can facilitate prairie restoration. |
