| Abstract: | The restoration of perennial grasslands in western North America often depends on effective weed control. We took advantage of a grassland restoration site on the Nature Conservancy's Agate Desert Preserve in southern Oregon (TNC 1997), where 3 sites had been previously burned, mowed, or both. At these sites we carried out a series of controlled, replicated experiments designed to test the effectiveness of 3 weed control measures: (1) sawdust, (2) glyphosate herbicide, and (3) herbicide plus an alfalfa mulch. All plots were seeded with a mix of 3 native perennial grasses. The soils of the 3 areas differing in previous vegetation management were similar, with the exception of total available soil nitrogen, which was significantly lower in the 2 burned sites. The sawdust treatment reduced total available soil nitrogen, but only in the unburned site, and only in the first few months after application. In all 3 areas the alfalfa mulch significantly increased total available soil nitrogen. However, none of these soil nitrogen differences significantly affected the success of weeds or planted perennial grasses. The herbicide treatment reduced exotic annual grasses and forbs and greatly increased the success of native forbs and the planted perennial grasses. The herbicide increased both initial establishment of the native grasses and their absolute cover and biomass. These results suggest that neither nitrogen impoverishment nor nitrogen enrichment was a useful restoration technique at this site, but weed control by herbicides can be of considerable assistance in restoring native perennial grasses. |
