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Green ash (Fraxinus pennsylvanica Marsh.) communities are valuable as sources of biological diversity and shelter for livestock in the Northern Great Plains. Excessive use of stands by livestock tends to convert these woodland communities to less valuable shrublands. We monitored 12 green ash stands in Theodore Roosevelt National Park (TRNP) from 1985 through 1996 to determine changes in species composition, plant density, and canopy coverage in green ash communities that were protected from livestock but exposed to foraging by native ungulates. Over the 12-year sampling period, density of choke cherry (Prunus virginiana L.) and Saskatoon service-berry (Amelanchier alnifolia Nutt.) in the tree stratum declined, shrub density showed no consistent trends, and canopy cover of grasses and forbs increased. The changes we observed were more likely attributable to succession and weather conditions than to impacts of native ungulates.