Long-term ecosystem impacts of an introduced grass in the northern Great Plains

Introduced plants can have negative effects on native species and diversity , but their impacts on ecosystem function are less apparent. At the norther n edge of the Great Plains, we examined five stands in each of undisturbed prairie, successional prairie (fields abandoned for similar to 50 yr that h ad undergone succession to native grasses), and abandoned fields of similar age planted with Agropyron cristatum, a C-3 perennial tussock grass introd uced from northern Asia. We attributed differences between successional pra irie and A. cristatum stands to the introduced species and not to cultivati on. A. cristatum fields contained few native species, resulting in signific antly lower species richness and diversity. Growth forms most similar to A. cristatum, i.e., C-3 grasses, were most likely to be excluded. Soils under A. cristatum had significantly less available N, total N, and total C than soils under successional prairie. A. cristatum fields had significantly hi gher shoot mass than successional prairie, but root:shoot ratios in A. cris tatum fields were less than half those under successional prairie, because root mass was significantly higher under native grasses. C:N ratios did not vary significantly between A. cristatum and native grasses, suggesting tha t the lower N and C content of soils under A. cristatum was not caused by d ifferences in nutrient concentrations. Instead, the relatively low root mas s of the introduced grass was probably responsible for decreased rates of N and C addition to the soil. Because total C was 25% less in soils under A. cristatum than under successional prairie, the planting of A. cristatum ov er millions of hectares of the Great Plains may have left 3.3-4.8 X 10(14) g of C in the atmosphere that would otherwise have been stored as soil orga nic matter by native grasses. In contrast to the large effects of A. crista tum, almost no significant differences were found between undisturbed and s uccessional prairie. Thus, the identity of the species dominating after dis turbance played a larger long-term role in determining ecosystem function t han did the disturbance itself. The results suggest that the effects of thi s introduced species extend beyond the displacement of native species and t he reduction of diversity, and include the alteration of pools and flows of energy and nutrients in the prairie ecosystem.