Long-Term Ecosystem Impacts of an Introduced Grass in the Northern Great Plains

Abstract

Introduced plants can have negative effects on native species and diversity, but their impacts on ecosystem function are less apparent. At the northern edge of the Great Plains, we examined five stands in each of undisturbed prairie, successional prairie (fields abandoned for ∼50 yr that had undergone succession to native grasses), and abandoned fields of similar age planted with Agropyron cristatum, a C3 perennial tussock grass introduced from northern Asia. We attributed differences between successional prairie and A. cristatum stands to the introduced species and not to cultivation. A. cristatum fields contained few native species, resulting in significantly lower species richness and diversity. Growth forms most similar to A. cristatum, i.e., C3 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 higher shoot mass than successional prairie, but root:shoot ratios in A. cristatum 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 that the lower N and C content of soils under A. cristatum was not caused by differences in nutrient concentrations. Instead, the relatively low root mass 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 over millions of hectares of the Great Plains may have left 3.3–4.8 × 1014 g of C in the atmosphere that would otherwise have been stored as soil organic matter by native grasses. In contrast to the large effects of A. cristatum, almost no significant differences were found between undisturbed and successional prairie. Thus, the identity of the species dominating after disturbance played a larger long‐term role in determining ecosystem function than did the disturbance itself. The results suggest that the effects of this introduced species extend beyond the displacement of native species and the reduction of diversity, and include the alteration of pools and flows of energy and nutrients in the prairie ecosystem.

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…More fires may act synergistically with N deposition and the invasion by exotics to alter vegetation structure [86]. For example, many drylands are being invaded by exotic species, a process that reduces ecosystem functioning [90] and is expected to be further worsened with global change [91]. Moreover, the productivity and cover of these exotics has also been reported to be enhanced with N deposition [92].…”

Non-indigenous grasses impede woody succession

“…In addition to sharing similar processes and dynamics, the processes of succession and species introductions may directly interact and affect one another. For example, if non-indigenous species significantly affect key ecosystem attributes or processes (Hobbs and Huenneke 1992;D'Antonio and Vitousek 1992;Vitousek et al 1997;Christian and Wilson 1999), they may affect historical successional patterns and dynamics.…”

“…The old fields and native environments differ in their dominant grass species, with the native savannas and grasslands dominated by native warm-season grasses and the old fields normally dominated by introduced European cool season grasses. Grass effects on woody plants can vary depending on the grass species involved (Gordon et al 1989;Rice et al 1993) and some non-indigenous grasses have been found to alter ecosystem properties (Christian and Wilson 1999). Since non-indigenous C 3 and native C 4 grasses differ in their photosynthetic pathways and wateruse efficiency, we hypothesized that the nonindigenous grasses may be depleting soil water levels resulting in reduced oak seedling growth and survival.…”

Invasion, competitive dominance, and resource use by exotic and native California grassland species

“…Although this conversion is often attributed to grazing, exotic annuals have maintained their dominance in many areas that have now been excluded from livestock grazing for decades (30). Along with the displacement of the native perennial flora, this invasion has potentially increased nitrate leaching, altered fire regimes, and decreased carbon storage (29,(31)(32)(33).…”