Is management of an invasive grass Agropyron cristatum contingent on environmental variation?

1. The control of invasive species is a challenge heightened by the dependency of management outcomes on environmental variation. This is especially true for plants invading semi-arid habitats, where growing season precipitation varies greatly among years. 2. Agropyron cristatum is an invasive grass widely introduced in the Great Plains of North America. We studied its demographic responses to management using field experiments and matrix population models. Plants were clipped to simulate grazing, treated with herbicide or left unmanaged, at three levels of water availability, for 2 years. 3. Growth rates (lambda) were high in unmanaged populations. Clipped populations were mainly stable, whereas lambda for herbicide-treated populations varied greatly with water availability and between years. Low lambda in clipped and herbicide-treated populations was mainly the result of low seed production, and these populations were the most sensitive to water availability. 4. Clipped populations produced no seeds in the second year, indicating a cumulative negative effect of defoliation. In general, seed production, germination and juvenile survival all increased with water supply, suggesting that invasion may increase under wet conditions. 5. Population projections revealed a steady increase in population size for unmanaged populations, whereas clipped populations were more stable. Herbicide-treated populations mainly decreased. Life-cycle stages associated with recruitment contributed the most to the rapid growth of unmanaged populations, whilst the persistence of managed populations relied on the survival of established tussocks. 6. Synthesis and applications. These results demonstrate strong management effects on A. cristatum invasion in spite of significant population responses to water availability. Management can therefore have large effects on the invasion of native grasslands regardless of among-year variability in precipitation.