Responses of the codominant grassland species Andropogon gerardii and Sorghastrum nutans to long-term manipulations of nitrogen and water

The availability and distribution of essential resources such as water and nitrogen are changing on a global scale and it is important to determine th e effects of such alterations on dominant and subdominant species in ecosys tems. We examined the effects of long-term additions of water and nitrogen on leaf-level ecophysiological characteristics of the tallgrass prairie cod ominants Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indian grass). Both species responded to irrigation with increased water potential s and to fertilization with decreased water potentials. Only in S. nutans, however, were net photosynthesis and stomatal conductance greater in respon se to both water and nitrogen addition. In contrast, A. gerardii, which cur rently is more abundant than S. nutans, was relatively unresponsive to reso urce manipulations. If net photosynthesis is positively related to plant pr oductivity, as has been shown for tallgrass prairie species, the differenti al photosynthetic responses of these two grasses suggests that their patter ns of abundance may change in the future if resource availability is altere d. These results are also inconsistent with the historical assumption that these two grasses are "ecological equivalents.".