Leaf-water relations of a native and an introduced grass species in the mixed-grass prairie under cattle grazing.

Both the native Pascopyrum smithii and introduced Poa pratensis are dominant plants in the mixed-grass prairie. Knowledge of leaf-water relations and water use strategies in these two grasses under animal grazing and drought is needed for understanding responses of the prairie to future climate change. We studied leaf-water relations traits of Pascopyrum and Poa by combined use of information from the pressure-volume analysis, leaf structural characteristics and leaf stomatal conductance as a function of leaf water status. Fieldwork was done in three grazed pastures and three exclosures in 2002 and 2003. With growing season drought, available soil water became lower in the grazed pastures than in the non-grazing exclosures. This had a more negative impact on leaf water status of the shallow-rooted Poa than the deep-rooted Pascopyrum. In both grasses, a significant decrease in leaf stomatal conductance occurred near the turgor loss water potentials of -2.42 MPa for Poa and -3.5 MPa for Pascopyrum, respectively. Both Poa and Pascopyrum responded to grazing with more negative osmotic potentials (and thus more concentrated cell solution) in months of peak growth. Specific leaf area and its two components, i.e., leaf density and thickness, had minimum responses to grazing for Pascopyrum, while in Poa, less dense leaves accompanied by a higher specific leaf area was observed, suggesting that long-term grazing encourages a higher potential growth rate but also a higher susceptibility to drought stress in Poa than in Pascopyrum.