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Populations of Pinus ponderosa and Juniperus virginiana are expanding into semiarid Sandhills grasslands in Nebraska. To evaluate the physiological basis of their success, we measured the seasonal course of leaf gas exchange, plant water status, and carbon isotope discrimination in these two native trees and two native C4 grasses (Schizachyrium scoparium and Panicum virgatum). Compared to the trees, grasses had higher net photosynthetic rates (Anet) and water use efficiency (WUE) and more negative predawn and midday water potentials (Ψ) in June and July. While leaf Ψ and rates of leaf gas exchange declined for all four species during August, the Ψmid of the grasses were significantly more negative than those of the two trees. The deeply rooted trees maintained water status during summer, in contrast to the grasses, which senesced. Juniperus virginiana in particular was well adapted to xeric conditions, with low stomatal conductance, high WUE, and positive Anet at low Ψ. The highest values of Anet were observed in May for J. virginiana and in May and September for P. ponderosa. Both species maintained low but positive Anet throughout the winter at temperatures above 0°C. Leaf carbon isotopic signature differed between tree and grass species but did not exhibit significant within-species seasonal variability. The semiarid grassland climate of Nebraska does not appear to limit P. ponderosa and J. virginiana, which use growth during the nongrowing season and access to deep soil moisture to compensate for growing-season drought.