We used the natural abundance of stable isotopic ratios of hydrogen and oxygen in soil (0.05-3 m depth), plant xylem and precipitation to determine the seasonal changes in sources of soil water uptake by two native encroaching woody species (Pinus ponderosa P. & C. Lawson, Juniperus virginiana L.), and two C(4) grasses (Schizachyrium scoparium (Michx.) Nash, Panicum virgatum L.), in the semiarid Sandhills grasslands of Nebraska. Grass species extracted most of their water from the upper soil profile (0.05-0.5 m). Soil water uptake from below 0.5 m depth increased under drought, but appeared to be minimal in relation to the total water use of these species. The grasses senesced in late August in response to drought conditions. In contrast to grasses, P. ponderosa and J. virginiana trees exhibited significant plasticity in sources of water uptake. In winter, tree species extracted a large fraction of their soil water from below 0.9 m depth. In spring when shallow soil water was available, tree species used water from the upper soil profile (0.05-0.5 m) and relied little on water from below 0.5 m depth. During the growing season (May-August) significant differences between the patterns of tree species water uptake emerged. Pinus ponderosa acquired a large fraction of its water from the 0.05-0.5 and 0.5-0.9 m soil profiles. Compared with P. ponderosa, J. virginiana acquired water from the 0.05-0.5 m profile during the early growing season but the amount extracted from this profile progressively declined between May and August and was mirrored by a progressive increase in the fraction taken up from 0.5-0.9 m depth, showing plasticity in tracking the general increase in soil water content within the 0.5-0.9 m profile, and being less responsive to growing season precipitation events. In September, soil water content declined to its minimum, and both tree species shifted soil water uptake to below 0.9 m. Tree transpiration rates (E) and water potentials (Psi) indicated that deep water sources did not maintain E which sharply declined in September, but played an important role in the recovery of tree Psi. Differences in sources of water uptake among these species and their ecological implications on tree-grass dynamics and soil water in semiarid environments are discussed.

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 C 4 grasses (Schizachyrium scoparium and Panicum virgatum). Compared to the trees, grasses had higher net photosynthetic rates (A net ) and water use efficiency (WUE) and more negative predawn and midday water potentials (C) in June and July. While leaf C and rates of leaf gas exchange declined for all four species during August, the C mid 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 A net at low C. The highest values of A net were observed in May for J. virginiana and in May and September for P. ponderosa. Both species maintained low but positive A net throughout the winter at temperatures above 0°C. Leaf carbon isotopic signature differed between tree and grass species but did not exhibit significant withinspecies 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.

While some studies aim to generalise the attributes of woody encroachers, examining their functional differences across biogeographic regions may also be instructive. Most of Texas is encroached by Prosopis glandulosa, but on the eastern Edwards Plateau, a limestone plateau with thin soils, P. glandulosa is rare and Juniperus ashei is dominant. We hypothesised that P. glandulosa is excluded from sites where bedrock at a depth of 1 m or less restricts the development of taproots, thus rendering this normally deep-rooted species too vulnerable to drought. To test this idea, we monitored the physiological status of the two species on a site where both species were encroaching and the soil was 1Ð5-2Ð5 m deep, thus relatively deep for the Edwards Plateau region but not for regions where P. glandulosa dominates. Data were collected across three tree size classes from May to November 2006. Stem water potentials were similar across species and water potentials and photosynthetic rates decreased with tree size. Based on isotopic evidence, the effective rooting depth of P. glandulosa increased with tree size, but not in J. ashei. P. glandulosa had a higher rate of leaf gas exchange overall as expected for a drought-avoider; except for large trees during a summer drought, when species differences in photosynthesis rates diminished and the stomatal conductance for J.ashei exceeded that of P. glandulosa. We speculate that restrictions of soil depth may limit the invasiveness of P. glandulosa in part through negative effects on large trees, which may lower fecundity.

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