1. An air‐injection method was used to study loss of water transport capacity caused by xylem cavitation in roots and branches of Pinus edulis (Colorado Pinyon) and Juniperus osteosperma (Utah Juniper). These two species characterize the Pinyon–Juniper communities of the high deserts of the western United States. Juniperus osteosperma can grow in drier sites than P. edulis and is considered the more drought tolerant. 2. Juniperus osteosperma was more resistant to xylem cavitation than P. edulis in both branches and roots. Within a species, branches were more resistant to cavitation than roots for P. edulis but no difference was seen between the two organs for J. osteosperma. There was also no difference between juveniles and adults in J. osteosperma; this comparison was not made for P. edulis. 3. Tracheid diameter was positively correlated with xylem cavitation pressure across roots and stems of both species. This relation suggests a trade‐off between xylem conductance and resistance to xylem cavitation in these species. 4. During summer drought, P. edulis maintained higher predawn xylem pressures and showed much greater stomatal restriction of transpiration, consistent with its greater vulnerability to cavitation, than J. osteosperma. 5. These results suggest that the relative drought tolerance of P. edulis and J. osteosperma results in part from difference in their vulnerability to xylem cavitation.

Predawn leaf water potential (Ψ) and xylem pressure potential (Ψ) are expected to be in equilibrium with the soil water potential (soil Ψ) around roots of well-watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Ψ, leaf Ψ, and xylem Ψ in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil-root hydraulic continuity. Most species failed to equilibrate fully with soil Ψ, depending on whether leaf Ψ or xylem Ψ was used as the measure of predawn plant water potential. The contribution of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem Ψ for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil Ψ. Plant xylem Ψ or leaf Ψ were significantly more negative than soil Ψ for 15 of 15 species where soil Ψ was measured. Predawn disequilibrium based on leaf Ψ was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Ψ is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Ψ and xylem Ψ may not reflect soil Ψ, particularly for woody plants and halophytes, even under well-watered conditions.

Predawn plant water potential (Ψw, measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Ψpc here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Ψw accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Ψw or Ψpc at predawn substantially more negative than the Ψw of soil accessed by roots. In the western Great Basin desert, the magnitude of PDD calculated as soil Ψw minus predawn leaf Ψw was as large as 1.4 and 2.7 MPa for two codominant shrub species, Chrysothamnus nauseosus and Sarcobatus vermiculatus, respectively. The magnitude of PDD calculated as soil Ψw minus predawn Ψpc was smaller, up to 0.6 and 2.1 MPa for Chrysothamnus and Sarcobatus, respectively. For both species, mechanisms contributing to PDD included nighttime transpiration and putative leaf apoplastic solutes, but not hydraulic conductance limitations. Hydraulic lift also occurred in both species and likely contributed to PDD for Sarcobatus. Finding large magnitude PDD in field populations emphasizes that species differences in predawn plant Ψw or Ψpc do not necessarily reflect differences in accessible soil Ψw and rooting depth, nor does a low predawn plant Ψw or Ψpc value necessarily mean that soil Ψw is also low. Mechanisms contributing to PDD affect relationships between plants and soil resources, as well as the potential for plant–plant interactions. Corresponding Editor: F. C. Meinzer

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