Species variation in water relations and xylem vulnerability to cavitation at a forest-woodland ecotone

Xylem vulnerability to cavitation and response of water potential (Ψ), stomatal conductance (g_s), and net photosynthesis (P _n) to drought are potentially important mechanisms of drought resistance. We compared Ψ, g_s, P _n, and cavitation vulnerability of shoot and root xylem among co-occurring ponderosa pine (Pinus ponderosa var. scopulorum Dougl. Ex Laws.), pinyon pine (Pinus edulis Engelm.), and Utah juniper (Juniperus osteosperma [Torr.] Little) at a forest-woodland ecotonal site in northern Arizona to elucidate drought resistance mechanisms of these species. Juniper shoots partly regulated Ψ during drought via stomatal closure, but regulation was weaker than that for ponderosa and pinyon pines, which had similar water relations and P n responses to drought. Midday g_s and P _n during summer drought were positive for juniper (g_s = 14.3 mmol m^-2 s^-1, P _n = 1.23 μmol m-2 s-1) but near zero for ponderosa (g_s= 0.7 mmol m^-2 s^-1, P n = -0.02 μmol m^-2 s^-1) and pinyon (g_s = 1.5 mmol m^-2 s^-1, P _n = -0.18 μmol m^-2 s^-1) pines. Cavitation vulnerability of shoots and roots was lower for juniper than for both pines. The water potential inducing 50% loss in xylem hydraulic conductivity (Ψ₅₀) for juniper was 5.0 MPa more negative for shoots and 3.9 MPa more negative for roots compared with the respective tissues of the pine species. Pinyon pine (Ψ₅₀ = -2.71 MPa) was slightly more vulnerable to cavitation than ponderosa pine (Ψ₅₀ = -3.42 MPa) for shoots, whereas root vulnerability was similar for both pines (Ψ₅₀ = -1.69 MPa for pinyon; -1.98 MPa for ponderosa). Roots of all species were more vulnerable to cavitation than shoots. Our results show an important role of cavitation vulnerability in the greater drought resistance of Utah juniper than pinyon and ponderosa pines but not for the presumed greater drought resistance of pinyon pine than ponderosa pine.