Physiological differences between piñon pine (Pinus edulis) and oneseed juniper (Juniperus monosperma) controlling hydraulic conductance may influence their differential mortality rates during droughts in the American Southwest. These species have essentially opposite strategies in regulation of water status and subsequent carbon gain, yet co-occur widely in semiarid regions projected to experience large precipitation changes. To test hypotheses regarding the mechanistic basis of growth, survival, and mortality responses of each species, we are imposing ecosystem-scale rainfall manipulations in the context of a hydraulic conductance model.  Because piñon is thought to be isohydric, while juniper is anisohydric, we predicted that in juniper leaf water potential (Ψ_leaf) would decline with soil water potential (Ψ_soil) while the stomatal maintenance of constant midday Ψ_leaf in piñon across treatments would severely limit transpiration. Further, we hypothesized that rainfall manipulations would disproportionately affect sap flux density (J_s) in piñon relative to juniper. We measured Ψ_soil, Ψ_leaf (pre-dawn and midday), and J_s before and after rainfall-removal, -addition, and control treatments were imposed in the fall of 2007. 

Results/Conclusions

Rainfall manipulations immediately changed the soil moisture environment in our plots, causing differential responses by the target species consistent with our predictions. Droughted trees had lower J_s than control trees for both species, while water addition initially increased J_s relative to other treatments for piñon but not for juniper. The differences between Ψ_pd and Ψ_md were smaller during drought for both species. Minimum juniper Ψ_md was -4.9 MPa during drought and, consistent with its anisohydric status, juniper Ψ_md’s did not converge after treatment imposition. In contrast, isohydric piñon exhibited strict Ψ_leaf regulation within seasons and across treatments. During summer 2007 minimum piñon Ψ_md was about -4 MPa; following treatment imposition, piñon Ψ_md was constant across treatments. These data are consistent with the hypothesis that piñon responds to drought with stomatal closure, limiting both the negative pressures within its tissues and its ability to gain carbon, while juniper maintains carbon gain in drought albeit at the risk of xylem cavitation.