Impacts of long-term precipitation manipulation on hydraulic architecture and xylem anatomy of piñon and juniper in Southwest USA.

Hydraulic architecture imposes a fundamental control on water transport, underpinning plant productivity, and survival. The extent to which hydraulic architecture of mature trees acclimates to chronic drought is poorly understood, limiting accuracy in predictions of forest responses to future droughts. We measured seasonal shoot hydraulic performance for multiple years to assess xylem acclimation in mature pinon (Pinus edulis) and juniper (Juniperus monosperma) after 3+years of precipitation manipulation. Our treatments consisted of water addition (+20% ambient precipitation), partial precipitation-exclusion (-45% ambient precipitation), and exclusion-structure control. Supplemental watering elevated leaf water potential, sapwood-area specific hydraulic conductivity, and leaf-area specific hydraulic conductivity relative to precipitation exclusion. Shifts in allocation of leaf area to sapwood area enhanced differences between irrigated and droughted K-L in pinon but not juniper. Pinon and juniper achieved similar K-L under ambient conditions, but juniper matched or outperformed pinon in all physiological measurements under both increased and decreased precipitation treatments. Embolism vulnerability and xylem anatomy were unaffected by treatments in either species. Absence of significant acclimation combined with inferior performance for both hydraulic transport and safety suggests pinon has greater risk of local extirpation if aridity increases as predicted in the southwestern USA. The unknown capacity of mature trees to acclimate their hydraulic architecture to climate forcing is a source of uncertainty in predicting the response of long-lived individuals to progressive climate change. We used a multi-year in situ precipitation-manipulation experiment to study hydraulic performance of mature pinon and juniper. When subjected to supplemental watering or experimental drought, neither species demonstrated significant adjustments in either hydraulic transport safety or efficiency. We conclude that the hydraulic architecture of mature trees in pinon-juniper woodlands is unlikely to acclimate to predicted climate changes, leaving trees more susceptible to the effects of increased temperature and water stress.