In recent years, the occurrence and severity of drought events in many semi-arid systems has been increasing. Prolonged drought can lead to tree mortality and canopy dieback, and stressed trees exhibit varying degrees of physiological recovery following drought cessation. A decline in plant hydraulic conductance and canopy gas-exchange is often observed in drought stressed trees, and the recovery of hydraulic function post-drought is critical for surviving trees to take full advantage of favorable growth conditions. Here, we report on the short-term physiological recovery of drought stressed piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees following large precipitation events (90+ mm) that fully alleviated plant water stress. This study is unique, in that our study trees had been exposed to 5+ years of experimentally imposed and naturally occurring drought preceding the precipitation events. To assess short-term physiological recovery post-drought, we examined multiple variables; including whole plant hydraulic conductance (K), plant leaf water potential (ΨPD), and soil water availability (VWC). Our objectives were; 1) to assess the hydraulic recovery of experimentally droughted trees compared to ambient control trees, 2) assess species specific patterns of post-drought recovery, and 3) assess if juniper recovery varied with soil depth.
Results/Conclusions
Following a precipitation anomaly (90 mm day-1) in summer 2012, water stress was alleviated with significant increases in VWC and ΨPD observed across the study site, regardless of pre-event drought severity (i.e., drought treatment versus ambient control). Pre-event (5+ year period), droughted pine and juniper both exhibited significantly lower average plant K as compared to ambient control trees. Post-event, K in droughted pine increased (to 0.6 mol m-2 s-1 MPa-1), but did not recover to levels observed in the control pine (1.1 mol m-2 s-1 MPa-1). In contrast, recovery of K in droughted juniper varied depending on soil depth. In plots with deeper soils (>1m), K of droughted juniper recovered to levels observed in ambient juniper (~1.0 mol m-2 s-1 MPa-1), while K of droughted juniper growing in shallow soil (<1m) was lower than ambient juniper post-event (0.45 versus 0.75 mol m-2 s-1 MPa-1 respectively). Our results indicate that severe drought induces hydraulic & physiological impairment in pine and shallow rooted juniper that persists post drought. Trees with reduced plant K are unable to take full advantage of increased VWC, which is detrimental to tree recovery in a semiarid system where trees often rely on short-term pulses in soil VWC.