Recovery of Populus tremuloides seedlings following severe drought causing total leaf mortality and extreme stem embolism.

In contrast with other native Populus species in North America, Populus tremuloides (aspen) can successfully establish itself in drought-prone areas, yet no comprehensive analysis has been performed on the ability of seedlings to withstand and recover from a severe drought resulting in complete leaf mortality. Here, we subjected 4-month-old aspen seedlings grown in two contrasting soil media to a progressive drought until total leaf mortality, followed by a rewatering cycle. Stomatal conductance (g(s) ), photosynthesis and transpiration followed a sigmoid decline with declining fraction of extractable soil water values. Cessation of leaf expansion occurred close to the end of the linear-decrease phase, when g(s) was reduced by 95%. Leaf mortality started after g(s) reached the lowest values, which corresponded to a stem-xylem pressure potential (Ψ(xp)) of -2.0 MPa and a percent loss of stem hydraulic conductivity (PLC) of 50%. In plants with 50% leaf mortality, PLC values remained around 50%. Complete leaf mortality occurred at an average stem PLC of 90%, but all seedlings were able to resprout after 6-10 days of being rewatered. Plants decapitated at soil level before rewatering developed root suckers, while those left with a 4-cm stump or with their stems intact resprouted exclusively from axillary buds. Resprouting was accompanied by recovery of stem hydraulic conductivity, with PLC values around 30%. The percentage of resprouted buds was negatively correlated with the stem %PLC. Thus, the recovery of stem hydraulic conductivity appears as an important factor in the resprouting capacity of aspen seedlings following a severe drought.