Lack of Hydraulic Acclimation in Response to Multiple Droughts and Recovery.

Human-caused climate change is expected to bring more frequent and extended droughts with shorter wet periods of relief in between for many regions of the world. Critical knowledge gaps remain around the extent to which tree species can acclimate physiologically to repeated droughts. Using a common garden experiment with saplings of the widespread and ecologically important trembling aspen (Populus tremuloides), we studied how repeat drought and recovery treatments impact plant functional traits and physiological responses. We utilised four treatments over the course of 3 years: a continuous control, a three consecutive drought year treatment, a two consecutive drought year treatment, and a treatment with two drought years with a well-watered recovery year in between. Trees that experienced a recovery period between two drought treatments exhibited less drought resilience compared to both the two and three-consecutive-year treatments. Trees in the recovery treatment also exhibited lower growth rates and increased xylem damage in their older rings. Additionally, we observed no evidence of hydraulic acclimation to severe drought over time. These findings indicate that, during severe and repeated droughts, aspen saplings are likely to experience increases in hydraulic vulnerability, and wet periods between droughts are not likely to improve overall drought responses.