A river runs through it: intraspecific variability in hotter-drought-related physiological traits across the...

Human activities are intensifying heatwaves and droughts, threatening forest ecosystems worldwide. However, understanding intraspecific variation in physiological traits that confer resistance to hotter drought is critical for predicting tree responses to climate change. In Pinus taeda L., long-term geographic isolation across the Mississippi River Valley after the last glacial maximum has resulted in distinct population genetic structures, likely reflecting historical adaptation to contrasting climatic conditions in western and eastern refugia. However, it remains unclear whether contemporary populations retain meaningful variation in traits that confer tolerance to hotter and drier conditions, such as hydraulic vulnerability and water-use regulation, which are critical for predicting responses to future climate extremes. Here, in a common garden of adult P. taeda trees planted in 2010, we characterized intraspecific variability in 22 physiological traits related to resistance to hotter droughts across 10 provenances originating from either west or east of the Mississippi River Valley. Key traits included xylem vulnerability to embolism, leaf and bark residual conductance, and leaf turgor loss point. We used an integrated indicator, Time to Hydraulic Failure (THF), predicted by a mechanistic hydraulic model, SurEau, to assess how trait combinations contribute to tree resistance to hotter droughts. While we hypothesized that western provenances would be more adapted to hotter drought, we found that THF was lower in western than eastern provenances. The THF values were negatively correlated with residual transpiration and leaf mass per area. These patterns suggest physiological differentiation between populations, although not one strictly determined by drought resistance alone. Overall, our findings demonstrate that intraspecific variation in physiological traits can inform forest management and breeding strategies, underscoring the complex interplay of hydraulic and residual transpiration traits in shaping drought resilience and emphasizing the need to integrate multiple physiological processes when predicting forest responses to climate change.