Physiology of Pinus taeda: an Ice Age legacy? Intraspecific variability in drought-related physiological traits among provenances from east and west of the Mississippi River Valley

Human activities are increasing temperatures and reducing water availability, intensifying climate variability and extremes including heat waves and droughts, which threaten forest ecosystems. Here, we characterize and model intraspecific variability in physiological traits related to resistance to hotter droughts in loblolly pine (P. taeda). For this, we use complementary approaches to evaluate trait variation and potential differences between two populations that have been geographically and spatially separated since the last glacial maximum (21,000–5,000 years before present). Adaptive variation was investigated by phenotyping provenances originating east and west of the Mississippi River Valley, where long-term geographic separation has resulted in distinct population genetic structure. 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. Measured physiological traits included xylem vulnerability to cavitation, leaf and bark residual conductance, and leaf turgor loss point, each of which is known to be essential for tree drought resistance. Surprisingly, results indicate a tendency for THF to be lower in western provenances compared to eastern ones. Time to Hydraulic Failure was negatively correlated with residual stomatal conductance and leaf mass per area. This pattern suggests a physiological differentiation between populations, although it is not only determined by traits associated with drought resistance. Ongoing work aims to leverage this intraspecific variation to guide selection within and among tree species for more drought-resistant forests under continued climate change.