COS 143-3 - Pine death: a global analysis of mortality drivers and physiological thresholds

Abstract: Anthropogenic climate change has resulted in intensified, more frequent, prolonged extreme weather events. Extreme drought, heatwaves, and combined heat-drought have resulted in widespread forest mortality globally. Recent global field observations of drought and heat associated tree die-off found 25/154 mortality-observed tree species to be in the genus Pinus (~1/4 of Pinus species). Pinus, a genus containing some of Earth’s oldest and most-resilient trees, is threatened by further climate change. We must determine lethal physiological thresholds for environmental stressors to better predict forest mortality globally. Our first objective was to analyze peer-reviewed studies of Pinus species to identify abiotic agents driving observations of widespread Pinus mortality in recent decades. Our second objective was to synthesize empirical evidence of abiotic stress tolerances for Pinus to determine physiological thresholds for survival. We conducted a review of existing peer-reviewed studies and unpublished findings acquired by data requests to determine the proportion of Pinus mortality events by driver (e.g., drought, heatwaves, combined heat-drought, or potential biotic agents atop abiotic stress). Secondly, we determined the proportion of Pinus species for which lethal physiological thresholds have been measured. Our synthesis reveals the variation in abiotic stress tolerances across measured Pinus species, within species, and within phenology. Of 46 Pinus mortality studies reviewed, ~2/3 were attributed to drought, ~1/3 were combined heat-drought, and half of Pinus mortality studies suspected potential biotic agent interactions. Nearly half (46%) of Pinus species do not have any abiotic stress tolerance measurements. Heat thermotolerance had the least studies with < 10% of Pinus species having any described lethal limits in our review. Drought tolerance ranked second (~33% of species measured P<sub>50 at least once), while cold thermotolerance was the most studied abiotic stress tolerance (~40% of species measured FR/LT₅₀ at least once). The interspecific range for heat thermotolerance (T_crit) in Pinus species was 41.0-48.2°C, while intraspecific range within P. taeda was 36.9-43.7°C. There was a 4°C difference in T₅₀ between immature (42.6°C) and mature (46.6°C) foliage of P. taeda, highlighting the importance of phenology and heat-wave timing on tree, population, and ecosystem responses. Further warming, both chronic and acute, is predicted to increase with the highest confidence of any climate metric. Given that many Pinus spp. have experienced widespread mortality under hotter-droughts, there is an urgent need to understand lethal limits for the 52 species which have never been measured. Our future work aims to determine lethal physiological thresholds across this climate-threatened genus.