Drought resistance and drought adaptation of Douglas-fir (Pseudotsuga menziesii) - A review

The rise in temperatures with climate warming exposes trees and forests on Earth to a triple threat through increased soil drought, enhanced atmospheric drought, and growing heat stress. Understanding which tree species are susceptible to mortality under a more arid future climate is urgent. Here, we review recent progress in our understanding of the drought and heat response of Douglas-fir (Pseudotsuga menziesii, DF) in all relevant fields of research, spanning from stomatal regulation and photosynthetic responses, plant water status dynamics and the vulnerability of the hydraulic system, to adaptive responses of the root system, the climate sensitivity of growth, and climate change-related tree vitality declines and die-off. The species' high productivity is linked to large leaf areas at maturity, which cause fairly high interception and transpiration rates and often result in effective soil moisture depletion and reduced groundwater recharge. While hydraulic safety is high in DF branch xylem, embolism seems common in the xylem of terminal branchlets and small roots during summer drought. The existing evidence suggests that the photosynthetic apparatus and growth rate of DF are fairly heat-sensitive in comparison to other tree species, with impairment of photosynthesis starting at temperatures of similar to 40 degrees C. A key growth-limiting factor is a high atmospheric saturation deficit, which causes partial stomatal closure and growth decline in summer, explaining high productivity in more humid maritime climates. We explore population, provenance and variety differences in the resistance to drought and heat and the related capacity to adapt, and compare the performance of Douglas-fir to that of other tree species. Across provenances and families, trade-offs between productivity and drought resistance, cold resistance and heat resistance have emerged, and more drought-resistant provenances are often less cold-hardy. Recent hot droughts have caused vitality decline and local stand-level die-off in DF in the drier parts of the U.S. and in some European regions. For the economically important coastal variety (P. m. var. menziesii), the existing evidence suggests considerable vulnerability to a future warmer and drier climate, especially in the warmer lowlands, while the interior variety (P. m. var. glauca) may perform better. Perspectives for the silviculture of Douglas fir in a warmer world are outlined.