The Effect of Atmospheric Demand and Soil Moisture Deficits on Tree Growth and Physiological Water Stress: Implications for the Future of Forest Management

Abstract:

Management plans designed to create climate resilient forests need to account for the drivers of drought stress in trees. Seasonal variation in soil and atmospheric conditions can change markedly between the early and late growing season in the Pacific Northwest, USA. Our research investigated how interannual variability in atmospheric and soil drought influenced the growth and physiological water stress in Douglas-fir trees in the western Oregon Cascades. The seasonal shift to drier conditions can cause stomatal conductance to decline. Therefore, we measured the width of latewood in annual tree rings, which is typically developed under seasonal conditions of increased atmospheric aridity and soil drought. Our preliminary findings suggested that increased vapor pressure deficits early in the summer growing season suppressed the annual latewood growth in Douglas-fir as measured by the latewood ring width index. Further, summer precipitation and soil moisture storage were also correlated to latewood growth. We used a soil-plant-atmospheric model to explore the physiological response of trees to summer drought conditions, specifically examining how transpiration responded to varying atmospheric water demand versus the amount and timing of precipitation. Disentangling the relative role of soil and atmospheric drought will help determine the best solutions for managing forest water stress under a warmer and drier climate in the future.