As air temperature increases, evaporative demand also increases, increasing vapor pressure deficit (VPD) and climatic water deficit (DEF; potential evapotranspiration minus actual evapotranspiration) and altering water availability in forest ecosystems. These integrated climate variables reflect the water and energy balance of terrestrial ecosystems. As deficits increase, water is lost to the atmosphere through plant tissues and from the soil, which affects water use efficiency, individual tree growth and vigor, and forest productivity. Using a spatially comprehensive network of Douglas-fir (Pseudotsuga menziesii) chronologies from 122 locations that experience distinctly different climate in the western United States, we correlated growth with temperature, precipitation, VPD, and DEF. By sampling throughout “climate space” at the continental scale, these data account for a large percentage of variability in growing environments for Douglas-fir which allows for a full assessment of the species response to climatic variability. Using an ensemble of global circulation models, we project an increase in both the mean VPD associated with the lowest growth extremes and the probability of exceeding these VPD values.
Results encompassing historical climate records (since 1916) indicate that temperature decreases growth via VPD and DEF. Increased VPD results in a steeper water potential gradient and stomatal closure, causing trees to lose water, and reduce carbon uptake and photosynthesis. Reductions in growth are observed at all latitudes, although the effects of VPD and DEF are most pronounced in the southernmost latitudes and at the lowest elevations. Results suggest that there is an 80-90% probability that the harshest growing conditions will be prevalent by 2080. As temperature continues to increase in future decades, we can expect deficit-related stress to increase and consequently Douglas-fir growth to decrease throughout its US range.