As climate change brings longer and drier growing seasons to the Inland Northwest, the regional mixed conifer forests are facing increasing water stress. To elucidate how six dominant species will deal with this challenge, we collected tree physiology, weather, and soil water data over the 2015 field season, during a severe drought. Fieldwork was conducted in the University of Idaho Experimental Forest in a stand containing Abies grandis, Larix occidentalis, Pinus ponderosa, Pinus monticola, Pseudotsuga menziesii, and Thuja occidentalis. Our objective was to quantify and compare the species’ responses to water stress and to determine which are best suited to persist in the area. The study began soon after prolonged rainfall in late spring. We measured gas exchange and leaf and branchlet water potentials every two hours throughout the day, 2-3 days per month from June through October, during which the stand received only 2.5 cm of precipitation. This allowed for a consistent and sustained decrease in soil water potentials for the duration of the season, giving us snapshots of the different species’ responses at four different levels of drought stress.
Results/Conclusions
For most of the species, the growing season ended by October due to water stress rather than temperature. P. ponderosa was the least sensitive to water stress, continuing to transpire in October at flux densities equal to those in August and September, despite predawn water potentials of -1.3 Mpa. T. plicata and A. grandis were the most conservative with their water use, showing no diurnal changes in transpiration and the lowest flux densities of the six species, despite being located in the areas with greatest soil water potential of the stand. While four of the species had significantly greater stomatal conductance in June than in the late season, only L. occidentalis continued at similarly high levels in July and September. All species had low stomatal conduce in August, when predawn water potentials were around -0.9 Mpa. In September, a small rain event occurred, resulting in increased (0.2 MPa on average) leaf predawn water potentials. However, there was no change in soil water potentials, suggesting that foliar uptake may have occurred. This alleviation of water stress appears to have allowed significantly greater stomatal conductance in September for P. ponderosa, P. monticola, and L. occidentalis.