As growing seasons in the pacific northwestern USA become longer, mixed conifer forests of the region face greater seasonal water loss. The growing season of 2015, which happened to be an extreme drought, offered a potential analog for future conditions. During that season, we measured gas exchange, leaf water potential, soil water, and hydraulic conductivity on six conifer species: Abies grandis, Larix occidentalis, Pinus monticola, Pinus ponderosa, Pseudotsuga Menziesii, and Thuja plicata. The effects of the water deficit on the trees were mitigated by the Palouse soils; the stand observed has a meter of ash-capped silty loam soil, giving it remarkable soil moisture storage. This prevented the trees from experiencing local drought conditions (i.e. gas exchange continued and leaf water potentials never went below –2.6 MPa), despite the region’s extreme drought. Using a hydraulic transport model (Sperry et al. 1998), we compared the predicted and critical transpiration fluxes and water potentials of the six species during dry years in two simulated systems, the Idaho system and a more common montane ecosystem with shallow soil. We hypothesized that, while all six can coexist in a Palouse system, the species’ differences would become more obvious in soil with reduced water holing capacity.
Results/Conclusions
While the six species reacted similarly to drought conditions in the Palouse system, the magnitude and implications of their responses in a more common montane system indicated different degrees of resilience to extended water stress. As predicted, L. occidentalis, P. ponderosa, and P. monticola maintained larger safety margins between their predicted transpiration and critical transpiration. Interestingly, L. occidentalis depleted soil water content earlier than the rest, while P. ponderosa maintained greater soil water content farther into the growing season. For all six species, the midday water potentials remained between - 2.0 and -2.5 MPa, but the predawn water potentials showed more variation. At the Idaho site, as measured, the model predicted predawn water potentials to remain high for over a month and then begin to become more negative, (~ -1.0 - -1.5 MPa). In the dry site, however, the predawn water potentials began to decline soon after precipitation stopped for the season. As the water potentials declined, the species’ safety margins between predicted and critical transpiration converged at different rates, indicating different degrees of resilience to unmitigated drought.