Characterizing variability in hydraulic traits in co-occurring western subalpine conifers to improve drought response predictions

Background: Western conifer forests are increasingly exposed to elevated water-stress caused by climate change. Although we have a strong understanding about the hydraulic traits controlling a tree's vulnerability to water-stress, we lack critical information about how these traits vary between western conifer species, and if such variation is important for maintaining critical ecosystem services and watershed function. Therefore, to strengthen our ability to model the vulnerability of two dominant Rocky Mountain subalpine forest species to future droughts, we characterized variation in selected hydraulic traits in Engelmann spruce and subalpine fir growing in the East River watershed of the Colorado Rocky Mountains. We measured maximum xylem conductivity of canopy branches, leaf turgor loss point, osmotic potential and bulk modulus of elasticity, and diurnal stomatal conductance during the summer seasons of 2018 and 2019.

Results and Significance: Engelmann spruce and subalpine fir have similar tolerances to water-stress at the leaf level with turgor loss points of -1.95±0.06 and -1.75±0.06 MPa, osmotic potentials of -1.50±0.05 and -1.30±0.04 MPa, and bulk modulus of elasticities of 14.7±0.9 and 10.0±1.0 MPa, respectively. Engelmann spruce has more efficient xylem transport where its maximum branch conductivity is about double that of subalpine fir. In contrast, stomatal conductance of subalpine fir (0.048±0.005 mol H₂O m^-2 s^-1) is about double that of Engelmann spruce (0.020±0.005 mol H₂O m^-2 s^-1) in the morning, however, it is similar and greatly reduced by midafternoon for both species(~0.010±0.005 mol H₂O m^-2 s^-1). The hydraulic traits and diurnal patterns of stomatal conductance measured in this study provide much needed information for parameterizing and benchmarking plant water-use algorithms in vegetation models. Improving these models holds great promise for making robust predictions about the risk more intense droughts pose for altering the function and services subalpine forests provide Rocky Mountain watersheds.