Inter-Specific Transpiration Differences Between Aspen, Spruce, and Pine in a Sky-Island Ecosystem of the North American Great Basin

Semiarid forests may face future challenges because of climate change-exacerbated soil droughts. Because the risk and rate of tree mortality from soil drying can vary widely between co-occurring species, inter-specific differences in tree response need to be evaluated for designing science-driven best-management strategies specifically tailored to iconic sky-island ecosystems. We analyzed sap flow of quaking aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), and limber pine (Pinus flexilis) for five consecutive years (2014–2018) at a remote, high-elevation site in the North American Great Basin. Our results revealed species-specific responses of sap flow to declining soil moisture. For quaking aspen, multi-year sap-flow trajectories mimicked growing-season depletion of soil moisture. Ample cool-season precipitation in 2017 diminished the summer drought impact on sap flow of Engelmann spruce and limber pine, indicating that the two conifers could tap a deeper soil water reservoir than the deciduous species. Since species-specific transpiration response to soil droughts was driven by shifting precipitation regimes, our findings suggest niche partitioning in the rhizosphere among coexisting tree species. In addition, spruce and pine rapidly downregulated canopy conductance with decreasing soil water availability, whereas aspen canopy conductance was insensitive to soil drying. This physiological characteristic allows quaking aspen to maximize resource acquisition when growing conditions are favorable, but poses a risk of hydraulic failure and subsequent mortality under soil drying. Overall, we found contrasting hydrological niches and physiological strategies between co-occurring tree species in semiarid, high-elevation ecosystems. We also emphasize the value of long-term, in-situ observations to determine species-specific susceptibility to environmental changes in remote mountain areas