PS 32-191 - Modeling the influence of species composition on forest ecohydrology

Abstract: Global change is expected to have direct and indirect effects on forest water cycling, leading to uncertainty in the future sustainability of ecosystem services. Species composition and structure of vegetation determine ecosystem responses to disturbance, but the simplified representation of hydrologic processes in forest landscape models limits our ability to link the effects of changing forests on water. We updated the LANDIS-II forest landscape model for hydrologic applications by incorporating species-level transpiration calculations into the Net Ecosystem Carbon and Nitrogen (NECN) succession extension. We conducted a multi-scale calibration of water fluxes, including sap flux measurements to capture relative species transpiration rates, flux tower measurements to capture ecosystem scale evapotranspiration (ET) and net ecosystem exchange (NEE), and a mass balance approach to capture watershed scale ET using reference watersheds at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains of the southeastern US. Using the calibrated model, we simulated ecosystem responses to future climate scenarios in this species-rich, topographically complex landscape. Model validation at the flux tower and watershed captured seasonal and interannual variation and improved the representation of ET along environmental gradients. Under future climate scenarios, the average annual ET increased while streamflow and net carbon uptake decreased. We show forest composition and functional traits will alter future ET, and thus water availability, highlighting the importance of considering changing species in future projections of water availability. Our findings demonstrate the model’s potential for simulating climate, disturbance, and management impacts on forest water dynamics.