Abstract: Water can limit the growth of trees across the western US while trees simultaneously influence the seasonality and availability of water resources within a watershed. Understanding this complex link between trees and water is both challenging and important as water resources and forest resilience are threatened by climate change. The dramatic decline of whitebark pine (Pinus albicaulis) is ecologically concerning considering that as of 2016, nearly half (46%) of its distribution has exhibited severe mortality within the Greater Yellowstone Ecosystem. Although it is well established that high-elevation tree species like whitebark pine impact snowpack and streamflow, the hydrologic role of whitebark pine remains poorly understood. To quantify how whitebark pine affects snowpack and streamflow, we utilized a water balance model to assess the partitioning of precipitation into streamflow and evapotranspiration using field data from two sites in Montana. Whitebark pine field sites were selected based on different canopy damage extents (low – high damage), different canopy cover densities (open – closed stands), and proximity to first order streams (top of watershed). At each field site (one “healthy” stand with more live whitebark pine and one “unhealthy” stand with more dead whitebark pine), we measured streamflow from March – October 2022. While analyses of field data are still ongoing, current observations indicate that the “healthier” stand maintained higher streamflow levels throughout the summer season. This initial finding suggests that healthy trees prolong streamflow later into the fall when water supply is most limited. In addition to inputting streamflow field data in the water model, we incorporated data from the Normalized Difference Vegetation Index (NDVI) to adjust evapotranspiration estimates of the water model. With this addition, we improved the water model where it now accurately predicts 85% of streamflow variation on a daily scale. The ability to better predict changes in streamflow as whitebark pine mortality is expected to increase will better inform water management and whitebark pine conservation planning in the Greater Yellowstone Ecosystem.
