Spatiotemporal Assessment of Forest Water Use Response To Climate Change

Abstract:

Water-based ecosystem services provided by forests are increasingly threatened by drought and climate change. Forest water use directly influences downstream water availability and is a key variable in assessing ecosystem health. Despite this, forest water dynamics remain unresolved across species and gradients in climate and topography. Leveraging imputed forest composition maps and two decades of remotely sensed MODIS evapotranspiration (ET), we assessed spatiotemporal forest water use across eastern US forests spanning energy to water limited ecosystems. Annual and interannual trends in evapotranspiration were quantified from 2000 - 2019 and correlated with changes in growing season length. The trends and drivers were further assessed across topoclimate gradients and by forest composition to highlight the impact of cross-scale interactions between regional climate and local forest composition and topography on ecosystem vulnerability to climate change, drought, and changes in forest water use. We identified increasing trends in annual ET, growing season ET, and growing season length (GSL) along a north to south gradient. Pixel-wise correlations between GSL and growing season ET and were positively correlated throughout the study region, except for the southwestern portion where correlations were largely negative. At the sub-continental scale, annual trends in ET were not strongly coupled with aridity or topographic gradients while the ET-GSL relationship was positively coupled with aridity and negatively coupled with slope, indicating ecosystem water use is increasingly dependent on climate driven shifts in vegetation dynamics in wetter and flatter regions. At local and regional scales, the strength and direction of the coupling between elevation and the ET-GSL relationship varied across space, highlighting regional differences in the role of topography mediating climate change impacts on forest water use. These preliminary results suggest that understanding forest water use responses with respect to changing vegetation dynamics over topoclimate gradients would improve our ability to predict changes in freshwater availability across eastern forest watersheds under global change.