Forest water use is increasingly decoupled from water availability even during severe drought
Figure 3 from the publication. “MODIS and Landsat ET anomalies averaged at drought peak (ETdp) for all (SPI ≤ − 1.3), moderate (− 1.6 < SPI ≤ − 1.3), severe (− 2.0 < SPI ≤ − 1.6), and extreme droughts (SPI ≤ − 2.0). Pixels with no data or that did not experience a drought at a given severity are in grey”Abstract
Key to understanding forest water balances is the role of tree species regulating evapotranspiration (ET), but the synergistic impact of forest species composition, topography, and water availability on ET and how this shapes drought sensitivity across the landscape remains unclear. Our aims were to quantify (1) the effect of forest composition and topography including elevation and hillslope gradients on the relationship between ET and water availability, and (2) whether the relationship has changed over time. We used remotely sensed Landsat and MODIS ET to quantify forest ET across the Blue Ridge ecoregion of the southeastern USA. Then quantified metrics describing ET responses to water availability and trends in responses over time and assessed how these metrics varied across elevation, hillslope, and forest composition gradients. We demonstrated forest ET is becoming less constrained by water availability at the expense of lateral flow. Drought impacts on ET diverged along elevation and hillslope gradients, and that divergence was more pronounced with increasingly severe drought, indicating high elevation and drier, upslope regions tend to maintain ET rates even during extreme drought. We identified a decoupling of ET from water availability over time, and found this process was accelerated at higher elevations and in areas with more diffuse-porous trees. Given the large proportion of forests on the landscape distributed across high elevation and upslope positions, reductions in downslope water availability could be widespread, amplifying vulnerability of runoff, the health of downslope vegetation, and aquatic biodiversity.
Mirela G. Tulbure
Professor
I am an Associate Professor with the Center for Geospatial Analytics at North Carolina State University (NCSU).