Investigating impacts of drought and disturbance on evapotranspiration over a forested landscape in North Carolina, USA using high spatiotemporal resolution remotely sensed data

Authors: Yang, Yun; Anderson, Martha; Gao, Feng; HAIN, C. - Goddard Space Flight Center; NOORMETS, A. - North Carolina State University; SUN, G. - Forest Service (FS); WYNNE, R.H. - Virginia Polytechnic Institution & State University; THOMAS, V.A. - Virginia Polytechnic Institution & State University

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2018
Publication Date: 5/1/2020
Citation: Yang, Y., Anderson, M.C., Gao, F.N., Hain, C., Noormets, A., Sun, G., Wynne, R., Thomas, V. 2020. Investigating impacts of drought and disturbance on evapotranspiration over a forested landscape in North Carolina, USA using high spatiotemporal resolution remotely sensed data. Remote Sensing of Environment. https://doi.org/10.1016/j.rse.2018.12.017.
DOI: https://doi.org/10.1016/j.rse.2018.12.017

Interpretive Summary: Forests provide many important service functions, including animal habitat, timber production and watershed water regulation. The decline of forests can be the result of many causes, relating generally to disturbance and changing climate. Disturbances include harvest, thinning, wind throw, fire, insect damage and land conversion. Climate, including temperature regime and drought, often affects forest conditions as an integrator of disturbances. For example, drought can make forest more susceptible to insect attack and wildfire, while prolonged drought can directly cause tree mortality. Evapotranspiration (ET) is a key hydrologic variable in assessing forest functioning and health, but it remains a challenge to accurately quantify ET at the landscape scale with the spatial and temporal detail required for effective decision-making. In this study, we apply a multi-sensor satellite data fusion approach to study the response of forest ET to drought and disturbance over a 7-year period. The model results were evaluated and showed good agreement with observed ET, with 8-13% relative errors at monthly time steps. Analyses demonstrate differential response to drought events from different land cover types, with young pine plantations showing larger impacts than mature plantations with significantly deeper rooting systems. The ratio between ET and potential ET appears to be useful in determining various levels of drought impact and can be used to quantify the rate of recovery in water usage after stand disturbance.

Technical Abstract: Forest ecosystem services such as clean water and timber supplies are increasingly threatened by drought and disturbances (e.g., harvesting, fires and conversion to other uses) that can have great impacts on the hydrologic cycle of forests. Hence, improved understanding of the hydrologic response to drought and disturbance at a high spatiotemporal resolution is important for effective forest management. Evapotranspiration (ET) is a key hydrologic variable in assessing forest functioning and health, but it remains a challenge to accurately quantify ET at landscape scale with the spatial and temporal detail required for effective decision-making. In this study, we apply a multi-sensor satellite data fusion approach to study the response of forest ET to drought and disturbance over a 7-year period. This approach combines Landsat and Moderate Resolution Imaging Spectroradiometer (MODIS) ET image time series from a surface energy balance model to generate a multi-year ET datacube at 30-m resolution and daily timesteps. The study area (~900km2) contains natural and managed forest as well as croplands in the humid lower coastal plains in North Carolina, USA, and the simulation period from 2006 to 2012 includes both normal and severe drought conditions. The model results were evaluated at two AmeriFlux sites (US-NC2 and US-NC1) dominated by a mature and a recently clearcut pine plantation, respectively. Simulated long-term ET showed good agreement with observed fluxes, with 8-13% relative errors at monthly timesteps. Transpiration (T) and a moisture stress metric given by the actual-to-reference ET ratio (fRET) were also estimated in the model and used to investigate changes in water use patterns in response to land cover type, forest stand age, climatic forcing and disturbance. T shows a clear ascending trend with the growth of young pine plantations. Forest thinning in NC2 in late 2009 leads to decreases in both leaf area index and T, as expected. Analyses demonstrate differential response to drought events from different land cover types, with young plantations showing larger impacts than mature pine plantations with significantly deeper rooting systems. Time series maps of fRET anomalies at 30-m resolution capture signals of drought, disturbance and the subsequent recovery after clearcut at the stand scale, and may be an effective indicator for water use change detection and monitoring in forested landscapes.