Drought can have pervasive and wide-spread impacts to forest health, as evidenced in several severe events occurring over the recent decades. Extensive forest die-off due to drought can impair the ecological functioning of forests, impacting habitat, water yield and quality from forested lands, and altering forest fire dynamics and intensity. Satellite remote sensing provides an effective means for detecting and monitoring spatial patterns of forest mortality over large areas, exploiting free and open long-term image archives available at a range in spatial and temporal resolutions. While remotely sensed surface reflectances and vegetation indices have been widely used to study optical response of forest canopies to drought events, retrievals of evapotranspiration (ET) derived from thermal satellite imagery – particularly at resolutions approaching crown scale - can provide insights into cumulative tree stresses that can incite disease and trigger mortality. In this study, we applied a multi-sensor satellite data fusion approach to estimate daily 30-m resolution ET and an associated Evaporative Stress Index (ESI) to study drought-induced mortality in a temperate forest at the Missouri Ozark AmeriFlux (MOFLUX) site, located in the central United States. The study covered the period from 2010 to 2014, including an exceptional drought year of 2012. Modeled ET agreed well with eddy flux measurements from the MOFLUX tower, with average monthly relative errors of 15%. Plot-scale ESI, describing temporal anomalies in the ratio of actual-to-reference ET, was used as an index of relative forest health to investigate relationships between forest mortality and drought severity. ESI showed good agreement with observed predawn leaf water potential, especially during the drought year. Furthermore, plot-scale ESI was also correlated with the subsequent year's tree mortality, suggesting the importance of considering the forest health condition prior to drought when studying drought-induced forest impacts. This study demonstrates the utility of multi-year ET remote sensing data at the stand or plot scale as an indicator of forest health and as a predictor of future mortality due to drought.

干旱会对森林健康产生普遍和广泛的影响，近几十年来发生的几次严重事件就证明了这一点。干旱导致的大面积森林死亡会损害森林的生态功能，影响林地的栖息地、水产量和质量，并改变森林火灾的动态和强度。卫星遥感提供了一种有效的手段来检测和监测大面积森林死亡率的空间模式，利用免费和开放的在空间和时间分辨率范围内可用的长期图像档案。虽然遥感表面反射率和植被指数已被广泛用于研究森林冠层对干旱事件的光学响应，来自热卫星图像的蒸散 (ET) 检索 - 特别是在接近树冠尺度的分辨率下 - 可以提供对可引发疾病和引发死亡的累积树木压力的见解。在这项研究中，我们应用多传感器卫星数据融合方法来估计每天 30 米分辨率的 ET 和相关的蒸发应力指数 (ESI)，以研究密苏里州奥扎克美洲通量 (MOFLUX) 站点温带森林中干旱引起的死亡率，位于美国中部。该研究涵盖了 2010 年至 2014 年期间，包括 2012 年异常干旱的年份。模拟的 ET 与 MOFLUX 塔的涡流测量结果非常吻合，平均每月相对误差为 15%。绘图尺度 ESI，描述实际 ET 与参考 ET 比率的时间异常，被用作相对森林健康的指数，以研究森林死亡率与干旱严重程度之间的关系。ESI 与观察到的黎明前叶水势非常吻合，尤其是在干旱年份。此外，地块尺度 ESI 还与随后一年的树木死亡率相关，这表明在研究干旱引起的森林影响时考虑干旱前森林健康状况的重要性。这项研究证明了多年 ET 遥感数据在林分或样地尺度上作为森林健康指标和未来干旱死亡率预测指标的效用。地块尺度 ESI 也与随后一年的树木死亡率相关，这表明在研究干旱引起的森林影响时考虑干旱前森林健康状况的重要性。这项研究证明了多年 ET 遥感数据在林分或样地尺度上作为森林健康指标和未来干旱死亡率预测指标的效用。地块尺度 ESI 还与随后一年的树木死亡率相关，这表明在研究干旱引起的森林影响时考虑干旱前森林健康状况的重要性。这项研究证明了多年 ET 遥感数据在林分或样地尺度上作为森林健康指标和未来干旱死亡率预测指标的效用。