Microwave radiometry can be used to measure vegetation water content through vegetation optical depth (VOD). VOD can vary due to changes in water stress alone, but also scales with aboveground biomass. Several studies have therefore interpreted VOD temporal anomalies as linearly proportional to biomass anomalies, neglecting the influence of water stress. Here, we explicitly test this assumption using a new annual-resolution data set of biomass derived from optical, radar, and lidar remote sensing. Both L-band and X-band VOD datasets are tested. Although VOD and biomass variations are highly correlated in space, their temporal anomalies are almost uncorrelated. At regional scale, it is as common for VOD anomalies to be more correlated to root-zone soil moisture anomalies (a proxy for water stress) than to biomass anomalies as for the opposite to occur. Care should therefore be taken when deriving biomass anomalies from VOD, especially in the absence of any large-scale biomass disturbances.

中文翻译：

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植被光深的年际变化是由于水分胁迫和生物量变化造成的

微波辐射测量可用于通过植被光学深度 (VOD) 测量植被含水量。VOD 可能仅因水分胁迫的变化而变化，但也会随着地上生物量的变化而变化。因此，一些研究将 VOD 时间异常解释为与生物量异常成线性比例，忽略了水分压力的影响。在这里，我们使用来自光学、雷达和激光雷达遥感的新的年度分辨率生物量数据集明确地测试了这一假设。L 波段和 X 波段 VOD 数据集都经过测试。尽管 VOD 和生物量变化在空间上高度相关，但它们的时间异常几乎不相关。在区域尺度上，VOD 异常与根区土壤水分异常（水分胁迫的代表）的相关性比与生物量异常的相关性更常见，反之亦然。因此，从 VOD 得出生物量异常时应小心谨慎，尤其是在没有任何大规模生物量扰动的情况下。