Interannual Variations of Vegetation Optical Depth are Due to Both Water Stress and Biomass Changes.

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. Plain Language Summary: Satellite measurements known as vegetation optical depth (VOD) measure how much water is in plants. Because plants with more mass can hold more water, changes in VOD are used to determine biomass changes, ignoring the fact that changes in how dry the plants are also affect VOD. We use an alternative data set for biomass to test this approach. At a given location, VOD and biomass do not vary in the same pattern from year to year and for many types of plants, VOD seems to follow soil moisture (which should co‐vary with the fraction of water in plants) better than it follows biomass itself. These results help clarify when VOD can and can't provide information about biomass. Key Points: We test the common literature assumption that biomass interannual variability (IAV) follows vegetation optical depth (VOD) IAVVOD‐biomass relationship is driven almost entirely by spatial correlation, rather than temporal similarityRegional‐scale VOD IAV is often more sensitive to soil moisture than biomass, due to influence of plant water stress on VOD [ABSTRACT FROM AUTHOR]