Higher Sensitivity of L‐Band Vegetation Optical Depth to Foliage Than Total Aboveground Biomass

Abstract L‐band vegetation optical depth (VOD) has been widely used to estimate spatiotemporal variations of forest biomass. However, the sensitivity of L‐VOD to different vegetation components, such as foliage and stems, remains poorly understood despite their distinct interannual variations. Here we investigate the differential sensitivity of L‐VOD to foliage biomass and aboveground biomass (AGB) at various spatiotemporal scales over contiguous US. Our analysis reveals that at interannual scale, L‐VOD's sensitivity to foliage biomass is 4.5–16.8 times greater than to AGB across various forest types. Given that foliage biomass typically exhibits larger interannual fluctuations than woody components, this suggests that L‐VOD may overestimate the interannual variability of total AGB by disproportionately responding to foliage dynamics. Although foliage constitutes only a small fraction of total aboveground biomass (∼2%), it accounts for 21.2% of the interannual variations in L‐VOD. However, when large‐scale disturbances reduce foliage and woody biomass proportionally, VOD effectively captures the AGB declines. These findings highlight the need to account for component‐specific sensitivities in VOD‐based models.