Sensing the Vertical Dynamics of the Amazon Rainforest Using Multifrequency Microwave Radiometry

Abstract

Vertical stratification is a defining feature of forest ecosystems, shaping their structure, function, and dynamics. Here, we apply a multifrequency microwave radiometry framework to monitor forest dynamics, highlighting the complementary information provided by L-, C-, and X-band vegetation optical depth (VOD). Using the Amazon rainforest as a case study, we show that L-band VOD (LVOD) is particularly sensitive to changes in lower and mid-canopy strata, capturing early signals of vegetation water content (VWC) decline that remain largely undetectable with higher frequency signals and conventional optical indices. Analysis of LVOD, C-band VOD, and X-band VOD from 2010 to 2023 reveals accelerated reductions in LVOD following the 2015–2016 El Niño event, with stress indicators appearing 5–10 years before similar trends are visible in the upper canopy. These results demonstrate the potential of multifrequency VOD analysis, particularly L-band measurements, to provide early warning indicators of forest vulnerability. Our study highlights the power of low-frequency radiometry in probing deeper forest strata and improving the long-term monitoring of tropical forest dynamics.