Multi-frequency microwave remote sensing reveals stratified forest responses to diverse drought stresses

Abstract

Under climate change, increasing frequency and intensity of droughts pose severe threats to forest water dynamics and ecosystem stability. However, layer-specific responses to drought and their global spatial patterns remain insufficiently explored. This study utilized multi-frequency vegetation optical depth (VOD) datasets (X-, C-, and L-bands) that are sensitive to canopy, sub-canopy, and woody tissue moisture, to explore stratified forest responses at the global scale. A drought-induced relative anomaly index (ΔVOD%) was integrated with the standardized precipitation evapotranspiration index to characterize moisture dynamics under different drought types. Vertically, the canopy exhibited the strongest response intensity (mean ΔVOD% in the X-band = 44.5 % ± 20.9 %), whereas the woody tissue layer showed weaker but more delayed and persistent responses (mean lag = 2.56 and 2.44 months, respectively). A clear vertical gradient was identified: response intensity decreased from the canopy to the woody tissue layer, whereas lag and duration increased. Spatially, stronger canopy and sub-canopy responses were concentrated in evergreen broadleaf forests and tropical-temperate regions, while woody tissue responses were more pronounced in savannas and cold regions. Evergreen needleleaf, deciduous broadleaf, mixed forests, and arid zones exhibited clear inter-layer differences in response lag, whereas others showed relatively small variations. By leveraging the penetration capability of microwave remote sensing, we elucidated global patterns of layer-specific response intensity, lag, and duration and examines the effects of drought type, forest type, and climate zone on moisture dynamics. These findings provide new insights into global forest water responses under drought stress and improve remote sensing-based monitoring of forest drought impacts.