Trajectories of Microwave-Based Soil and Vegetation Water Content Underlying Wildfire Dynamics in Africa

Wildfires are a major factor influencing vegetation dynamics and biogeochemical cycles. Soil moisture (SM) and vegetation optical depth (VOD) control fuel availability and flammability, but their interactions and feedbacks with wildfire dynamics on large spatial scales remain insufficiently understood. To investigate soil and vegetation water dynamics in the vicinity of wildfires, we employ a multi-sensor remote sensing approach that combines long-term (2000–2020) microwave-based SM and VOD data with optical fire observations across Africa. In addition to characterizing regional SM and VOD anomaly patterns in the vicinity of fire activity, this study also examines whether wildfire alters the coupling between SM and VOD dynamics during dry-down periods—an aspect that has not yet been addressed at the continental scale. Our results reveal strong regional differences in pre-fire trajectories: in the Southern Sahel, both variables exhibit positive anomalies 5–6 months before a fire, indicating above-average conditions associated with vegetation growth, whereas Southern Africa exhibits a continuous decline prior to the fire. Across varying land cover classes, regions with sparse vegetation show a multi-year increase in SM and VOD prior to fires, while areas with high biomass do not exhibit long-term fuel accumulation. Our post-fire analysis reveals an accelerated loss of SM and enhanced recovery of VOD during comparable initial conditions. These results demonstrate that wildfires not only alter the soil and vegetation water states but also modify the coupling between SM and VOD during dry-down periods. The drying of SM and the gain of VOD following a fire are accelerated, indicating intensified water exchange between the soil and vegetation, as well as likely faster SM uptake by post-fire vegetation. This leads to temporary shifts in the ecohydrological functioning of African ecosystems.