Large-scale fire events substantially impact plant-soil water relations across ecosystem types

Wildfire is a global scale ecosystem phenomenon with substantial impact on the carbon cycle, climate warming, and ecosystem resilience. Fire and the hydrological cycle are strongly interlinked, with water availability determining the amount and combustibility of fuel, and fire influencing infiltration, runoff rates and evapotranspiration. Consequently, understanding soil moisture (SM) and vegetation water content (VWC) dynamics pre- and post-fire is fundamental for predicting fire occurrence, fire severity, and ecosystem recovery. Fire can modulate SM and VWC dynamics by influencing interception of rainfall, soil porosity, plant water uptake, and runoff; however, much evidence for fire effects on the hydrological cycle is obtained at the field- to watershed-scale. Therefore, we ask the following research question: What are the effects of large-scale fire events on SM and VWC dynamics across biomes globally?

Here we use over six years of global SM, VWC and vapor pressure deficit (VPD) derived from different remote sensing datasets to investigate the effects of large-scale fires on SM and VWC dynamics. We apply a dry down framework, only analyzing consecutive observations of decreasing soil moisture, to describe post-fire response rates for SM, VWC and VPD relative to a pre-fire reference state.

We find large scale evidence that the post-fire rate of change of SM over time is more negative, indicating faster water loss. Vegetation recovery, indicated by a positive change in VWC over time, exceeds the pre-fire reference state, which suggests that post-fire recovery is predominantly faster than undisturbed seasonal vegetation growth, likely due to succession of fast-growing plant species. Furthermore, fire affects ecosystem hydrology on shorter timescales as well, reducing diurnal VWC variation over a wide range of SM and VWC conditions. Our findings confirm several trends previously only observed at smaller scales and suggest global remote sensing of SM and VWC can substantially contribute to understanding the dynamics of post-fire plant and soil water status.