What controls fire size in the South American Gran Chaco? Exploring atmospheric and landscape drivers through Remote Sensing

Abstract. Wildfires are key ecological agents in the Gran Chaco, one of the world's largest tropical dry forest systems. We analyzed more than 100 000 fire patches across the Wet, Dry and Very Dry Chaco between 2001 and 2022, to quantify environmental and anthropogenic controls on fire size. Fire sizes were strongly right-skewed: more than 80 % were smaller than 5 km2, yet large and extreme fires dominated total burned area. Megafires (>100 km2) occurred in all subregions, while gigafires (>1000 km2) were rare but concentrated in the Dry Chaco. Fire Weather Index–burned area correlations exhibited strong spatial contrasts, reaching values of up to r=0.7 in the Wet Chaco and showing weaker, more heterogeneous relationships in drier regions. Meteorological conditions during fires, particularly persistent strong winds, were associated with larger and more elongated patches. Random Forest models showed that topography and land cover composition together accounted for about 60 % of total SHAP importance, whereas demographic variables had very low SHAP contributions in the models. Human pressures shape ignition timing but showed limited direct influence on fire size once landscape structure was included in the models. These results provide a quantitative basis for improving regional fire danger assessments in the Gran Chaco.