What controls fire size in the South American Gran Chaco? Exploring atmospheric, landscape, and anthropogenic drivers

Abstract. Wildfires are key ecological agents in the Gran Chaco, one of the world's largest dry forest ecosystems, where fire regimes are increasingly shaped by human pressure and climate variability. However, the drivers of fire size variability remain poorly understood. We analysed over 100,000 fire patches (2001–2022) from the FRYv2.0 database to assess environmental controls on fire size and morphology across the Wet, Dry, and Very Dry Chaco. High-resolution fire polygon data were combined with ERA5-Land reanalysis, vegetation and topographic metrics, and anthropogenic layers. Fire sizes were highly skewed: >80 % were <5 km², yet large events (Megafires >100 km², Gigafires >1000 km²) dominated burned area (BA). Gigafires were rare but mostly confined to the Dry Chaco, whereas the Wet Chaco had the highest BA, fire frequency, and Megafire count. Fire Weather Index (FWI)–BA correlations reached r = 0.7 in the Wet Chaco but were weaker and spatially fragmented in drier subregions, where fuel continuity and ignition context played larger roles. Lag analyses showed that in drier areas, wet-season biomass buildup (4–6 months prior) increased subsequent fire activity, while in wetter areas short-term dryness (1–3 months prior) was more predictive. During-fire meteorology, especially persistent strong winds, better explained fire morphology than pre-fire conditions. Random Forest models ranked static landscape features (elevation, land-cover evenness, slope, tree cover) highest in size prediction. Our results reveal region-specific fire–environment couplings, clarifying the interplay of meteorological, ecological, and anthropogenic factors, and providing actionable insights for fire risk forecasting and management in the Gran Chaco.