Day of burning maps and burn severity landscape metrics in the southwestern United States 2002-2020

Extreme fire spread events rapidly burn large areas with disproportionate
impacts on people and ecosystems. Such events are associated with warmer
and drier fire seasons and are expected to increase in the future. Our
understanding of the landscape outcomes of extreme events is limited,
particularly whether or not they burn more severely or produce spatial
patterns less conducive to ecosystem recovery. To assess relationships
between fire spread rates and landscape burn severity patterns, we used
satellite fire detections to create day-of-burning (DOB) maps for 623
fires comprising 4,267 single-day events within forested ecoregions of the
southwestern United States. We related satellite-measured burn severity
and a suite of high-severity patch metrics to the daily area burned.
Extreme fire spread events (defined here as burning >4900 ha/day)
exhibited higher mean burn severity, a greater proportion of area burned
severely, and increased like adjacencies between high-severity pixels.
Further, increasing daily area burned also resulted in greater distances
within high-severity patches to live tree seed sources. High-severity
patch size and total high-severity core area were substantially higher for
fires containing one or more extreme spread events than fires without an
extreme event. Larger and more homogenous high-severity patches produced
during extreme events can limit tree regeneration and set the stage for
protracted forest conversion. These landscape outcomes are expected to be
magnified under future climate, accelerating fire-driven forest loss and
long-term ecological change.