Human-ignited fires spread faster and kill more trees in Californian forests

Forest fires increased over the last decades across California, resulting in loss of human lives and damages to infrastructure and ecosystems. Most attention has been paid to changes in area burned. However, much less is known regarding fire behavior while it is closely tied to the impact fire has on the ecosystem. Here we present a homogenous database on daily fire spread across California (2012-2018) based active fire detections from VIIRS satellite data for 214 individual fires. We used this database to answer the following questions: 1) Is there a difference in fire behavior between human and lightning ignited fires. 2) Does fire Rate-of-Spread influence fire severity and can we quantify this relationship, and 3) Do human-caused fires impact Californian ecosystems differently when compared to natural ignitions.

We find that fire Rate-of-Spread shows a skewed distribution, characterized by a high frequency of fire with very low spread rates (< 0.5 km d-1) and infrequent fast spreading fire days (>5 km day-1). We observe a difference in fire behavior between human-caused and lightning caused fires, with human caused fires exhibiting a more explosive growth in the first few days following ignition. We show that fire spread rate is strongly related to fire ecosystem impact, with tree mortality being a factor 3 higher for fast-moving fires (>1 km /day) compared to slow-moving fires (<0.5 km/day). As a result, large human-caused fires have a significantly larger impact on the ecosystems when compared to lightning-caused fires. As the 10% fastest fire days cause 55% of the total burnt area, our findings have important implications for wildfire management strategies and suggest that limiting human-caused fires during extreme weather conditions is key to ecosystem conservation under scenarios of future change.