Evaluating the impacts of fuel treatments on burn severity across the Front Range

Abstract

Understanding how wildland fuel treatments interact with more frequent extreme wildfire events is an increasingly pressing issue. Quantifying the effectiveness of fuel treatments is difficult for many reasons including dynamics between treatment type, time since treatment, weather, climate, the fuels present, and topography. This study investigated wildfires along the Front Range from southern Wyoming to northern New Mexico to evaluate under what conditions treatments reduce the ecological impacts of fire, as measured by remotely sensed burn severity. We first evaluated methods and metrics to measure burn severity on the Front Range landscape. We found the Parks et al. (2018) Google Earth Engine (GEE) method and the relativized burn ratio (RBR) to have the highest correlation with field data. Additionally, the GEE method provided the advantage of allowing us to include small wildfires, which have historically been underrepresented in the data. We then determined (1) factors influencing the relationship between treatments and burn severity, (2) how burn severity differed across forest and treatment types, and (3) how extreme fire weather conditions influenced burn severity across treatment types. Across the Front Range, lower elevation forest types burned at lower severities compared to higher elevation forest types. Treatment effects varied across forest types but treatments generally had lower burn severity in lower elevation forest types compared to spruce ‒ fir and lodgepole pine forests. Areas that previously experienced low to moderate severity wildfire had the lowest burn severity outcomes across forest types and in extreme conditions. Intentional surface fuels reduction treatments (i.e. prescribed fire, removal plus surface fuel reduction, and removal plus fire) had a relatively minor impact across our study area. That said during non-extreme conditions, treatments that included previous fire (prescribed burning or low to moderate severity wildfire) had lower subsequent burn severities. Higher elevation forests (spruce – fir and lodgepole pine) burned at high severity regardless of intentional treatment effect. This better understanding of the outcomes of treatment efforts will help land managers strategically utilize resources and employ adaptive management strategies that account for changing wildfires.