Evaluation of Energy Release from Wildfires Across the Elevation Gradient
Authors: Butler Isabelle Rose 
Published: December 2022

Wildfires are an integral process in vegetative terrestrial land which shape ecosystem functions. A warming climate, however, has increased the size and severity of fires with significant ecosystem and societal implications. Furthermore, warming has changed characteristics of wildfires enabling a median upslope advance of 252 m in high-elevation forest fires from 1984 to 2017, allowing wildfires to burn in areas that were previously too wet to burn frequently. This exposed an additional 81,500 square kilometers (11%) of western US montane forests to fires.

In this thesis, I test the hypothesis that wildfires burn more intensely in high-elevation mesic forests than low-elevation dry forests. To this end, I assess fire intensity, which refers to how much heat energy is released during a fire, across the elevation gradient. I use satellite-observed fire radiative power (FRP) that measures the amount of radiant energy released from burning vegetation during a wildfire event as a proxy for fire intensity. FRP data are acquired from the MODIS sensor aboard Terra and Aqua satellites for fires between 2000 and 2021 which are then paired with elevation data using digital elevation maps. I derive this data for the 15 mountainous ecoregions of the western US and conduct various hypothesis tests to determine whether or not there is a statistically significant trend in FRP as a function of elevation. I will also assess whether or not the distribution of FRP for high-elevation and low-elevation wildfires are equal.

Among the 15 studied mountainous ecoregions, 12 ecoregions are associated with a statistically significant increasing FRP as a function of elevation, 1 is associated with statistically non-significant increase in FRP as a function of elevation, and 2 were associated with statistically significant decreasing FRP as a function of elevation. I note the limitations of satellite-derived FRP, including twice-a-day overpass of satellite over fires, limitations of the MODIS sensor in capturing small fires, and algorithmic errors in inferring FRP from thermal anomaly observations. Nevertheless, long-term (20+ years) observation of FRP provides unparalleled opportunities for geospatial and temporal analysis of trends in fire intensity. Furthermore, quantile regression analysis revealed that higher intensity fires increase at a higher rate compared to lower intensity fires as a function of elevation. Finally, my analysis showed that 10 of the studied ecoregions are associated with statistically significant increase in FRP as a function of year (i.e., fires are intensifying in recent years), 1 has a statistically non-significant increasing trend, 2 ecoregions don’t show any trend in FRP as a function of year, and 2 are associated with statistically non-significant trends in FRP versus year.

High-elevation wildfires and their intensity are important for societal and ecological systems that are affected by wildfires. They impact, for example, quantity and quality of water resources for 70% of the western US population that depend on high-elevation areas as their source of water. Understanding this phenomenon can inform wildfire and land management in a warming climate.