By Nicole King | May 16, 2023

Aspen Fire Severity Research in Utah

Dr. Jim Lutz and Dr. Joseph Birch reveal trends in fire-related tree mortality following medium-sized fires in Utah aspen and Douglas fir forest types.

Using remote sensing data, researchers have observed trends over nearly 40 years. This data includes information about fire size and severity – the overall effect of the fire on vegetation and soil.

According to Dr. Lutz, fires can be divided into sizes with different management implications: big (>1,000 acres), medium (100 - 1,000 acres), and small (<100 acres) fires. It is relatively easy to decide how to treat big fires (put them out before they destroy something valuable), but medium fires can be more complicated. Medium-sized fires may have desirable impacts on the site, such as fuel management and the promotion of fire-dependent species.

This research will provide forest managers with more tools to understand how medium-sized fires burn in Utah. In large fires, roughly half of the aspen stands experience high mortality. In smaller fires, that number drops to nearly a quarter. More information about the behavior of medium-sized fires might help encourage more burning to reduce the possibility of megafires.

“The conclusion from [this] data is that large fires burn through aspen stands with a relatively high proportion of area showing high mortality, whereas [smaller] fires burn through aspen stands with much lower levels of tree mortality.” -Dr. Lutz

Four graphs with proportion on the Y-axis and dNBR on the X-axis. The images include medium aspen, large aspen, medium Douglas-fir, and large Douglas-fir
Fire severity distributions for all medium-sized (40 – 400 ha; 100 – 1000 acres) and large (>400 ha; >1000 acres) fires burning in aspen and Douglas-fir forest types in Utah from 1984 to 2021. Red color indicates a high level of tree mortality, whereas dark green, light green, and yellow indicate tree persistence through fire. Large fires in both aspen and Douglas-fir forests burned at a high severity that killed many trees. Medium-sized fires left many trees alive. Forest types taken from LANDFIRE definitions. Severity is defined as the differenced Normalized Burn Ratio (dNBR) calculated from LANDSAT data.

Dr. Jim Lutz is a professor of Forestry at Utah State University. Dr. Lutz received a MS and a PhD in Ecosystem Analysis from the University of Washington. He can be contacted at

Dr. Joseph Birch is a Research Associate at Michigan State University. Dr. Birch has a PhD in Forest Biology and Management from University of Alberta and a BSc in Conservation and Restoration Ecology from Utah State University.