Friday Flash - Evaluating Flammability of Reburns in the Boreal Forests of Interior Alaska

JFSP Final Report: 19-1-01-43

Researcher: Brian Buma (University of Colorado Denver) and Katherine Hayes (University of Colorado, Denver)

Temperatures in high-latitude environments are rising quickly, leading to increases in the frequency and intensity of wildfires. This trend is especially important in the boreal where fire return intervals have shrunk from between 100-300 years to sometimes less than 20 years. 

This study utilized field observations of fuel composition, structures, and spatial arrangement in a natural experiment whereby different plots had differing frequencies of fire histories, from a single burn and recovery, similar to historical norms, to three fires in the last 60 years, similar to many projections of future conditions. These datapoints are important for management in terms of both fire likelihood and behavior, as well as from an ecological perspective as future habitat, carbon stocks, and future functioning are greatly altered by these short interval fire events. The study then parameterized a high resolution, physics based model take can estimate the effect of these observations of novel structures and realistically generate future fire behavior under multiple wind and weather scenarios.

Fuel composition and structures change substantially across the burn frequency gradient, with ample regeneration of the historical forest type seen after a single burn. This is expected, as fire is key to the boreal forest cycle. After two burns, loss of conifers is apparent, and the composition shifts to deciduous species. Lowland and upland forests diverge at this point, with upland forests having dense birch and lowland forests becoming more open. Grass peaks after two burns, about 40 years apart, connecting the landscape for fire. After three, the trees are shorter, as are all the fine fuels, and there is more open soil and short moss coverage. In the modeling framework, this does seem to enable more ground fuel spread, a new behavior that may result in more extensive fires and a more connected firescape. Future work should consider expanding on the plot sizes here to determine the scale and scope of these connections and intensifying the modeling effort to better characterize the range of thresholds at which fire behavior and feedbacks change.