Background: Wildfires produce pyrogenic carbon (PyC) through the incomplete combustion of organic matter, and its chemical characterization is critical to understanding carbon (C) budgets and ecosystem functions in forests. Across western North American forests, fires are burning through landscapes with substantial tree mortality from insect outbreaks, often referred to as short-interval disturbances. These short-interval disturbances often result in beetle-killed snags (snags that subsequently burn) that are heavily charred, while their fire-killed (alive at time of fire) counterparts are lightly charred. We investigated whether the chemical signature of PyC on heavily charred beetle-killed snags differs from lightly charred fire-killed snags.

Results: We evaluated differences in PyC chemical concentrations and condensation on beetle-killed (n = 3) and fire-killed snags (n = 3) with the benzene polycarboxylic acid (BPCA) molecular marker method. Beetle-killed snags exhibited over five times the grams of BPCA per kilogram of C, and over seven times the grams of BPCA per kilogram of material compared to fire-killed snags, indicating higher concentrations of PyC.

Conclusion: PyC concentrations were greater in samples from beetle-killed snags compared to fire-killed snags, pointing to an important compound effect from the interaction of beetle outbreak and wildfire in forest ecosystems. While this study was limited to a single fire event and sampling in individual trees, it provides insight into PyC on snags with different disturbance histories. The abundance of heavily charred snags from a fire burning through landscapes with high levels of existing mortality from drought, insect outbreaks, and previous wildfire events is becoming a more common legacy for contemporary western North American forests. Accordingly, the distinct wood-based PyC legacy could influence ecosystem functions: C sequestration, water purification, soil nutrient supply, and resources for wildlife.