Disentangling how the opportunistic parasitic fungus Armillaria affects the flammability of coarse deadwood in exotic pine plantations

Parasitic wood fungi are important to forest carbon cycles globally. However, whether or how they affect the flammability of coarse deadwood is poorly understood. Given the predicted climate-driven increase in wildfires and associated carbon emissions into the atmosphere, potentially amplifying climate warming, filling this knowledge gap should have high priority. We thereto investigated coastal plantations of the exotic black pine, Pinus nigra J.F. Arnold, in the Netherlands, which are widely suffering from Armillaria infection. We hypothesized that branches from forest stands with a visible Armillaria infection will burn differently compared with branches from stands without a visible Armillaria infection, due to Armillaria infection having an additional effect on the branch traits. We tested this hypothesis by burning coarse Pinus nigra branches across a range of densities from infected and uninfected forest patches under standardized conditions in a fire lab and by measuring Armillaria biomass (via ddPCR), deadwood traits and key flammability parameters. Armillaria infection did enhance the flammability of Pinus nigra branches (e.g. more ignitable, longer flame duration and higher percentage mass loss). This higher flammability originated from both direct Armillaria influences, e.g. via changing wood structure (before and/or after wood death), and indirect influences, e.g. by facilitating nitrogen fixation in wood, thereby increasing wood decomposability and consequently reducing wood density. Our findings also have important implications for understanding the role of pathogens in fire regimes more broadly.