Shifting tree species composition of upland oak forests alters leaf litter structure, moisture, and flammability

•We compared oak and non-oak leaf litter traits, fuel moisture, and flammability.•Non-oak litter was thinner and smaller than upland oak litter.•Flammability decreased and fuel moisture increased as non-oak litter increased.•Increases in non-oaks may challenge reintroduction of fire to upland oak forests. In historically open-canopied and fire-dependent upland oak (Quercus spp.) forests of the central and eastern United States, fire exclusion is contributing to an increase in competing non-oak tree species that are often shade-tolerant and fire-sensitive. As these non-oak species encroach and oak abundance declines, forests are becoming denser and will likely become cooler, moister, and less flammable through a hypothesized feedback loop termed mesophication. To better understand how this gradual shift in forest composition could affect flammability of leaf litter, the primary fuel in these systems, we measured leaf litter traits and moisture dynamics of two pyrophytic oaks and three non-oaks, and implemented experimental burns on plots (~3-m2) with fuel beds comprised of single- and mixed-species leaf litter. Non-oaks produced thinner, smaller leaves with a greater specific leaf area compared to oaks, and had a higher initial fuel moisture content, traits associated with low flammability. Our experimental burns confirmed that non-oak leaf litter flammability was lower than that of oaks and that flammability decreased linearly with increasing non-oak leaf litter contribution to the fuel bed. These species-driven changes in fuelbed flammability may provide a mechanism whereby encroaching non-oak tree species create self-promoting conditions that are less favorable for regeneration of fire-dependent upland oaks. Thus, without other management interventions, our ability to reintroduce fire into these systems as a management tool to prevent further compositional shifts and improve oak regeneration will likely decline as non-oak species’ contribution to the fine fuel bed increases.