OOS 61-1 - Tree traits drive fuel conditions, fire behavior, and mesophication of southeastern U.S. forests

Across the central and eastern U.S., open upland oak and pine forests persist through adaptations that facilitate their dominance in fire-prone environments while simultaneously contributing to a feedback loop that promotes fire. This loop can create a seemingly stable steady state where open oak and pine forests self-replace following frequent surface fire disturbances; however, alterations to the natural fire regime via anthropogenic fire exclusion may cause a rapid transition to an alternative state by allowing tree species that are fire-intolerant and shade-tolerant to outcompete oaks and pines in the absence of fire. This new state is hypothesized to be driven by a mesophication process, which is a positive feedback loop between (1) anthropogenic fire exclusion, (2) the spread of fire-sensitive, shade-tolerant tree species (i.e., mesophytes) that create cool, moist conditions in the forest understory, and (3) a subsequent reduction in flammability. Ultimately, without fire disturbances, mesophytes may foster their own proliferation through a variety of mechanisms, while increasing vulnerability and decreasing resilience of upland oak and pine forests.

Over the last decade, we have conducted a variety of experimental studies evaluating whether traits of encroaching species impact forest flammability, thus the mesophication process, across the southeastern U.S. We found that many encroaching species exhibit leaf litter traits associated with reduced flammability, such as thinner, smaller leaves that compact at higher bulk densities with higher moisture holding capacities and decomposition rates. In plot-based experimental burns and/or lab trials, several encroaching species (e.g., sweetgum, winged elm) clearly reduced flammability (rate of spread, flame height, fuel consumption), but a small amount (10%) of pine needles in the fuel bed was sufficient to negate the fire suppressing traits of leaf litter from mesophytic species. We found that leaf litter traits are relatively constant within a species despite regional climatic variations but often vary depending on fire history, with species normally not considered pyrophytic exhibiting more flammable traits when more regularly exposed to fire. We have also shown that restoring a flammable, pyrophytic state once mesophytic species reach the overstory requires more than simply removing midstory encroachers. Combined, our data suggest that tree species clearly vary in leaf litter fuel traits, leading to a gradient of fire-suppressing capacities that are difficult to overcome without aggressive management (e.g., overstory plus midstory thinning) once mesophytic species occupy the overstory.