A prescribed fire spreading through a mixture of post oak and blackjack oak in north Mississippi (Chickasaw County). Photo by Morgan Varner.
Throughout their range, oak trees have long represented a close relationship between forest and fire. Fire exclusion throws this relationship off balance, and over time open oak woodlands can undergo big changes in composition and structure, sometimes leading to forest plant communities that have never existed before.
Fire ecologist Morgan Varner has been studying how widespread fire exclusion and land-use change has altered oak-hickory forests of the Southeast United States. Without fire, these forests shift away from diverse, fire-adapted plant communities toward shade-loving, fire-sensitive communities. Varner and his colleagues sought to evaluate how these oak-hickory forests become less flammable when other plant species encroach and change the forest floor “litter” – the loose leaves, branches, bark and other vegetation that sit above the soil.
Varner’s team collected fresh litter from across an oak–hickory forest at Strawberry Plains Audubon Center in Mississippi, USA. The team saturated the samples, dried them in a lab, and then burned them across a drying-time gradient to test their flammability. Litterbeds with higher proportion of shade-tolerant, fire-sensitive plants retained more moisture and were less flammable than the fire-resistant species.
Their results highlight changes wrought by these encroaching shade-tolerant trees. The litter they produce reduces flammability by slowing water loss and dampening the fire intensity of fuels available on the forest floor. Sweetgum poses a particular challenge, given its widespread establishment in fire-excluded uplands in the Southeast, and its dampening effect on flammability.
These changes may result in losses of ecological function and threaten understory biodiversity in oak-hickory forests. The impacts of the shift in flammability from fire-adapted oaks to fire-inhibiting species like sweetgum can also hamper the effectiveness of prescribed fire, which is an important tool for many land managers in the region. Results from this study can be useful in prioritizing harvest of invading non-flammable trees and promoting those with high flammability to help reinitiate the frequent fires that generated the high biodiversity of the region.
Kreye, J.K., J.M. Varner, G. Hamby, and J.M. Kane. 2018. Mesophytic litter dampens flammability in fire-excluded pyrophytic oak-hickory woodlands. Ecosphere 9(1): e02078.