srs.fs.usda.gov - 12-01-2022
Historically, fires burned North American forests very frequently.

Many tree species - developed resistance to fires. However, fire has been excluded for forests for a long time—and new research suggests some traits that once made trees resistant to fire may now make them more susceptible to it.
Oaks, hickories, pines, and other fire-adapted trees once dominated Southern Appalachian forests and are still abundant. However, fire-intolerant species such as tulip poplar and maples have become more common. All of these tree species have fungal friends called mycorrhizae. The fungi grow in or on the plant’s roots and benefit the tree.
Fire-intolerant species typically associate with arbuscular mycorrhizal fungi (AM). Fire-adapted trees usually associate with ectomycorrhizal (ECM) fungi. Without fire, a thick layer of duff builds up underneath ECM trees. As the duff—decomposing leaves, twigs, and needles—accumulates, fine roots grow into it. When fires return and duff is burned, these roots can be damaged or consumed, and the trees can be seriously injured, as a recent Southern Research Station study shows.
Trees that associate with ECM were more likely to die after the 2016 wildfires that struck the Southern Appalachian Mountains—even though these trees are typically considered fire-adapted. Because prescribed fire consumes duff, the study suggests that controlled burning could mitigate risks associated with duff burning in wildfires and promote long-term health and survival of ECM tree species.