Without fire, trees become more susceptible to it

Low intensity fire behavior typical of much of the area burned in the Rock Mountain Wildfire. USDA Forest Service photo by Joseph O’Brien.

Historically, fires frequently burned Southern Appalachian forests. Many tree species evolved traits that aided survival in this fire environment. However, with the exclusion of fire from these forests for many decades, new research suggests traits that once made trees resistant to fire may now make them more susceptible to it.

USDA Forest Service scientists Melanie Taylor, Mac Callaham, E. Louise Loudermilk, and Joseph O’Brien teamed up with Nina Wurzburger and her graduate student, Dana Carpenter, from the University of Georgia and Kevin Hiers from Tall Timbers Research Station to investigate this phenomenon using data from the Rock Mountain wildfire that burned during the extreme drought in the Southern Appalachians in 2016.

Nearly all plant species have coevolved with fungal friends called mycorrhizae. The fungi grow in or on the plant’s roots and form beneficial relationships. Oaks, hickories, pines, and other fire-adapted trees usually associate with ectomycorrhizal (ECM) fungi, while fire-intolerant species typically associate with arbuscular mycorrhizal fungi (AM).

Fire-dependent ecosystems tend to be nutrient poor, because fire routinely consumes ground layer vegetation, leaf litter, and some organic material. ECM fungi benefit fire-dependent species by helping with nutrient conservation. But when fire is suppressed, ECM continue to slow the rate of nutrient cycling and even suppress the activity of microbes involved with decay.

In these ecosystems, leaves are resistant to decay, meaning they are available as fuels for a fire. When fire is absent, duff — decomposing leaves, twigs, and needles — accumulates underneath ECM trees.

As the duff builds up, fine tree 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 and ultimately die. This is especially concerning during droughts, like those that occurred in the Southern Appalachians in 2016, when duff is dry and more available to be consumed along with the fine roots of the trees.

Thick duff layer found in long unburned pine-oak forests of the Southern Appalachians. USDA Forest Service photo by Melanie Taylor.

“What we have learned from this work is that the novel fuels caused by long-term fire exclusion led to negative effects, upon the reintroduction of fire, on tree species that we generally consider to be fire-adapted,” says Taylor. “Those novel fuels are deep organic horizons, or duff, on the forest floor that are driven by the slow decay of oak/hickory leaf litter, coordinated by ECM fungal associations, and are allowed to accumulate due to absence of fire.”

Trees that associate with ECM were more likely to decline than those associated with AM after the 2016 wildfires—even though these species are typically considered fire-adapted. Because prescribed fires are conducted under planned and manageable conditions – when duff is wetter, for instance they gradually consume duff over the course of several fires, rather than all at once. This allows time for roots to move deeper into the duff or soil and thus protecting them from fire.

The study suggests that prescribed burning could mitigate risks associated with duff burning in wildfires, promote long-term health and survival of ECM tree species, and help to restore these fire-dependent ecosystems.

Read the article in Ecosystems.

For more information, email Mac Callaham at mac.a.callaham@usda.gov.

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