Post-Fire Mortality for Southern Hardwoods

Toward a model of tree attributes and prescribed fire effects

Drive down Highway 7 in northern Arkansas, winding through the Ozark National Forest, and you may glimpse evidence of recent fire: scorched grass, darkened tree bark, maybe even a lingering wisp of smoke.

Traces of prescribed burning can be seen throughout the South. Prescribed fire is a critical tool for forest restoration.

A new study led by USDA Forest Service research forester Tara Keyser examines the effects of prescribed burning on the mortality of ten upland hardwood species found in the eastern United States.

Prescribed burns are often low intensity and used to create more open conditions in the understory. This makes space for target restoration species – such as oaks or shortleaf pine – and helps them survive to larger sizes.

nps-prescribed-burn
Prescribed burn sites included the Great Smoky Mountains National Park, Natchez Trace Parkway, and Ozark National Scenic Riverways, shown here. Photo courtesy of NPS.

Does prescribed burning affect the intended species as expected? Land managers use fire effects models to understand how forest structure and composition may change post-fire – but most current models lack data about hardwood tree species that are common in the South.

“Post-fire mortality can be estimated with data about tree morphology, like diameter and bark thickness, and fire effects, like the severity of char on a tree’s bole or bark,” says Keyser.

Larger trees, with correspondingly thicker bark, are able to withstand more heat and suffer less damage to the cambium during a fire.

Existing research suggests a strong correlation between stem size and the maximum height of char for some eastern hardwoods like red oak, red maple, and pignut hickory.

Keyser and her colleagues, including SRS forestry technician Virginia McDaniel, designed their study to use easily-obtained data on those parameters.

“We had two hypotheses: that, regardless of species, larger-sized trees would be more likely to survive a prescribed burn, and that greater fire effects would contribute to greater mortality,” adds Keyser.

The study arose from an ongoing collaboration with partners at the National Park Service (NPS). NPS had conducted prescribed burning across 13 different locations and offered to share this large dataset on post-fire effects.

Keyser and McDaniel focused on analyzing those data, and their findings were published in the International Journal of Wildland Fire.

char-height
Relatively low maximum bole char height is a desired outcome of low-intensity prescribed burning. Photo by Tara Keyser, USFS.

The dataset included 201 prescribed fire plots on land units with no history of active forest management. No fire events had been documented during the last 20 years.

Before burning, NPS tagged individual trees and recorded their species, diameter at four-and-a-half feet above the ground (dbh), and status – live or dead.

Prescribed burns were conducted between 1997 and 2012, mostly during the late dormant to early growing season. Within one month of each burn, direct fire effects were measured for each tagged tree: the highest extent of char. Plots were revisited after two years to catalog changes in tree status.

“These were low intensity burns, so it follows that the maximum char height was fairly low. Most were between one and one-and-a-half feet,” says Keyser. “Mortality at two years ranged from a low of 6.9 percent for white oak to a high of 58.9 percent for sassafras.”

For all ten species, post-fire mortality was significantly influenced by both stem size and char height.

Overall, smaller diameter trees had a higher probability of mortality than larger diameter trees. Even for fire-sensitive species, like red maple, the probability of mortality was very low for trees with dbh greater than eight inches.

“The results are pretty basic, but they’re still valuable,” notes Keyser. “We have quantitative evidence of fire effects on common southern deciduous trees. The model relationships are more representative.”

When land managers design prescribed burn prescriptions, they can use the findings to address specific management goals. “If the goal is Quercus (oak) regeneration, then fire can help control competition in the smallest diameter size classes. For bigger stems, they may need to include mechanical thinning,” says Keyser.

“We would like to revisit the plots and see what happens next, after the main tree stem was killed. Many of these species are able to re-sprout,” adds Keyser. “More research is needed on how repeated burning might affect mortality.”

Forest managers often use prescribed fire every three to five years, and there could be an interaction between fire frequency and severity for upland hardwoods.

Read the full text of the article.

For more information, contact Tara Keyser at tkeyser@fs.fed.us.

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