Fire Frequency & Hardwood Regeneration

Shade-tolerant species compete with oaks and hickories

The mighty oak is a critical component of southern forests—for wildlife habitat, acorn production, and hardwood timber—but forests are changing, and its future is uncertain.

shade-tolerant-trees
The shady environment under dense, mature oak-hickory canopies has allowed shade-tolerant species like red maple to thrive in the understory. Photo by Tara Keyser, USFS.

A long-running U.S. Forest Service experiment studied the use of prescribed fire to control competition from shade-tolerant tree species like red maple, American beech, and blackgum.

The study area, located on the Daniel Boone National Forest in eastern Kentucky, is dominated by mature, upland oak-hickory forests. Fire has been absent throughout the last century of growth. More than 80 percent of the trees in the overstory are oaks and hickories.

However, the lack of disturbance has resulted in a midstory brimming with shade-tolerant species such as red maple.

“After a disturbance in the overstory, the individuals most likely to secure a spot there are large seedlings and small saplings in the woody regeneration layer,” says Tara Keyser, SRS research forester.

Keyser, along with retired SRS research forester David Loftis and Mary Arthur, professor of forest ecology at the University of Kentucky, examined different prescribed fire regimes to see how they affected seedlings and saplings.

The study is one of only a few to measure the effects of repeated burning on regeneration in mature oak-hickory forests, and the results were published in Forest Ecology and Management.

Traditional silviculture methods often fail to regenerate oak-hickory forests on all but the driest of sites, like ridgetops. Keyser and her colleagues established a series of experimental plots across a range of moisture conditions, from wetter, more productive sites to drier and less fertile.

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Frequent prescribed burning can control shade-tolerant competition, but fire-free periods are needed for oak-hickory seedling and sapling recovery. Photo by Tara Keyser, USFS.

At the center of each tenth of an acre plot, Keyser, Loftis, and Arthur counted small seedlings (less than two feet tall), large seedlings (between two and four feet tall), and small saplings (more than four feet tall and less than one and a half inches in diameter). The trees were counted in 2002, before any prescribed burns were conducted.

The team conducted all prescribed burns during the spring, before leaves appeared, with different treatments representing a range of frequencies and durations. The low fire frequency treatment involved two prescribed burns that occurred over a six year period, while the frequent fire treatment included five prescribed burns over a nine year period. No prescribed burning was conducted on the control plots.

After the 13 years of fire treatments, Keyser and her colleagues sampled the plots again in 2015. They expected that more frequent fire would result in more oaks and hickories–and fewer shade tolerant species–in the regeneration layer. They also expected to see the greatest effects on drier sites.

large-oak-seedling
Silvicultural treatments such as the oak shelterwood method may help to recruit oak-hickory seedlings into the canopy. Photo by Tara Keyser, USFS.

The results suggest that repeated burning can help oak and hickory seedlings reach more competitive sizes.

The low frequency fire treatment provided the greatest benefit to oaks and hickories. It resulted in a significant increase in both large seedlings and small saplings.

More frequent burning can result in top-kill of seedlings and saplings. When this happens, some trees can recover by resprouting from their roots, but increased mortality is also possible. Fewer seedlings and saplings then leads to fewer trees of larger, more fire-resistant sizes.

Although prescribed fire increased the number of oak and hickory seedlings, fire also favored competing species such as red maple. The number of shade-tolerant seedlings or saplings was not affected by either of the fire treatments, across any of the different moisture conditions.

“At this point in the study, prescribed fire in the dormant season does not appear to be enough to recruit competitive oak and hickory saplings in forest stands with a long history of little or no disturbance,” adds Keyser. “We will continue this research to learn if additional treatments, either mechanical, chemical, or continued fire, can help to control shade-tolerant competition in the forest understory.”

Read the full text of the article.

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

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