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  • Lindsey Hosier

One Fire, Two Trees, and a Lesson in Fire Ecology

Updated: Apr 24

After performing a prescribed burn in the Big Mountain area of the Monongahela National Forest, Melissa Thomas-Van Gundy with the USDA Forest Service Northern Research Station describes the unexpected fire effects between chestnut oak and northern red oak.

In 2019, the Forest Service used prescribed fire in the Big Mountain area of the Monongahela National Forest in Pendleton County, WV for sustainable forest management. Across much of the Monongahela and the eastern United States in general, foresters and ecologists are seeing forest overstories become dominated by one type of tree but the understories, often the next generation of trees, is of a different species. Oak forests are slowly being replaced by maple species in a process called mesophication. In this process, lower fire frequency creates conditions where a species not well-adapted to fire gains a foothold and then flourishes as fire is excluded for an extended time. Prescribed fire was used at the Big Mountain area to create change in the understory and the forest floor, which would help reverse the mesophication process. After the fire, a timber harvest was planned to remove some of the overstory trees.

Before the formation of the Monongahela National Forest, much of WV’s forests were changed by the exploitative, extractive logging of the turn of the 20th century. At that time, little thought was given to more than one harvest in these forests, or to what trees were left behind. The slash, tree tops, branches, and other unmerchantable material left in the woods dried out and became fuel for fires that were uncharacteristically large in size and intensity. As the forests of WV regrew, this legacy was a vivid caution of the destructive nature of fire. Fire suppression and cessation became a driving forest management practice and policy.  Without fire for approximately 100 years, mountain laurel in the Big Mountain area regrew, likely to densities and heights that it would not have reached without fire suppression. 

The prescribed burn did successfully remove the understory as planned. Overstory trees were unexpectedly affected, including oak trees that had been planned for harvest. Dead and alive trees were chosen from the burn units for an on-site demonstration of fire effects to overstory trees. Butt logs from trees in the burn units were sawed into rough boards to see how far into the tree the fire damage had occurred, if at all. Among the logs to be sawn were a chestnut oak and a northern red oak.

In fire ecology terms, chestnut oaks have more fire-adapted traits than northern red oak including thick bark to protect the living part of the tree bole. Trees grow taller from the tips of branches, from structures called apical meristems, and grow fatter from lateral meristems. Lateral meristems along branches, roots, and trunk, just under the bark of a tree, and are divided into two different tissues called vascular cambium and cork cambium. Vascular cambium produces phloem tissue (food-carrying) on its outer surface and xylem cells (water-carrying) on the inner side. In the cork cambium, cork cells are produced on the outer side and starch-storing cells on the inner. When cork cells die, they become bark. 

Chestnut oak produces more cork cells than other oaks and certainly more bark cells than a red maple. Considering that this takes energy and represents a trade-off for the tree from the production of other cells and tissues, there is likely an evolutionary basis for this difference. This thick bark protects the living cambium from harm from a variety of causes – drying out during a drought, rubbing from deer horns, and direct heating from fire. Species are adapted to a fire regime, defined in part with an intensity and frequency, and not just one fire. Oaks, in general, are adapted to low to moderate intensity fires that occur fairly frequently.

Like other oaks, chestnut oak seedlings can resprout from buds just under the ground line where living tissues are protected from the heat of fire by soil. Although the top of a small tree or seedling is killed by fire or drought, small oaks can regain height rapidly

after this type of disturbance. Not only is their ability to recover their height rapidly connected to soil protected buds, but also by the growth habitat of their root systems. Oaks tend to place more energy into growing a deep, tap-root.  A short-statured oak seedling may have a thick root, indicating an older seedling than might be expected based on height alone. In this thick root, food is stored for the inevitable need to resprout the top in response to the loss of the top part of the seedling. While most oaks have some measure of fire-adaptation the greatest difference between chestnut oak and northern red oak is bark thickness.

Oak seedling growing in the understory of a mature oak stand. Note the difference in the length of the top to the length of the root and the thickness of the root.

Based on this short introduction to fire ecology, you might predict that the chestnut oak in the Big Mountain controlled burn area would have less fire damage than the northern red oak. That’s what I thought I’d see at the saw demonstration. However, even before the saw cut into the logs the differences in fire effects were clear. The whole 10-foot section of the chestnut oak log was black, with the scorch covering the whole circumference. The northern red oak had a scorched area of approximately 3 feet tall and only on one side of the tree.

When these trees were chosen for our demonstration, the northern red oak was alive and the chestnut oak dead. As we talked about what we were seeing during the demonstration, the foresters who had harvested the trees noted that the chestnut oak had been growing in a patch of dense and tall mountain laurel. Mountain laurel is a fire-adapted shrub of the heath family with flammable, evergreen leaves. The northern red oak grew in a more mesic spot not surrounded by flammable shrubs.

This buildup of fuel may explain why my initial thoughts on fire effects between the two trees were incorrect. Under an altered fire regime, the chestnut oak growing on a drier spot in the forest stand ended up surrounded by tall mountain laurel. The tree’s thick bark may not have been thick enough to protect the cambium from the fire’s heat and direct flame contact. The northern red oak, having found a slightly wetter spot to grown on, didn’t have the threat of increased fuels since mountain laurel doesn’t compete as well on the site with more moisture and shade. Even though the bark of the northern red oak is not as thick and protective as the bark of the chestnut oak, the fire had less heat in this spot.

We found that when the logs were cut into boards, neither log showed much damage beyond the first 1” to ¾” thick board. The current guidelines for using prescribed fire to help oak forests remain oak-dominated can include increasing light levels in the mid story for the oak seedlings that respond to the fire. In some areas it may be possible to do this by commercial timber harvest. As such, the wounding of trees by fire and the potential of value loss is an important consideration. The scorch seen on the logs may also be a concern for those assuming a loss of timber value. Quantifying these impacts to timber value is a next step for researchers and managers on the Monongahela National Forest.

Melissa is a Research Forester at the USFS Timber and Watershed Laboratory. Her research includes stand and landscape-level projects focused on restoration and sustainable management of forests in the eastern US. These projects include investigating the role of fire in oak forests, using witness trees to understand historical forest conditions, restoration of red spruce forests, and restoration of American chestnut.

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