By: Melissa Thomas-Van Gundy, Research Forester
USDA FS Northern Research Station, Parsons, WV
In spring 2019, the Monongahela National Forest conducted a prescribed burn in the Big Mountain area in Pendleton County, West Virginia near the town of Cherry Grove. A prescribed burn is defined as fire applied at a set location, under specific weather conditions, to accomplish predetermined, well-defined objectives. The Big Mountain prescribed burn went through a thorough planning process to identify the specific conditions under which it could take place. The potential effects of the prescribed fire were documented and disclosed in an analysis prepared to meet the National Environmental Policy Act (NEPA). After this documentation and approval process, a burn plan was created where resource objectives and specific conditions under which the burn could take place are described in detail.
With this prescribed fire, land managers mainly wanted to change the forest understory (that part of the forest below the main canopy of mature trees) and the forest floor. Across much of the Monongahela and the eastern United States in general, foresters and ecologists are seeing a trend where forest overstories are dominated by one type of tree but the tree regeneration in the understory is dominated by a different species. Case in point are oak forests, which are slowly being replaced by maple species in a process called “mesophication”. In this process, a change in the frequency of a disturbance (in most cases, a change in fire occurrence) has created conditions in which species that are not well adapted to the disturbance gain a foothold and then flourish as the disturbance is excluded for an extended time.
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.
USDA Forest Service photo by Melissa Thomas-Van Gundy.
Oak-dominated forests of the eastern United States have changed significantly over the past 100 years. Drivers of this change include the loss of the American chestnut, extractive logging at the turn of the 20th century, fluctuations in white-tailed deer (an aggressive browser), and the reduction in fire occurrence. This loss of fire is thought to have a large influence on the types of trees found in forests.
The Big Mountain prescribed burn was successful, causing desired changes to the forest understory. The burn also resulted in the death of some overstory, mature trees, which allowed more light to the understory. In the larger goals for the area, trees within the burned area were planned to be removed through a timber harvest.
Chestnut oak boards from Big Mountain burn. Note that the entire 10’ log shows scorch. This tree was killed by the fire, however only the first (outer) board showed evidence of fire injury.
USDA Forest Service photo by Kelly Bridges.
Trees, both dead and alive, were chosen from the Big Mountain burn for a demonstration of fire effects to large, timber trees in November 2019. The lower part of the trees (what foresters call butt logs) were brought to the Greenbrier Ranger District office in Bartow, West Virginia, to be sawed into rough boards. A portable sawmill from the Forest Service’s Northern Research Station office in Princeton, West Virginia, was used to cut the logs into boards to see how far fire damage penetrated, 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 that a northern red oak. These traits include thick bark to protect the living part of the tree trunk.
Trees grow taller from the tips of branches from structures called apical meristems and widen from lateral meristems. Meristems are groups of cells that multiply and are undifferentiated, that is, they can divide and become different tissues or organs within the plant. The lateral meristems along branches, roots, and trunk, just under the bark of a tree, are divided into two different tissues called vascular cambium and cork cambium. Vascular cambium produces phloem tissue (the food-carrying tissue) on its outside and xylem cells (water-carrying cells) on its inside. Bark is formed on the exterior from cork cambium.
Chestnut oak trees produce more cork cells than other oaks and certainly more bark cells than a red maple. This thick bark protects the living cambium from harm from a variety of causes – drying out during a drought, rubbing from deer antlers, and direct heating from fire. Chestnut oak bark is blocky and deeply grooved, its thickness signifying it is well-adapted to fire.
Like other oaks, chestnut oak seedlings have the ability to resprout from buds just under the soil. If top-killed by fire, drought or deer browse, small oaks can rapidly regrow after this type of disturbance. Soil-protected buds and deep root systems allow for rapid height recovery after disturbance. As a group, oaks tend to place more energy into growing a deep tap-root. Roots serve multiple purposes: to anchor plants, collect water, and for starch storage. A short-statured oak seedling may have a thick root, indicating it is older than might be expected based on size alone. In this thick root, food is stored for the inevitable need to resprout in response to the loss of the top part of the seedling.
The leaves of oaks also show adaptations to fire and drought being relatively thick with waxy coatings, which slow moisture loss during drought. These characteristics result in leaf litter that is slow to decompose, forming a dry and loose layer that encourages fire ignition and spread through the understory. In comparison, maples leaves are thin, decompose quickly after leaf fall, forming a moist litter layer that absorbs moisture. Oak leaf litter is loud and crunchy when you walk over it and slippery, either wet or dry, due to the waxes that coat the leaves.
While all oaks have some measure of fire-adaptation, the greatest difference between chestnut oak and northern red oak is bark thickness. Based on this short introduction to fire ecology and oaks, it might be expected that the chestnut oak in the Big Mountain controlled burn area would have less fire damage that the northern red oak. However, even before the saw cut into the logs the exterior differences in fire effects were clear. The whole 10-foot section of the chestnut oak log was black with fire scorch covering the whole circumference, while the northern red oak had scorch only 3 feet in height and on one side of the tree.
As the foresters, fire-crew members, and others from the Monongahela and WV Division of Forestry observed the demonstration, the people who had harvested the trees noted that the chestnut oak had been growing in a patch of dense and tall mountain laurel, a fire-adapted shrub of the heath family with evergreen leaves that are flammable. The northern red oak grew in a moister spot that was not surrounded by flammable shrubs. When these trees were chosen for our demonstration, the northern red oak was alive and the chestnut oak dead.
The Big Mountain area is located in a part of West Virginia where fire likely was more common in the past. In addition to lightning as an ignition source, Native Americans frequently used fire for hunting, ceremony, to reduce insect density, and to encourage specific edible plants. It is also likely that any accidental fires from Native Americans were not aggressively suppressed and fires would have burned until weather, fuels, or topographic changes caused them to go out. Based on the mix of tree species found in old deeds of the area, and on the mix of fire-adapted traits of those species, the forests in the Big Mountain area were a mixture of fire-adapted and fire-sensitive species. So when fires occurred, the variation in local topography and existing species likely kept the burn area to a small area, maybe hundreds to a thousand acres, compared to areas where more fire-adapted trees were found.
Red oak boards from a tree in the Big Mountain prescribed fire. Note the relatively low amount of scorch on the bole of the tree.
USDA Forest Service photo by Kelly Bridges
Species are adapted to a fire regime, not simply fire. A fire regime has, at a minimum, a typical intensity (how “hot” a fire burns) and a frequency or return interval. Oaks and pines, in general, are adapted to low to moderate intensity fires that occur fairly frequently. The variability in fire occurrence determines when an oak seedling or sprout can grow into trees and reach the overstory instead of being reset by fire.
Before the formation of the Monongahela National Forest, much of West Virginia’s forests were changed by the exploitative, extractive logging of the turn of the 20th century. Little thought was given to the remaining forest and 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 uncharacteristic in size and intensity. As the forests of West Virginia regrew, this legacy was a vivid caution of the destructive nature of fire. Fire suppression became a driving forest management practice and policy. Without fire for approximately 100 years, the mountain laurel in the Big Mountain area regrew, likely to densities and heights that it would not have reached had fire remained as an active disturbance at a frequency as historically occurred.
This buildup of understory fuel may explain why the chestnut oak tree died and the red oak tree had not. Under an altered fire regime, the chestnut oak growing on a drier spot in the stand ended up surrounded by tall mountain laurel. The tree’s thick bark may not be thick enough to protect it from the fire’s heat and direct flame. The northern red oak, having found a slightly wetter spot to grown on, did not have the threat of increased fuels since mountain laurel doesn’t grow 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 particular spot.
Back to the sawmill demonstration. We found that when the logs were cut into boards, neither log showed much damage beyond the first three-quarters to one inch thick board. The current guidelines for using prescribed fire to help oak forests remain oak-dominated include giving them the growing space and light needed to regenerate. 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 some leading to the assumption that value has been lost. Quantifying these impacts to timber value is a next step for researchers and managers on the Monongahela National Forest.
