By Brandon Howell, Area Biologist, Division of Wildlife and Freshwater Fisheries
Prescribed fire is a useful tool to help manage for timber and wildlife if implemented appropriately. While blackening of a tree's bark is perceived by the public as damaging to the tree, this is generally untrue and only aesthetically unpleasant for a short time.
To cause true permanent injury to a tree, the cambium layer – located between the bark and wood – must be heated to 120 F for about one hour. If the cambium temperature reaches 130 F, tissue damage results in six minutes; at 140 F the damage occurs in 30 seconds, and at 147 F tissue death is instant. A tree’s bark is the only defense it has to protect against damage from fire.
All bark is not created equal, however. Bark from different trees looks different and the insulating qualities are different. In general, trees with the thickest bark are most resistant to fire damage. An astute observer in the forest will notice that older trees have thicker bark than younger trees of the same species. This leads to the conclusion that older trees are more resistant to damage from fire.
A good rule of thumb concerning plants and fire is if a tree is resistant to fire damage it will produce by- products that help carry a fire such as the resinous needles of pine trees. On the other hand, a tree that lacks good fire resistance produces by-products that are difficult to burn. Pine trees produce bark that is thicker and has better insulating qualities than hardwood trees. For this reason, many managers still prefer to only use fire within pine stands where the thick bark creates a relatively effective barrier to protect the inner cambium layer from high temperatures that accompany flames.
Some land managers have begun using control burning in mixed pine-hardwood stands that were traditionally considered off limits to accomplish goals such as fuel reduction and to kill undesirable small hardwoods in a mature forest. Conducted properly under certain conditions, controlled burning is a valid management approach to protect the long-term health of the forest.
A fire in hardwoods during the late summer or fall will be detrimental to the trees, especially if ambient air temperatures are already in the high 80s and 90s. Under these conditions there is a great probability that the flame would raise the temperature of the cambium layer 30-plus degrees to the point where damage would occur. A proper fuel reduction burn within a hardwood stand should be done on a very cold day when the ambient air temperature is helping keep the tree cool as the fire passes. An air temperature of 40 F would provide an 80-degree zone of safety before damage is initiated.
It should be noted that no hardwood tree is classified as highly resistant to fire damage. In fact, the only tree native to Alabama considered highly resistant to damage is the longleaf pine. Most of the other pines are considered resistant. The most resistant hardwood, ranking as moderately resistant, is the chestnut oak, followed by yellow popular, black oak and post oak. Intermediate resistance is obtained by red oak, hickory, sweetgum and white oak. Trees classified as having low resistance to damage from fire include sugar maple, scarlet oak, yellow birch, black cherry and cedar. If the cambium is damaged it will cause fire scars that result in rotting in the trunk. This will hurt the aesthetics and productivity of the tree in addition to severely reducing its value when the time comes to sell the timber.
The best option for cleaning up a hardwood and pine forest is to initiate burns in the winter months when the correct parameters of humidity, ambient air temperature and wind speed converge to produce a good burning day for this specialized forest management tool. Do not compromise on having the desired weather conditions for a burn in hardwoods. Mistakes potentially damage the forest for decades resulting in less productive trees for wildlife and drastically reduced values on the timber market. The trees are less susceptible to damage in winter months when they are in a type of dormant stage.
Always use backing fires when possible and make sure all firebreaks are adequate. Burning during the winter months is not just for pines anymore. Fires in mixed hardwood forests can clean up leaf litter, logging slash and stumps. This allows more sunlight to reach the forest floor encouraging the seed bank to grow plants that may have not been seen in decades. These fresh green sprouts will encourage countless species of wildlife to utilize the new growth.
Always be careful when burning at any time or place, but be especially careful in pine-hardwood forests. If temperatures are too high or fire weather forecasts call for erratic conditions, wait until another day. Have your burn plan looked over by a professional forester or prescribed burn manager. Make sure the weather conditions call for low winds and relative humidity in the 40-45 percent range. It is also helpful if there is a forecast of rain in the next few days. The results will be a “cool” fire that doesn’t kill trees or bushes but simply cleans up the forest floor by removing litter that would otherwise takes years to decompose. The wildlife in the area will be respond positively to the changes in the habitat.