Effects of Prescribed Fire on Timber

Prescribed fire is a proven management tool that helps maintain the health and resilience of oak-dominated woodlands in Ohio. When applied under controlled conditions, fire reduces competition from shade-tolerant and fire-intolerant species, recycles nutrients, and promotes the regeneration of oak trees—key components of maintaining healthy, oak-dominated ecosystems. While fire can positively influence timber quality and growth, careful planning ensures the benefits of prescribed fire for long-term woodland sustainability outweigh the potential risks. Understanding these effects is essential for landowners and managers committed to sustaining oak-dominated woodlands for wildlife habitat, timber production, and ecological integrity.

Understanding the Relationship Between Oak and Fire 

Oaks display numerous adaptations to fire that assist them in dominating woodlands:

• thick, corky bark
• rapid wound sealing
• deep root systems that promote vigorous resprouting
• rainwater absorption
• thick, curly leaves that decompose slowly

Oaks have thicker bark than most trees species. This allows them to better withstand the intense heat produced by fire, which could otherwise result in damage or death of the tree. The sometimes corky and thicker bark of oak also absorbs more rainwater, providing additional resistance to fire. However, if intense fire does damage the tree, oaks are highly adept at compartmentalizing the wound, which helps keep potential rot from spreading and causing additional damage (Figure 2). This compartmentalization allows oak trees to respond to infected wounds with both chemical and physical changes to limit decay (Shigo & Marx, 1977). Additionally, oaks have deep root systems that store carbohydrates and water. These reserves enable the tree to recover from fire damage by producing new shoots and foliage. As a result of this deep root system, oak seedlings and saplings vigorously resprout after being top-killed by fire. This adaptation ensures survival and rapid growth even following multiple fires in successive years.

Not only do oaks possess features that demonstrate adaptation to fire, but they also have characteristics that help ignite and spread fire. Because the bark of oak can absorb more rainwater than other trees, the vegetation and soil around the oaks tends to be drier, making fire either more likely to spread or burn more intensely. Oak leaves are also thicker than many other species, allowing them to decay slower and remain as viable fuel on the forest floor for a longer period. Fallen oak leaves as fuel for fires is enhanced by the leaves' natural ability to curl as they dry, which decreases contact with the soil and slows down decomposition rates. This curling of the leaves also allows more airflow, aiding in fuel dryness and fire spread.

How Fire Impacts Trees

Fire behavior is primarily measured by intensity (amount of heat generated) and severity (amount of damage to the resource). A high intensity fire may be low in severity. For example, grassland fires typically generate an intense amount of heat. However, the fine, flashy fuels in grassland fires allow fire to spread quickly. This decreases fire severity by minimizing the residency time of the fire. Alternatively, low intensity fires may be considered high severity when branches and stems (timber slash) are piled near the base of a tree. These fuels do not necessarily burn intensely, but because they burn for an extended amount of time, their impact (severity) on the area is greatly increased.

When evaluating the timber value of a hardwood tree, defects such as knots, rot, bark char, and staining result in a lower value because there is a reduction in quality (grade). Wounding and mortality have a negative impact upon a tree’s economic value. Once a tree dies, the tree must be quickly processed into lumber before staining from decay or minerals lowers the quality of the tree. Additionally, dead trees are more likely to be impacted by wood-boring insects that cause defects.

Two principal ways fire can decrease the timber quality in a woodland:

1. Wounding
2. Mortality

Fire may damage a tree’s cambium layer, which is protected by outer and inner bark. The cambium layer is often referred to as the “living layer of the tree” because it produces new wood and bark. The death of individual cells in a tree occurs when the cambium layer is heated to 145 degrees Fahrenheit (Hood & Varner, 2019). The heat produced by fire is many degrees higher but, cambium cells are protected by bark, which provides insulation from heat. Mortality may occur if a significant amount of the base of the tree experiences charring, although this varies by species and bark thickness, or if a significant percentage of the canopy is scorched. Since most prescribed woodland fires occur during the dormant season, the risk of crown scorch is low for deciduous trees. As you might expect, the outer layer of a leaf does not provide much protection from the heat of fire. Therefore, leaves can be severely damaged by fire during the growing season. The degree of damage is a function of the amount of heat generated by the fire and the distance between the fire and the leaves. Thus, leaves high up in the canopy will be less likely to be impacted compared to leaves near the forest floor (Resop, et al., 2024).

Damage to the cambium layer that is not significant enough to cause tree mortality may still lead to the formation of wounds.  Figure 3 shows examples of common wounds caused by prescribed fire. Tree wounds most commonly occur near the base of the tree, and most often occur on the uphill side of a tree growing on a slope. This occurs because leaf litter often accumulates on this upper side of the tree, and these fuels may burn at higher intensity or for longer periods of time. A study by Brose and Van Lear (1999) found that 50%–60% of overstory oaks retained following a shelterwood harvest were undamaged by fire. The same study indicated that nearly all the severely damaged overstory oak, hickory, and yellow-poplar (5%–20%) had timber slash (branches and treetops left in the forest following harvesting) within 3 feet of their base. Twenty percent to 25% of the remaining oak trees had small wounds usually confined to one side of the tree. These wounds were low enough on the tree to not impact the trees' value or volume. Removing slash from around the base of oak, hickory, and yellow-poplar trees should eliminate most of the potential risk during a prescribed fire. Brose (2009) developed a photo guide to help forest managers recognize trees at risk of wounding or mortality from prescribed fire. This guide can help forest managers decide if it is appropriate to take preventive measures to protect trees in advance of a prescribed fire.

Depending upon the size of the wound and length of time between fires, individual trees may or may not have the ability to completely over grow or “seal” the wound. When open wounds (i.e., catfaces) are exposed to additional fire(s), expansion of the wound is likely. A study by Marschall, et. al., (2014) found that most value loss in red oaks is due to decreases in lumber quality, not from the loss of volume. They found that only minimal value loss was expected for fire damage within 20 inches of the ground, regardless of the fire scar size. Red oak sawlogs harvested within 5 years post-fire had little to no loss in value because their fire scars were not usually contained within the center portion of the log used to produce lumber. Figure 4 depicts a black oak that appears heavily defected while standing. However, once harvested, its butt log value loss was 8% and its volume loss was 2.8% (Marschall, et. al., 2014). The value loss for the entire tree was 4.4% (Marschall, et. al., 2014). One of the reasons why value and volume losses for individual trees with fire scars is limited is because of the necessity of “squaring the log” (Figure 4). This is a process where the slabs (outer cuts from the log) will be thicker on the large end of the log and thinner on the small end.

In a multistate study that included Ohio, Saunders, et. al., (2023) assessed relationships between wounding and tree grade after prescribed fire. Their study found that additional prescribed fires within a forest caused more wounding, with wound likelihood increasing by approximately 10% with each successive fire. However, no significant increase in wounding occurred after the third fire. Their study also showed that 10% of the trees in long-unburned control areas had wounding around the base, indicating that some level of natural wounding occurred within the woodlands. A similar study across the same geographic region assessed the impact of prescribed fire by analyzing how the number of prescribed fires related to board-feet-per-acre and how tree value was reduced (Mann, et. al., 2020). Data from this study is provided in Table 1.

Key Variables Influencing Fire Behavior 

Three factors determine fire behavior:

1. Fuels
2. Weather
3. Topography

Fuels impact fire behavior through their type (leaf litter, timber slash, grasses), amount, moisture content, arrangement, size, and shape.

Weather impacts fire behavior primarily through wind, temperature, relative humidity, precipitation, and atmospheric stability.

Topography impacts fire behavior through slope, aspect, elevation, and shape.

By understanding factors that influence fire behavior, fire managers can prescribe fire to a woodland to help meet the specific goals for the area. For example, by setting parameters for when it is acceptable to use fire, such as temperature, relative humidity, wind speed and direction, time since last precipitation event, etc., fire managers can predict the fire behavior and then match the anticipated fire behavior with their desired outcomes. If 1-foot flame lengths were desired, which would represent a relatively cool surface fire, parameters could be set to achieve those results. Figure 5 shows a backing fire (fire burning against the wind or slope), with approximately 1-foot flame lengths, moving toward a much faster-moving head fire (fire burning with the wind or slope), with approximately 3-foot flame lengths. It is important to note that more extreme fire behavior (10–15 foot flame lengths), such as those that may be created during a wildfire (fire not intentionally set) could easily result in mortality of many if not all the overstory trees. While these more extreme wildfires are not necessarily common in Ohio, there are times when weather and fuel conditions can combine to trigger severe scenarios.

References 

Brose, P. H. (2009). Photo guide for predicting fire risk to hardwood trees during prescribed burning operations in eastern oak forests. Gen. Tech. Rep. NRS‑GTR‑44, U.S. Department of Agriculture, Forest Service, Northern Research Station.
nrs.fs.usda.gov/pubs/gtr/gtr_nrs44.pdf

Brose, P., &Van Lear, D. (1999). Effects of seasonal prescribed fires on residual overstory trees in oak-dominated shelterwood stands. Southern Journal Applied Forestry, 23(2), 88–93.
research.fs.usda.gov/treesearch/36734

Hood, S. M., &  Varner, J. M.  (2019). Post-fire tree mortality.  Encyclopedia of Wildfires and Wildland-Urban Interface (WUI) Fires. 836–844.
doi.org/10.1007/978-3-319-51727-8_252-1

Mann, D, Wiedenbeck, J. K., Dey, D. C., & Saunders, M. R. (2020).  Evaluating economic impacts of prescribed fire in the Central Hardwood Region. Journal of Forestry, 118(3), 275–288.
doi.org/10.1093/jofore/fvaa004

Marschall, J., Guyette, R., Stambaugh, M., & Stevenson, A. (2014). Fire damage effects on red oak timber product value. Forest Ecology and Management, 320(15), 182–189.
doi.org/10.1016/j.foreco.2014.03.006

Resop, L., Demarais, S., Strickland, B. K., & Iglay, R. B. (2024). Fire season matters for midstory control: Impacts of firing season and firing technique on plant communities. Journal of Environmental Management. 363, 121297.
doi.org/10.1016/j.jenvman.2024.121297

Saunders, M. R., Mann, D. P., Stanis, S., Wiedenbeck, J. K., Dey, D. C., & Schuler, T. M. (2023). Prescribed fire causes wounding and minor tree quality degradation in oak forests. Forests, 14(2), 227.
doi.org/10.3390/f14020227

Shigo, A. L., & Marx, H. G. (1977). Compartmentalization of decay in trees. Agriculture Information Bulletin No. 405. U.S. Department of Agriculture, Forest Service.
nrs.fs.usda.gov/pubs/misc/ne_aib405.pdf