This is a common question regarding burning in hardwood forests in the eastern U.S. The short answer is yes, fire can kill hardwoods, but fire can also kill pine trees or any other type of tree. The key to hardwood tree mortality is fire intensity and duration. Low-intensity fire with flame lengths of 6-12 inches typically will not harm upland hardwood species unless woody debris has accumulated at the base of the tree. Use a conservative prescription, on the first burn in particular, to limit fire intensity and duration. Before burning your forest or woodland for the first time, walk through the area you intend to burn and remove any slash or accumulated woody debris from around the trunks of high-value trees, whether for wildlife or timber. Accumulated woody debris, when ignited, may retain heat long enough to damage the cambium (inner bark layer) and injure or stress the tree. Unless there has been a recent windstorm, ice storm, or other such event, most trees will be clear of debris and clearing can be done fairly quickly.
Fire frequency (or fire-return interval) is the most important factor with regard to how prescribed fire influences vegetation composition and structure. Fire intensity and season of fire are two other factors that can also influence plant communities and thus wildlife communities.
Managing plant succession is one of the main reasons managers use prescribed fire. Plant succession is the orderly process of plant communities changing following some disturbance event, such as fire, that leads to some final stage that is relatively stable until another disturbance event occurs. Whether our objective is to manage land for a particular wildlife species, ecological restoration, or increased forage for livestock, we use fire to influence the plant community. Fire can be used to set back succession or maintain a particular successional stage, and the effect of fire is influenced by how we use it. In general, and to a certain point, the hotter the fire, the greater the chance of killing vegetation. Most woody plants are killed when the cambium layer (the inner bark) reaches approximately 145⁰ F. Of course, all fires are hotter than 145⁰ F. However, tree bark is a good insulator and protects woody plants from many elements, including fire. Chances of raising the temperature of the cambium layer to a lethal temperature are increased with a hotter fire or by slowing the spread of fire (a longer residence time). It is important to understand that many woody stems may only be top-killed when the cambium reaches 145⁰ F. That is, the roots of the plant may continue to survive and the plant will sprout from the roots.
Fire intensity is influenced by environmental conditions, such as fuel load (amount of leaf and grass litter), litter moisture, litter size, air temperature, relative humidity, and wind speed, as well as the firing technique used. Fire intensity can be controlled by burning within certain environmental parameters and by using certain firing techniques. Within a given set of environmental conditions, headfires are hotter than backfires (headfires move with the wind and backfires move against the wind). However, the residence time of a backfire is considerably longer than that of a headfire. Thus, it is possible for a slower moving and less intense backfire to heat the cambium layer of a small tree or shrub to a higher temperature than a hotter and faster moving headfire. It is also generally safer to use a backfire than a headfire becasue a backfire is slower and typically has shorter flame lengths. Nonetheless, a relatively hot headfire is normally required to kill relatively large (>4 inches ground diameter) woody stems because the bark is thicker on larger trees and more resistant to relatively cool backfires.
Across most of the US, fire may occur in any season. Although late winter is when most people implement prescribed fire, considering other times of the year will not only extend opportunities for burning, it could also influence plant composition and help meet land management objectives. In general, research has shown that within a given fire-return interval, burning during the growing season (especially the late growing season) tends to kill woody species and decrease woody composition more than burning during the dormant season.
It is interesting how fire intensity may be greater, on average, when burning during the dormant season than during the growing season, but the effect in killing woody species is greater when burning during the growing season. Fuels tend to be drier and burn hotter during the dormant season than the growing season (the exception being during summer drought conditions). However, greater fuel moisture content during the growing season also means a longer residence time. Thus, timing of fire and fire intensity can be related.
It should be clear that fire frequency has a greater influence on plant composition and structure than timing of fire. However, prescribed fire does not have to be implemented during the dormant season. Consider burning during other seasons of the year and adjust fire intensity to help meet land management objectives.