Fire and Woodland Management
By Rob Chapman
Is fire a four-letter word when considering its use in woodlands? The answer is complicated and undoubtedly depends on the specific management objectives of a particular piece of woodland.
Since the retreat of the Wisconsinan glacier some 13,000 years ago, the vegetation communities occupying Indiana’s landscape have seen remarkable change. Pollen cores taken from the natural lakes of northeast Indiana help assist scientists in recreating what our woods looked like through time. Our woods were dominated by conifers, primarily spruce and pine, for the first 4,000 years post glaciation. Hardwoods were present, but were minor components of the forest. Hardwoods began dominating the landscape from about 8,000 to around 100 years ago, with oaks being the predominate trees. Annual plants currently represent the dominate pollen deposition in natural lakes, not surprisingly given the dominance of agriculture on our landscape. Today’s woodlands are currently undergoing another major transition.
Forestry Inventory Analysis conducted by the United States Forest Service throughout the Central Hardwoods indicate that, while oak is still a dominate overstory tree in many woodlands, the mid- and understory are composed mostly of shade tolerant species, including maples and poplars. These surveys indicate relatively few, if any oaks, are naturally regenerating. Further compounding the regeneration issue is the preponderance of exotic shrubs invading the woods. If oak was such a dominate component of our woods for so long, why are we having an oak regeneration problem today?
We know that oak is a disturbance species, requiring adequate amounts of sunlight in order to regenerate. The standard silvicultural practices of disturbing woodlands to promote sunlight reaching the forest floor, and therefore attempting to stimulating oak regeneration, has been tree harvesting. What we are seeing, however, is that tree harvesting alone has not reversed the oak regeneration problem.
The likely disturbance mechanism impacting the historic species composition of our woodlands was likely fire. Lightning induced fire is relatively rare in the eastern United States, as most thunderstorms are accompanied by rain. Native Americans would have been the ignition source of such fire. Humans have occupied Indiana since the glaciers retreated, and have had substantial impact on natural communities. We know that Native Americans burned for many reasons. They used fire as a means of clearing land for agriculture and to facilitate travel. They used it in warfare, burning out their enemies and to keep fuel loads low around their villages to keep enemies from burning them out. Perhaps the most widely use of fire by Native Americans was for accessibility to food. Fire was used to improve forage for wild game as well as to increase yields of, and accessibility to, berries, nuts, seeds and other wild vegetable foods. Fires were used as a hunting tool, to improve visibility and drive game species.
Because oaks have a long history of association of fire, they have evolved several adaptations to living in a fire environment. They have thick bark, making it more difficult for heat to penetrate the cambium layers. Should a fire get hot enough to damage part of the cambium, oaks can quickly seal the damage through compartmentalization. Compartments created by fire leave scars that are used by dendrochronologists to determine how frequently high intensity fires occurred on the landscape. The fire return interval for southern Indiana was estimated to be between 5 and 10 years. Oaks also exhibit deep and wide rooting allowing for vigorous resprouting. When acorns germinate, their cotyledons are often placed below ground. This places the root collar below the soil surface where they are more protected from fire.
Oak leaves have further fire adaptations. They are thick and contain tanins, both of which increase decay resistance. Oak leaves tend to be drier, making them more flammable than other hardwoods, and they curl which increases fire spread. While these characteristics promote hot fires, the fires often move quickly through the understory. The low residence time of fast burning does not impact mineral soil, reduces damage to mature trees, and promotes the recycling of mineral back to the soil.
So, I will ask my original question again, in different terms. Can we use fire for woodland management in Indiana? The answer is certainly yes. But the answer depends on the specific management objectives of a particular woods and the land manager’s ability to incorporate fire in an effective and safe manner. Should you use fire and how often you burn often dictates the vegetation community of the forest floor. Burning once every three to five years favors an understory of grasses, sedges, and fall blooming wildflowers. Native legumes will also be more abundant following a frequent fire prescription. A 10-25 year burn interval results in fewer grasses and sedges and promotes spring wildflowers.
Research on fire use in mixed oak woodlands has shown that a single, low intensity fire will do little to promote oak regeneration. However, a single high-intensity fire increases oak regeneration. This research has resulted in the following guidelines for fire use to manage oak woodlands:
1. Fire is best used 3-5 years following a shelterwood harvest in which about 50 percent of the overstory is reduced. It may be necessary to protect valuable residual trees by clearing the area around those trees of leaves or other fuels. Leaf blowers work well in this situation.
2. Burns can be conducted anytime from leaf fall to leaf expansion (late October to mid-Spring).
3. Where insufficient oak seedlings exist, fire can create conditions suitable for oak establishment by reducing litter and competition in the under-and mid stories. Burns would ideally be conducted prior to an abundant acorn crop. Do not burn immediately following an abundant acorn crop or when oak seedlings are small or recently established.
4. Following a regeneration cut, a burn will give oak a competitive advantage when other species occur in higher densities. Burns should occur 3 to five times for the first ten years, then decreased to once every 10-25 years.
5. Deer browse could increase as oak seedlings vigorously sprout after burning. In areas with high deer densities, it may be necessary to increase hunting pressure. Exclusion activities may also be necessary to further reduce browsing pressure.
Fire is an ecological process that helped shape the composition and structure of our woods and certainly has a place in the land manager’s toolbox. Fire is inherently risky and should only be conducted by following a carefully prepared prescribed fire plan. Such a plan will include specific objectives, design and location of fire breaks, the season and weather conditions necessary to safely and effectively conduct the fire, and a contingency plan should a fire escape. It is important that private landowners participate in a prescribed fire workshop given by the Indiana Department of Natural Resources. Cost share for prescribed burning may be available under some Farm Bill programs. If you have any questions about prescribed fire, please feel free to contact me at Rnchapman@purdue.edu. Contact your DNR District Forester or District Wildlife Biologist for technical assistance and information on cost share options and availability.
Rob Chapman is an Extension wildlife specialist in the Department of Forestry and Natural Resources at Purdue University