The late “hard” frosts that killed new growth of hardwood species in much of South Carolina in the middle of April may provide an uncommon chance to deliver a “double-punch” to these hardwoods in grassland and other ecosystems where they are generally considered undesirable.
In deciduous species like sweetgum, red maple, persimmon and oaks, the year’s first growth is produced by nutrient reserves (mostly carbohydrates) that have been stored in the roots during the winter, according to Johnny Stowe, past chairman of the S.C. Prescribed Fire Council and forester and wildlife biologist with the S.C. Department of Natural Resources. These reserves were produced and sent to the roots the preceding fall, and are key to “priming the pump” for the spring’s new growth.
The late frost, by “killing back” the new growth, will force the trees to send up additional nutrients to produce another set of leaves. So the trees will be stressed more than usual. Once the second flush of leaves develops, a timely prescribed growing-season burn this year, that again kills the leaves and buds—while also, ideally, girdling the stems—may provide the added stress needed to kill unwanted hardwoods, roots-and-all.
Growing-season burns are superior to dormant-season burns (those conducted during the winter) when it comes to controlling hardwoods. Often, the former will kill the trees roots-and-all, while the latter frequently only “top-kills” (kills only some or all of the aboveground part of the tree, leaving the roots intact, resulting in sprouting).
“Sometimes, even growing-season burns fail to kill some trees’ roots,” Stowe said, “but this year an opportunity has arisen to, in effect, hit the trees with a one-two punch. It will be much like conducting two growing season burns in one year.”
Prescribed burns should only be conducted by appropriately trained and experienced land managers, who must obtain a notification approval from the S.C. Forestry Commission prior to setting the fire. Growing-season burns require more expertise than other burns, in part because it is much easier to kill desirable vegetation. Also, some forest types should generally not be burned during the growing season. For instance, young longleaf pine stands, which have trees sending out very long new shoots (candles) in the spring, are very vulnerable to damage or mortality. Also, stands of any age that are fire-suppressed are extremely sensitive to fire-damage even in the dormant season, but even more so during the growing season. These stands are best burned during winter, as are stands that have been stressed by recent thinning or other factors.
Stowe said landowners should also take great care in burning around any large hardwoods that they do not wish to kill. These trees may have withstood growing season burns in the past with no harm, but might not be able to take such a fire this year, without damage. Many landowners wish to control small hardwoods in their quail woods, but have certain, special, individual large oaks or other hardwoods that they value. If land managers burn around these trees, they should take care to keep fire well outside the outer drip line of the branches.
Because much of the Southeastern United States has not received much rainfall this spring, prescribed fire managers must consider the effect that the dearth of rain has had on forests. In some areas, the peat in Carolina bays and other isolated freshwater wetlands is not inundated, as it usually is this time of year. If dry peat catches fire, it produces large amounts of smoke, and can be difficult or impossible to extinguish. These fires can burn for weeks or months, causing smoke hazards by limiting the view on roads, as well as creating problems for folks who have respiratory or other ailments. The drought can also make it much harder to contain fires within their breaks. So anyone burning this spring and summer should exercise extreme caution.
“In many areas the only safe way to conduct a prescribed burn will be to wait until we receive significant amounts of rainfall,” Stowe said. “On appropriate sites, where a growing-season burn can be safely conducted, however, this year may be a prime opportunity to ‘knock back’ undesirable hardwoods.”