Thermal thinning and pruning with prescribed fire benefits longleaf pine stands
The benefits of prescribed fire in the Southern "pineywoods" are many and varied—and because of the passion of folks who love and believe in these legal and carefully-planned controlled fires and their results—they are becoming increasingly well-known and appreciated, according to a wildlife biologist and forester with the S.C. Department of Natural Resources.
Johnny Stowe, a Heritage Preserve manager with the S.C. Department of Natural Resources (DNR), and also a certified wildlife biologist and forest, said the benefits of prescribed fire are many. These benefits include: 1) reducing fuel loads that would otherwise create potential for dangerous wildfires; 2) limiting damage from disease and insect pests such as brown-spot needle blight and red-headed saw-flies (with proper timing); 3) creating, restoring, or enhancing habitat for wildlife and desirable plant species while yielding aesthetically pleasing landscapes; and 4) preserving the ancient and multi-cultural tradition, ritual, and heritage of woods-burning as a landscape management tool—a natural process that has shaped the character of landscapes, as well as people and societies for many millennia. People have been using fire for this purpose for more than 1.5 million years.
According to Stowe, prescribed fire can also be used in more sophisticated ways—for example, to prune and thin in stands of longleaf pine. The ability of fire to kill or topkill (which inhibits growth) of hardwoods such as red maple, sweetgum, water oak and other species that are generally undesirable in longleaf woodlands and savannas, is well known. Less well-known is the artful use of fire to prune the lower limbs of young longleaf pines; to reduce the number of trees in longleaf stands that are too dense; and to remove other pine species, particularly loblolly, from these stands.
The need for thinning longleaf stands and for removing wildings (pines that that have ―volunteered into longleaf stands) is common and usually obvious. Sometimes longleaf was planted thicker than it needed to be, and perhaps mortality of planted long-leaf was expected but did not happen, or longleaf or other pines seeded into planted stands from nearby mature trees, or too many seedlings arose in sites being naturally regenerated. On some occasions, landowners' management values and objectives change, or simply their perspective—what looks like a nice stand of grass-stage long-leaf today, may later seem far too dense.
Dale Wade, veteran fire researcher with the U.S. Forest Service (now retired) and ardent promoter of prescribed burning, says Fred Wetzel of the U.S. Fish and Wildlife Service introduced him to the terms thermal pruning and thermal thinning in the 1980s.
The propensity of pines to produce significant numbers of large, low-to-the-ground limbs in open-grown situations is evident in fields or pastures along most any roadside and is a function of exposure of the bole to sunlight. Dense stands of trees generally do not have persistent, low limbs. These limbs may initially develop when the trees are very small, but over time they are shaded out and generally die in such stands.
For various reasons, many landowners don't want dense pine stands. Moreover, some government cost-share or other incentive programs aimed at improving early successional wildlife habitat, mandate that pines be planted at a wide spacing in order to encourage development of groundcover. This approach promotes good early successional habitat, but it may also lead to larger limbs that are retained for a longer period of time, if not for the life of the tree. A stand of open grown longleaf that is not properlyburned (all burns that can be conducted on a given site are not equal) will retain low limbs that may produce as much shade and inhibit groundcover as much as the tightly packed trees in a high density planting. Open stands of longleaf pine that have nice cylindrical boles (main stem of tree) with few limbs are a result of either proper prescribed burning practices and/or stands that grew densely at one time but were later thinned.
By burning under the right conditions, Stowe said, land managers can kill or inhibit the retention of lower limbs. This practice makes the first log more valuable by making it clear of knots. Moreover, it allows more sunlight to reach the ground, and sunlight is a vital, often the limiting, factor in the quality of groundcover that grows on a site. Groundcover such as native grasses and legumes provide valuable cover and food sources for wildlife, fuel for subsequent fires, and many species are attractive and desirable for their own sake. Plus, legumes fix nitrogen. For landowners keen to produce timber products as well as wildlife habitat, all else being equal, a tract of land can grow more and higher quality timber while still producing early successional habitat when the trees lack low limbs. Historical photographs often suggest this phenomenon—dense stands of large, clean-trunked longleaf or shortleaf pines of impressive and valuable timber volumes, yet with an apparently continuous and intact groundcover. In other words, one need not choose between maximizing financial returns and the maintenance of prime wildlife habitat and aesthetic attributes, especially when managing longleaf pine on longer rotations.
Fire can also be used to kill unwanted pines that have volunteered into a site, or those that were planted too densely. Given that loblolly is less fire tolerant than longleaf, fire of a given intensity will kill young loblolly while only pruning longleaf. Thinning some longleaf seedlings or saplings while maintaining others, is trickier and not exact, but can be done.
The benefits of burning and the techniques used in prescribed fire are science-based, but the successful and repeated use of fire requires a practitioner more akin to an artist, than a scientist. Only so much can be usefully written down in a prescribed burn plan. The art of fire management, based on inherent skill plus experience, tends to come into play during the burn, when on-the-spot customizing of ignition techniques can increase efficiency and help one better reach pre-determined objectives. If a prescribed fire manager is comfortable making these adjustments, it is vital that she or he build that flexibility into the written prescribed burn plan, since these plans should be followed during a burn.
It is common across the region for loblolly pine seed to be blown into longleaf plantations from some distance away. Loblolly is a prolific seed producer and often can swamp longleaf plantings with thousands of wild seedlings per acre, crowding and suppressing the longleaf. Bill Boyer, renowned U.S. Forest Service longleaf researcher, once said that regenerating loblolly naturally might be achieved if 10 good loblolly seed trees per acre are pre-sent. He added that if 10 aren't present, five would be enough…if five weren't present, then one would be sufficient, and if one wasn't there, that would still be enough! Loblolly is truly an aggressive pioneerspecies. Whether planted or naturally regenerated, young longleaf will tolerate fires that will kill unwanted seedlings, be they loblolly, slash, sand, Virginia pine, or other undesirable pine species.
Prescribed fires aimed at thinning or pruning longleaf often results in significant needle scorch, and even some consumption, of needles. This is not necessarily anything to worry about—in fact, scorch and even some degree of consumption, in longleaf, are often good indicators that the burn was hot enough to achieve objectives. Conversely, lack of needle scorch in longleaf may indicate the fire did not get hot enough to do the intended job. Depending on time of year of the burn, many sites burned hot enough to scorch and even consume many needles of most long-leaf end up being just what is needed to achieve objectives. But this pushing the envelope with fire is tricky and the difference between meeting pruning and thinning goals and killing too many trees is often a fine and nebulous line. High rates of needle scorch and especially of needle consumption may inhibit long-leaf growth in the short run, but the various benefits may often make that possibility a more-than-worthwhile sacrifice, especially considering the long-run results of increased tree growth because of competition control and the benefits of removing lower limbs.
"Foresters and land managers tend to be conservative," Stowe said. "It has been hard for many of them to accept the notion of burning in the growing season, and I have run into opposition and criticism for prescribing and carrying out really hot fires that scorch or even consume needles in longleaf, but, as the Brits say, the proof is in the pudding. Now that I have been lighting such fires for more than a decade on South Carolina's Heritage Preserves, as well as on private tracts, I run into much less criticism, at least from folks who have seen the results firsthand and on-the-ground."
Most land managers today realize that old rules of thumb such as never burn when air temperature is over 65 degrees, don't burn after the last of March, or pine needles should never be scorched, are archaic and were based on observations of higher intensity wildfires, narrow and outdated notions of perceived landowner values, and inaccurate perceptions of fire effects, according to Stowe. "Yes, we need more research to quantify the effects of fire intensities on ecosystem attributes," Stowe said, "but until we get those data, we should continue to carefully work outside the narrow confines of dormant season and low intensity burning, when the results are obvious and beneficial."
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