326 IV. Restoration

BOX 10.1

Prescribed Burning forUnderstory RestorationKenneth W. OutcaltSouthern Research Station,USDA Forest Service, Athens,Georgia 30602

Role of Prescribed Burning. Because the long-leaf ecosystem evolved with and is adaptedto frequent fire, every 2 to 8 years, pre-scribed burning is often useful for restor-ing understory communities to a diverseground layer of grasses, herbs, and smallshrubs. This restoration provides habitat fora number of plant and animal species thatare restricted to or found mostly in longleafpine communities. Burning can also be usedto reduce the midstory layer, which catchesshed needles and serves as a ladder to carryunderstory fires into the crowns of the treesresulting in catastrophic wildfires that cankill vast areas of pines. Prescribed burningalso recycles nutrients by releasing thosetied up in litter and duff and significantlyreduces brown spot needle blight, which at-tacks longleaf seedlings.

Terms and Techniques. Prescribed burn-ing is the application of fire by trainedprofessionals following a well-developedplan to obtain desired management objec-tives. Restoration is often done with un-derstory burning or underburning, whichis prescribed burning under a forest canopy(McPherson et al. 1990). The fuel for thesefires is the understory rough that consistsof the accumulated living and dead grasses,forbs and shrubs plus draped needles andthe litter layer. The litter layer, the toplayer of the forest floor, is composed ofrecently fallen and largely intact dead need-les, leaves, twigs, and branches. A duff layer,composed of partially decomposed litter orfermentation layer and decomposed hu-mus, lies between the litter and mineralsoil.

Underburning can be done using head-ing, backing, flanking, or spot fires, or acombination of these techniques. Headingfires are fire fronts ignited to spread with thewind while backing fires are ignited so thefire front spreads against the wind. Flank-ing fires are ignited in a line into the windand thus spread at approximately right an-gles to wind direction. Spot fires are a seriesof separate ignition points that are allowedto spread in all directions and thus containheading, backing, and flanking fires at eachspot. Both heading and backing fires can beset as a series of strip fires. Strip heading firesare used to control how fast the fire spreadsand thereby the fireline intensity, i.e., therate of heat energy release (Box A Fig. 1).Placing strips closer together reduces therate of spread and intensity. Backing fireshave low intensities but move slowly andtherefore require a lot of time to burn eachunit. In addition, backing fires under cer-tain conditions may be quite severe, i.e.,cause much of damage to the site, becauseof excess duff consumption. Internal fire-breaks can be constructed for strip backingfires to significantly reduce time to completethe burn. An alternative is to use flankingor spot fires to reduce intensity but speedup the burn without internal fire breaks.These techniques require considerable ex-perience, especially spot firing as you mustcontinually adjust both the spacing and thetiming between spots to obtain the desiredintensity with changing fuel and weatherconditions (Wade and Lunsford 1989).

Burning Prescriptions

Sandhills. Prescribed burning can be usedfor restoration across the range of sites thatlongleaf can occupy from dry sandhills towet savannas. Burning prescriptions de-pend on the ecosystem type and its cur-rent condition. Reduced fire frequency inmany xeric and subxeric sandhills longleafareas has resulted in the development of amidstory layer of native scrub oaks: turkey(Quercus laevis), bluejack (Q. incana), sand

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FIGURE 1. Schematic of typical strip heading fire showing securing downwind side with backfirefollowed by sequential ignition of strip heading fires with a flanking fire to widen fuel free zonealong plow line.

live oak (Q. virginiana var. geminata), andsand post oak (Q. stellata var. margaretta).Because these sites are very droughty andnutrient limited, even in the absence offrequent burning, they do not developa continuous closed canopy of midstoryhardwoods. Therefore, although greatly re-duced, some of the understory grasses dosurvive. These grasses along with needlelitter from longleaf pines furnish sufficientfuel to carry at least a patchy prescribedburn. Repeated applications of prescribedfires during the growing season, i.e., begin-ning in March, in southern latitudes, andending in July, can be used to restore thesesites by gradually reducing the density ofthe midstory scrub oaks (Glitzenstein et al.1995) and promoting the growth of un-derstory grasses and herbs. Managers havefound that fire causes wounds on the stemsof hardwoods, which are enlarged by subse-quent fires, and eventually the top breaks orthe stem is girdled and the top dies. Sprouts

emerge from the roots of many top-killedstems, but these can be kept in check bysubsequent periodic burns.

Flatwoods. On flatwoods and wet lowlandpine types restoration means increasing un-derstory diversity in longleaf communitiesthat have been captured by woody speciesand in many cases developed a substantialmidstory layer. The goal is to reduce woodyunderstory and midstory species and allowthe grasses and forbs to increase and even-tually become at least co-dominant. Pre-scribed fire can be used to accomplish thistransition. Research shows that althoughgrowing season burns are sometimes moreeffective, dormant season burns can alsobe used to readjust understory composi-tion (Waldrop et al. 1987). For areas notburned for 10 years or more, a couple ofdormant season burns should be used to re-duce fuel loads before switching to grow-ing season burns. In addition, it is usuallybest to have these burns close together, i.e.,

328 IV. Restoration

2 years or less, to minimize fuel accumu-lations between burns. Miller and Bossuot(2000) recommend these initial burns beconducted when the drought index is below250. On sites dominated by saw palmetto(Serenoa repens), if burning alone is going tobe used for understory restoration, then aseries of closely spaced prescribed burns isrequired. Frequency of burns is more im-portant than season with annual burns themost effective but biennial burns will re-duce palmetto-dominance and increase theherbaceous component. It is important notto miss a burn, as this can result in a signif-icant regrowth of palmetto.

Uplands. There also exist upland long-leaf types, mostly in Alabama, Mississippi,Louisiana, and Texas, and montane sites inAlabama and Georgia, which have devel-oped unnaturally dense hardwood midsto-ries, and suppressed and impoverished un-derstories. Because these are the most pro-ductive longleaf sites, they change the mostrapidly, quickly developing midstory layersin the absence of frequent fire. In addi-tion to a very dense midstory and a shrub-dominated understory, these sites also ac-cumulate significant quantities of fuel. Asnoted for other longleaf ecosystem types,a series of dormant season burns is of-ten necessary to gradually reduce fuel lev-els before switching to a growing sea-son regiment. However, frequent and mul-tiple growing season burns will be re-quired to reduce the hardwood rootstocks(Boyer 1990), thereby providing condi-tions favorable to understory grasses andforbs.

Potential Negative Impacts. As with all burn-ing, there is the potential for negative im-pacts. The most obvious damage is directtree mortality that can result from exces-sively hot burns that are too intense and killtree crowns, including the buds. Tree mor-tality can also occur with low-intensity buthigh-severity fires that slowly consume ac-cumulated forest floor duff, and because oftheir long residence time heat root and stem

cambial cells beyond the lethal temperature.Trees suffering from such injury often retaina healthy-looking green crown for sometime following the burn, but will eventuallydie. Longleaf communities needing restora-tion burning rarely have many seedlings. Ifsignificant numbers of seedlings are present,however, excessive seedling mortality canresult from burning during the bolting stage.If burning must be done with seedlings atthis stage, then burning should be done inthe dormant season or early spring prior tothe candle stage when seedlings would bemost susceptible to fire-caused damage andmortality.

Precautions. In all longleaf types that havenot been burned for 10 years or more, thereis an excessive buildup of litter and duffaround the base of trees. Reintroduction ofburning in these stands without excess mor-tality is best accomplished by a series of dor-mant season burns. Apply burns when onlythe litter is dry enough to burn and the duffis too wet to ignite. On upland or sandhillssites, fast-moving heading or flanking firespushed by a good wind are better than slowbacking fires that may dry the duff layerand promote smoldering combustion. Flat-woods sites with palmetto-dominated un-derstories should be burned with headingfires, but will also require a light wind, cooltemperature, and higher humidity for thefirst burn. The objective on all sites is toconsume the dry top litter layer while thewet lower duff layer will protect the rootsand root collar. Space burns as closely to-gether as fuel to carry the fire will allow.Be cautious in your prescriptions because apatchy burn is preferable to a more com-plete hot burn that could result in exces-sive tree mortality. The objective is to grad-ually reduce the duff layer at the base oftrees over a cycle of four or five fires andkeep tree mortality at an acceptable level.Once the excess duff layer is removed, ap-ply a growing season burn, again as soonas there is sufficient fuel to carry a goodfire.

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The most important factor in accomplish-ing the goal of successful restoration withfire while minimizing the negative conse-quences is experience. Only through train-ing and practice can you become proficientat selecting proper conditions of tempera-ture, humidity, wind, fuel moisture, and fir-ing techniques keyed to existing fuel typesand loads that are likely to produce thedesired outcomes. This means obtaining acontract burner from a consulting forestrybusiness or an experienced crew from anonprofit or government agency until yougain knowledge and experience needed tobe a certified burner. A source of informa-tion is the U. S. Department of Agriculture,Forest Service, Southern Research Station(http://www.srs.fs.usda.gov), which has anumber of relevant publications. Consider-able advice and guidance is also availablefrom state forestry agencies, forestry unitsof southern universities, and local extensionagents.

ReferencesBoyer, W.D. 1990. Growing-season burns for

control of hardwoods in longleaf pine stands.

USDA Forest Service, Southern Forest Exper-iment Station, Research Paper SO-256, NewOrleans, LA.

Glitzenstein, J.S., Platt, W.J., and Streng, D.R.1995. Effects of fire regime and habitat on treedynamics in north Florida longleaf pine savan-nas. Ecol Monogr 65(4):441–476.

McPherson, G.R., Wade, D.D., and Phillips, C.B.1990. Glossary of Wildland Fire ManagementTerms Used in the United States. Washington, DC:Society of American Foresters.

Miller, S.R., and Bossuot, W.R. 2000. Flatwoodsrestoration on the St. Johns River Water Man-agement District, Florida: A prescription tocut and burn. In Proceedings of the 21st TallTimbers Fire Ecology Conference, pp. 212–215.Tall Timbers Research Station, Tallahassee,FL.

Wade, D.D., and Lunsford, J.D. 1989. Aguide for prescribed fire in southern forests.USDA Forest Service, Southern Region,Technical Publication R8-TP 11, Atlanta,GA.

Waldrop, T.A., Van Lear, D.H., Lloyd, F.T., andHarms, W.R. 1987. Long-term studies of pre-scribed burning in loblolly pine forests of theSoutheastern Coastal Plain. USDA Forest Ser-vice, Southeastern Forest Experiment Station,General Technical Report SE-45, Asheville,NC.