Silviculture To Restore Oak Woodlands And Savannas - Usda

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SILVICULTURE TO RESTORE OAK WOODLANDS AND SAVANNASDaniel C. Dey, Benjamin O. Knapp, and Michael C. StambaughAbstract—We present a perspective on how to approach developing silvicultural prescriptions for restoring oakwoodlands and savannas. A large degree of success depends on selecting appropriate sites for restoration.We discuss historical landscape ecology, fire history, detecting legacies of woodland/savanna structure, andmodels of historical vegetation surveys. Ultimately, site selection for restoration is determined by integratedmanagement goals and objectives. We discuss silvicultural practices for restoration including prescribed burningand thinning by mechanical or chemical methods or timber harvesting. We provide an overview of fire effects onvegetation and stress how the timing and sequencing of the various practices can be used flexibly dependingon site restrictions, initial vegetation condition, and threats such as invasive species. We review various fireregime attributes that managers can control in moving the vegetation toward the desired future condition. Weconclude by giving a perspective on developing the restoration prescription using a holistic, integrated resourcemanagement approach.INTRODUCTIONOak savannas and woodlands were oncesignificantly more prominent on eastern landscapesin the United States and Canada. Since Europeansettlement, their loss has decreased landscape diversityand resilience, and diminished our ability to conserveand sustain native biodiversity and promote ecosystem/landscape productivity and health. We lack silviculturalstrategies, prescriptions, and tools to restore thesenatural communities. However, we have a strongfoundation in hardwood forest silviculture that we maydraw upon to develop plans for restoring oak woodlandsand savannas. In addition, a strong understanding of oakwoodland and savanna ecology forms the critical basisfor developing the silviculture needed to manage thesesystems. This paper discusses silviculture strategies,practices, and tools for restoration of oak naturalcommunities including (1) positioning restoration on thelandscape, (2) developing the silvicultural prescription—available practices, and (3) the restoration prescription—setting the quantitative targets.POSITIONING RESTORATIONON THE LANDSCAPELandform, Topography, and SoilsLandscape patterns of oak forests, woodlands, andsavannas and prairies resulted, in part, directly from thephysical characteristics of landform, geology, soils, andtopography, and indirectly by the effect of the landscapeon disturbance regimes (Anderson and others 1999,Batek and others 1999, Nigh and Schroeder 2002,Stambaugh and Guyette 2008). Important environmentaland physical site variables that influence speciesdistribution and natural community type can be identifiedthrough landscape modeling with spatially explicithistoric vegetation information (e.g., Bolliger and others2004; Hanberry and others 2012, 2014a, 2014b, 2014c).For example, oak species that are typical of woodlandsand savannas were more likely to occur on increasinglyxeric sites as defined by variables such as elevation,slope, parent material, wetness index, and solar radiationin the Missouri Ozarks according to Hanberry andothers (2012), who modeled species occurrence andcommunity tree structure using General Land Office(GLO) witness tree survey data. Managers can also usevegetation and ecological classifications developedby ecologists who have reconstructed or modelledthe location and extent of historic vegetation types forStates and physiographic regions (e.g., Anderson andothers 1999, Curtis 1959, Nigh and Schroeder 2002,Schroeder 1982).Tree density and canopy cover may be limited on sitesthat are seasonally flooded (e.g., oak flatwoods) anddroughty where (1) soils are shallow in depth, or whererooting volume is limited by the presence of claypansand fragipans, (2) soils are coarse textured and extremelywell-drained such as those derived from sandstones,Author information: Daniel C. Dey, Research Forester, Northern Research Station, USDA Forest Service, Columbia, MO 65211;Benjamin O. Knapp, Assistant Professor, School of Natural Resources, University of Missouri, Columbia, MO 65211; andMichael C. Stambaugh, Research Associate Professor, School of Natural Resources, University of Missouri, Columbia, MO 65211.Citation for proceedings: Clark, Stacy L.; Schweitzer, Callie J., eds. 2019. Oak symposium: sustaining oak forests in the 21st centurythrough science-based management. e-Gen. Tech. Rep. SRS-237. Asheville, NC: U.S. Department of Agriculture Forest Service,Southern Research Station. 192 p.Oak Symposium: Sustaining Oak Forests in the 21st Century through Science-based Management125

and (3) soils have high rock content that limits waterholding capacity and increases internal drainage. All ofthese soil-site features may restrict the developmentof forests and promote woodland and savannacommunities as defined by Nelson (2010). Collectively,these site characteristics often increase the likelihoodof droughts that limit tree development and fires thatcan create oak woodlands and savannas. Certain oakspecies such as post oak (Quercus stellata), white oak(Q. alba), bur oak (Q. macrocarpa), and chinkapin oak (Q.muehlenbergii) were dominant overstory trees in thesesystems and on these sites because of their adaptationsto fire and drought, ability to resist wood decay, andlongevity (Abrams 1990, Bahari and others 1985). Givena set of physical site characteristics, the nature of the fireregime would then determine the outcome, i.e., whetheroak forests, woodlands, or savannas formed.Topography/landform influences various attributesof a fire regime including fire frequency and size(fig. 1). Frequent fires—sometimes annual fires—werecharacteristic of the once extensive prairies in theEastern United States (Anderson 2006, Transeau 1935).In the past, prairies occurred most often on level togently rolling terrain (e.g., 4 percent slope) where firescould spread rapidly and extensively. As topographybecomes more dissected, topographic roughnessincreases, and fire frequency and size decrease (Guyetteand others 2006, Stambaugh and Guyette 2008). Prairiestransition into savannas as the topography becomesgently rolling and headwater drainage ways begin toform. Steeply dissected topography creates landscapesthat have more natural barriers to fire spread such aswaterways and protected mesic north-to-east slopes,which either physically oppose the advance of fire,Figure 1—Topography had a strong influence on fire frequency and size before the fire suppressionera. In plains and relatively flat terrain, fires were more frequent and larger in size resulting in prairie,or oak-pine savanna and open woodlands. In more severely dissected terrain, fire frequency and sizewere reduced due to the increase in natural fire breaks in the form of streams and north-east aspectsthat were less conducive to burning. Oak-pine woodlands were a dominant type on moderatelydissected terrain. On the leeward side of major waterways, fires burned relatively infrequently, andmesic forests were able to develop. This example from the upper Current River watershed in theMissouri Ozarks illustrates the interaction of fire and topography that resulted in distinct, spatiallyexplicit patterns in tree composition and structure (based on Batek and others 1999).126Silviculture to Restore Oak Woodlands

or modify fire weather, fuel dynamics (e.g., continuity,loading and moisture), and site hydrology to restrictfire spread. Although fires can burn rapidly and moreintensely up exposed, xeric, south-west slopes, therate of fire spread—especially that of low-intensitybackfires—is reduced on north-east slopes due to coolerand moister conditions, increased fuel moisture, changesin fuel loading and flammability, and increased treedensity among other factors. With less frequent fire onprotected and mesic sites, trees increase in dominanceand density, and forests develop complex verticalstructure with the formation of a midstory canopy andshrub/tree understory while the more flammable grassesand heliotrophic forbs are greatly diminished. Litter fromthe more shade-tolerant trees and shrubs has relativelylow flammability; forms a compact, flat fuel structure;and decomposes quickly. These conditions act to reducethe probability of fire ignition, the fire intensity, and therate of spread. This mesophication that occurs in theabsence of fire, or in areas of infrequent fire, creates apositive feedback that promotes succession and forestdevelopment toward more fire-resistant conditions(Nowacki and Abrams 2008). In heavily dissected terrain,oak woodlands prevailed and true forests formed onmesic aspects and lower toe-slope positions, and on theleeward side of major natural fire barriers such as riversand lakes.communities that can be used to identify sites that oncewere savanna or woodland, and still have the potentialto respond well to silvicultural restoration practices(e.g., Bader 2001, Farrington 2010, Packard and Mutel1997). Floristic inventory of the candidate restorationarea can reveal the presence and coverage of desiredindicator species, which indicates the site’s potential torespond well naturally to the reintroduction of fire andreduction of tree density. Ecological indices developedby plant ecologists such as floristic quality index andcoefficient of conservatism can be used to identify thosesites that have the capacity to recover through naturalregeneration of the ground flora following restorationpractices (Swink and Wilhelm 1994, Taft and others1997), and to anticipate ahead of time the need forsupplemental artificial regeneration of desired groundflora to meet management objectives on degraded sites.Knowledge of land use history is useful for consideringhow degraded sites may be. Severely degraded sites arethose that were cultivated annually or overgrazed, andallowed to erode severely. They are of low floristic qualityhaving lost native seed bank and remnant vegetation,and are dominated by weedy, invasive exotic or nativegeneralist species. Historical photos or local journalsprovide anecdotal insights on historical conditions andland uses.Historic Legacies in VegetationComposition and StructureKnowledge of local fire history can provide insight on theoccurrence of savannas, woodlands, and forests. Sitespecific fire histories are increasingly being developedthroughout eastern North America (e.g., Brose andothers 2013a; Guyette and Spetich 2003; Guyette andothers 2003; Hoss and others 2008; Stambaugh andothers 2006a, 2011). A landscape model predictinghistoric fire frequency for the continental United Stateshas been produced by Guyette and others (2012) andis a useful guide for areas that lack local fire historydata. Annual and biennial fire regimes are more closelyassociated with prairies and savannas (Andersonand others 1999, Anderson 2006, Nelson 2010). Lessfrequent mean fire return intervals favor woodland( 5 years) and forest ( 10 years) development. Firehistories provide a wealth of information on past fireregimes such as variation in fire occurrence at a givensite, which is equally important as average fire frequency.Infrequent but extended fire-free periods of from 10to 30 years are needed to permit recruitment of treesaplings into the overstory to replace the overstory(Arthur and others 2012, Dey 2014). Infrequent highseverity fires are capable of killing much of the overstoryand provide regeneration and recruitment opportunities,or fundamentally change oak natural communities fromone type to another. Variation in fire seasonality resultsin higher plant diversity in the long term. Frequent fireregimes that lack variability in fire-free periods trendtoward producing savanna or prairie communitiesStand structure and characteristics of individual treesmay indicate previous savanna or woodland conditions.The presence of large “wolf” trees with low and widespreading crowns and large lateral branches indicatesthat previously the trees had grown in the open in lowdensity savannas or open woodlands. Sometimes thesetrees are also surrounded by dense thickets of smallerdiameter saplings or pole-sized trees that invaded thesavanna or woodland following fire suppression. Oaksoften dominate the initial recruitment of trees in savannasand woodlands during an extended fire-free periodfollowing a long-term history of frequent fire (Stambaughand others 2014). Encroachment of mesophytic treespecies such as red maple requires longer periods of firesuppression (Fei and Steiner 2007, Johnson and others2009, Nowacki and Abrams 2008). Final confirmationof natural community location on the landscape maybe gained by a study of historic photos or journalsthat provide anecdotal information on local naturalcommunity types and composition and insights on firehistory and land use practices to better understandvegetation change since European settlement.Remnant ground flora may indicate more open woodlandand savanna conditions once occurred in the past. Manypublic and non-profit conservation organizations havepublished lists of indicator plant species for naturalKnowledge of Local Fire HistoryOak Symposium: Sustaining Oak Forests in the 21st Century through Science-based Management127

because they suppress tree recruitment into theoverstory and as overstory tree mortality occurs thenstand density decreases.Management Goals and ObjectivesForestry organizations, agencies, and industriesusually develop a hierarchy of plans and prescriptionsto guide management at the forest level (e.g., MTNF2005, OSFNF 2005), project level (e.g., MTNF 2015),and stand level (e.g., Wisconsin DNR 2013). They oftenuse a synthesis of the above mentioned criteria insetting forest, management unit, and stand goals andobjectives. However, selection of areas and sites foroak woodland and savanna restoration also considersthe character of modern-day landscapes, priority needsfor wildlife habitat, landscape and stand resiliencegoals, forest health risks and threats, rural communitysustainability, and demands for goods and servicesfrom the public. Hence, sites may be designated forrestoration regardless of historic patterns, disturbances,and processes.DEVELOPING THE SILVICULTURALPRESCRIPTION – AVAILABLE PRACTICESIn the absence of fire, savannas and woodlandsbecome forests, and many landscapes have come tobe dominated by mature forests of similar composition.Forests today have two to three times more trees thanformer woodlands and savannas did in the early 1800s(e.g., Hanberry and others 2012, 2014b, 2014c). Themost common starting condition in areas designatedfor savanna or woodland restoration is the matureforest state. In mature oak forests in Eastern NorthAmerica there is an absence or lack of large oakadvance reproduction, and the ground flora is sparse,being dominated by woody species and shade-tolerantforbs, with low diversity. Light levels in the understoryare extremely low, often inhibiting survival and growthof all but the most shade-tolerant species (Dey 2014).This forest condition provides habitat that favors wildlifespecies associated with complex forest structure.In the absence of an invasive species problem (seebelow for further discussion), an initial silviculturalobjective is to reduce stand density. This can beachieved in a number of ways using timber harvesting,prescribed fire, mechanical cutting and girdling, orherbicide application, or a combination of thesepractices. There are several sources that can be usedto set a reasonable range in stand structure metrics thatdefine desired future conditions at intermediate stagesin restoration, or endpoint conditions that representthe beginning of the maintenance phase in sustainablenatural community management. In some areas, modelsof historic natural communities at specified historic timescan be used to guide setting of quantitative ranges intree density, size, stocking, and canopy cover (e.g.,table 1). Alternatively, matrices can be developed byexperts based on ecological principles and knowledgethat define key attributes of the range of naturalcommunities (e.g., table 2).Prescribed Fire Effects on VegetationSince fire was instrumental in creating and sustainingoak savannas and woodlands historically, it is naturalto think about reintroducing fire by prescribed burningto begin restoration. Prescribed fire is often conductedin winter and spring seasons, when it burns with lowintensity and severity. These types of fires are capable ofkilling or topkilling hardwood stems that are 4–6 inchesdiameter at breast height (dbh) and thereby begin theprocess of reducing stand density (Arthur and others2012, Dey and others 2017). The midstory canopy inmature forests can be diminished or eliminated by oneor more low-intensity dormant season fires (fig. 2) (Deyand Hartman 2005; Fan and Dey 2014; Hutchinson andothers 2005a; Knapp and others 2015, 2017). Removalof the hardwood midstory canopy increases light levelsto about 15 percent of full sunlight depending on thecomposition of the overstory, i.e., northern hardwoodvs. oak vs. pine, with light levels being highest underpine overstories (Lockhart and others 2000, Lorimerand others 1994, Motsinger and others 2010, OstromTable 1—Setting structural targets for oak woodlands and savannas using historicalconditionsa in the Missouri OzarksDensitytrees per a3323–44143522–442516–305138–60Open ed atural communitybBasal areaft2/acStockingpercentCanopy coverpercenta Estimates are based on models of witness trees from General Land Office surveys done in the early 1800s.b Natural communities were defined as: savanna—10 to 30% stocking, 20 to 40 tpa; open woodland—30 to55% stocking, 40 to 71 tpa; closed woodland—55 to 75% stocking, 71 to 101 tpa; forest— 75% stocking, 101 tpa (based on Hanberry and others 2014b).128Silviculture to Restore Oak Woodlands

Table 2—Quantitative attributes defining desired conditions for natural community types on the Mark TwainNational Forest, Missouri Ozarks (MTNF 2005)Overstory treesShrubsGround cover layerTreecanopypercentBasalareaft2/acShrub layerpercentGround organic layerGroundcoverpercent 10NA 10Scattered grasses, sedges, and forbs90–100Savanna20–4040–6050Scattered grasses, sedges, and forbs;60–80% leaf litter cover30–50Open woodland40–7040–7020–40Scattered grasses, sedges, and forbs;30–50% leaf litter cover30–40Closed woodland70–9080–1005–10Scattered sparse grasses, sedges,and forbs; 100% leaf litter cover20–30Upland forest90–10080–10050% in 2-acreopenings/wind gaps; 5 % elsewhereModerately deep leaf litter; sparseground coverNatural communitytypesPrairieBottomland forest 3090–10090–100Multi-layered; unevenage; few gapsDeep leaf litter; ephemeral herbs50–70Fen 10NAVariableShallow marly to deep muck90–100All glade types 20NA 40Sparse to dense thatch of grasses;mineral soil sometimes exposed30–90grassesdominantFigure 2—Low-intensity prescribed fires done in the spring (March to April) are effective in eliminating or significantlyreducing density of the hardwood midstory in mature oak forests in the Missouri Ozarks (A). Annual fires (B) or periodicfires (every 2–3 years) (C) over 10 years can topkill saplings that form the midstory and inhibit redevelopment of themidstory by causing mortality of understory stems and repeated topkilling of survivors. Overstory density is relativelyunaffected by these burning regimes, and growth of hardwood sprouts is reduced by shade of the closed-overstorycanopy. (photographs courtesy of Daniel C. Dey, USDA Forest Service)and Loewenstein 2006). This improves environmentalconditions for oak regeneration development andincreases ground flora diversity and coverage, but furtherincreases in light levels are often needed to achievedesired future conditions in both tree regeneration andground flora (Dey 2014, Hutchinson 2006, Kinkead andothers 2017). However, an incremental approach anddeliberate progress toward the final desired conditionmay be prudent when transitioning from a forestcondition with aggressive competing tree species suchas yellow-poplar (Liriodendron tulipifera L.), sweetbirch (Betula lenta L.), and aspen (Populus spp.), ordisturbance-adapted invasive species. Low-intensityfires have little effect on overstory mortality (Fan and Dey2014, Fry 2008, Horney and others 2002, Hutchinsonand others 2005a).Oak reproduction is generally favored over otherhardwood species in a regime of frequent fire (Broseand others 2013b). Oak seedling and sapling sproutscan grow and increase root mass with adequate lightunder a regime of frequent fires, i.e., every 2 to 5 years(Brose 2008, Brose and others 2013a, Dey and Parker1997, Rebbeck and others 2011). However, young andsmall-diameter oaks are vulnerable to mortality whensubjected to low-intensity, dormant season prescribedOak Symposium: Sustaining Oak Forests in the 21st Century through Science-based Management129

burns (Johnson 1974). Survival of oak advancereproduction is relatively high (80 to 90 percent) afteronly one fire regardless of species, but mortality amongoak species varies with repeated fires, and scarletoak is one of the most fire-sensitive species (Dey andHartman 2005). Differential mortality rates between oakand its competitors in a frequent fire regime give oaka competitive advantage over time (Brose and others2013b, Dey and Hartman 2005).Acorns are recalcitrant seed and must maintain highmoisture content to remain viable (Korstian 1927). Seedlying on the forest floor or mixed in litter is vulnerableto desiccation over the winter, especially in regionswithout permanent snow cover. Some leaf cover (e.g.,1 to 2 inches) helps to maintain moisture content inacorns located beneath the litter and in contact withmineral soil, but deep litter ( 2 inches) can reducegermination and seedling establishment (Barrett 1931).Prescribed fire is an effective tool for reducing leaf litterdepth but must be applied periodically (e.g., 4 years)to consume additional litter input and maintain optimallitter depth (Barrett 1931, Stambaugh and others 2006b).Fire conducted before acorn drop and in forests withlittle to no oak advance reproduction can reduce leaflitter, midstory density, understory height structure, andcanopy coverage, and begin to manage competitorseed in the forest seed bank (Schuler and others 2010).Repeated fire is important in controlling seed bankand young germinants of competing or undesirablespecies (Schuler and others 2010). Once a good acorncrop is on the ground, fire should be delayed until oakseedlings are well-established (e.g., for 2 to 3 years)because fire can kill 70 percent or more of the acorncrop (Auchmoody and Smith 1993, Greenberg andothers 2012). Sufficient light in the understory promotesoak seedling development and shortens the time thatfire should be avoided to minimize mortality in small oakadvance reproduction.In addition to using fire to restore and maintain thewoody structure of savanna ecosystems, the restorationof fire as a disturbance that shapes the ground floracommunity is important. Fire may increase ground floradiversity and promote its development by breakingchemical or thermal seed dormancy, thus increasinggermination in some species (Hutchinson 2006). Firemay improve establishment of herbaceous plants byreducing thick litter layers that act as a physical barrier toseedling establishment by inhibiting roots from reachingmineral soil, or emerging shoots growing beneath litterfrom reaching the light of day. Litter removal elevatessoil temperature that promotes germination and earlyseedling growth. Fire releases nutrients when litteris consumed, which promotes plant growth. It alsoincreases available light for herbaceous plants by topkilling woody trees, shrubs, and vines. Fire frequency,season, intensity and other attributes of the fire regime130Silviculture to Restore Oak Woodlandscan be set by the manager, and various combinations ofthese attributes can dramatically direct plant dominanceand community succession.The use of prescribed fire alone in mature forestsgenerally increases herbaceous species coverage,richness, and diversity, but improvements are smallin magnitude due to the shade from a closed-canopyoverstory (Lettow and others 2014; Ralston and Cook2013; Taft 2003, 2005). These improvements in theground flora are also ephemeral unless burning isrepeated to control the sprouting and regrowth oftrees and shrubs in the understory (Abrahamson andAbrahamson 1996, Bowles and others 2007, Glasgowand Matlack 2007, Kuddes-Fischer and Arthur 2002,Vander Yacht and others 2017). It may take 20 to40 years of annual burning to eliminate most of theunderstory woody species (Knapp and others 2015,Waldrop and others 1992). Each woody and herbaceousspecies is unique, but some generalities in howfunctional groups of plants respond to changes in thefire regime are worth noting. Annual fires increase grassdominance, and biennial fires promote forb speciesrichness and cover, especially in open environmentssuch as prairies, savannas, and open woodlands(Anderson and others 1999, Burton and others 2010,Haywood and others 2001, Nelson 2010, Peterson andothers 2007). Fires separated by 3 to 5 years or longerfavor trees, shrubs, and vines (Briggs and others 2002,Burton and others 2010, Haywood 2009, Peterson andothers 2007). Ground flora diversity is relatively lowerin prairies where tallgrasses can dominate or in forestswhere woody species are the major competitors thanin savannas and open woodlands where there is thegreatest heterogeneity in environmental conditions thatsupport high plant diversity (Haywood 2009, Petersonand others 2007, Peterson and Reich 2008, Towne andKemp 2003).Controlling the season of burning can help promotespecific plant functional groups. Burning in the spring(March to April) favors warm season grasses andforbs; may promote flowering and biomass productionof late summer flowering species; and diminishessurvival, growth, and vigor of cool season grassesand forbs (Copeland and others 2002, Glen-Lewinand others 1990, Howe 1994, Peterson and Reich2008, Taft 2003, Towne and Kemp 2003). Promotingthe dominance of warm season grasses may actuallydecrease total plant diversity due to the ability of warmseason grasses to suppress all subdominant vegetation(Biondini and others 1989, Copeland and others 2002).Compared to winter or spring fires, summer burns(mid-July to early August) can increase cool seasongrass and forb diversity, cover, and density; reduce toa greater extent woody trees, shrubs, and vines; andincrease perennials that are able to flower before thesummer burn (Haywood and others 2001, Haywood

2009, Howe 1994, Nelson 2010, Waldrop and others1992). Summer burns done during a time when warmseason grasses are actively growing can reduce theirdominance, thus releasing subdominant vegetation andincreasing total species richness (Biondini and others1989). The outcomes of fall (September to October)fires are somewhat inconsistent but show a tendencyto decrease cool season exotic grasses; reduce woodycover; increase perennial forb cover; and either increaseor have no effect on warm and cool season grasses andforbs (Biodini and others 1989, Bowles and others 2007,Copeland and others 2002, Howe 1994, Towne andKemp 2003, Weir and Scasta 2017). Dormant season(December to February) fires have the least impact onherbaceous or woody species, even when repeated fordecades (Haywood 2009, Hutchinson 2006, Waldropand others 1992, Weir and Scasta 2017). Consistencyin application of prescribed fire tends to createhomogeneity in the vegetation community. Thereforeit can be beneficial to vary the frequency, intensity andseason of burning to sustain plant species diversityand provide a variety of habitats for wildlife, insects,pollinators, and other taxa (Hiers and others 2000,Howe 1994, Nelson 2010, Peterson and Reich 2008).An important factor that governs the initial response ofground flora to burning is the abundance of propagulesin the seed and bud bank. These may be diminishedon sites degraded from decades of agricultural landuse or from years under heavy forest shade and deeplitter (Ralston and Cook 2013). Restoration of severelydegraded sites may require artificial regeneration of theground flora by seeding and planting (e.g., Packard andMutel 1997).Prescribed Fire, Woody Structure,and Ground Flora InteractionsHeavy tree canopy cover is a major limiting factorcontributing to low diversity and productivity of groundflora (i.e., grasses, forbs, and legumes) (Peterson andReich 2008, Ratajczak and others 2012, Vander Yachtand others 2017, Zenner and others 2006). Applicationof low-intensity dormant season fires is effectiveat creating closed-woodland structure (Hutchinsonand others 2005a, Knapp and others 2017, Waldropand others 1992), but other practices are needed tocreate open-woodland and savanna structure andpromote their ground flora. Increasing fire intensity toproduce moderate to high-severity fires is generallynot the preferred method for reducing overstorydensity. Thinning by herbicide or mechanical methodsor harvesting using a modified shelterwood systemis preferred for managing stand density, regulatingtree density in time and space, controlling speciescomposition, and receiving income from timber sales.The shelterwood system is flexible to accommodateincremental adjustments to tree density and stocking,can be used to achieve a wide range of final desiredtree density and stocking, and can be applied as anirregular or uniform pattern to create heterogeneity instructure. The application of the shelterwood methodin restoration of woodlands and savannas differs fromthe traditional forestry application in that the finalshelterwood is retained for the long term in restoration,

of forests and promote woodland and savanna communities as defined by Nelson (2010). Collectively, these site characteristics often increase the likelihood of droughts that limit tree development and fires that can create oak woodlands and savannas. Certain oak species such as post oak (Quercus stellata), white oak (Q. alba), bur oak (Q. macrocarpa

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