Reversing the decline of oak forests : are low-intensity treatments effective?

Altering successional trends in oak forests:19 year experimental results of low- andmoderate-intensity silvicultural treatments

Nicholas A. Povak, Craig G. Lorimer, and Raymond P. Guries

Abstract: Intensive silvicultural treatments can sometimes prevent the conversion of an oak (Quercus spp.) forest to a for-est composed of mesophytic competitors following harvest, but the required labor is a disincentive for many private land-owners. In this study, shelterwood removal, commercial clear-cutting, understory control, and oak underplanting wereconducted on mesic and dry–mesic sites in southwestern Wisconsin to evaluate the effect of these treatments on forestcomposition and to identify the least intensive combination needed for successful oak regeneration. Commercial clear-cut-ting, with or without prior herbicide spray of low vegetation and oak underplanting, resulted in nearly complete dominanceby a wide array of non-oak species on both mesic and dry–mesic sites. In contrast, 153–903 ha–1 of the oaks that wereunderplanted on shelterwood – understory removal plots successfully achieved dominant or codominant status by age 19.Control of tall understory saplings was essential for successful oak regeneration on both sites. On the mesic site, oakunderplanting was an additional necessary treatment, whereas natural regeneration was adequate in shelterwood plots onthe dry–mesic site. The study suggests that successful oak regeneration can be obtained on productive sites in this regionafter a single application of a moderately intense silvicultural treatment, although the effort required for understory controlmay still be an obstacle to widespread application without external incentives.

Resume : Les traitements sylvicoles intensifs appliques apres une coupe peuvent parfois prevenir la conversion de foretsde chene (Quercus spp.) en formation vegetale composee de competiteurs mesophytes, mais la quantite de travail requisdissuade plusieurs proprietaires de foret privee. Dans cette etude, la coupe progressive, la coupe a blanc commerciale, lamaıtrise de la vegetation de sous-bois et la plantation de chene sous couvert ont ete realisees sur des stations mesique etmesique-seche dans le sud-ouest du Wisconsin pour evaluer leurs effets sur la composition forestiere et pour determiner lacombinaison la moins intensive permettant de regenerer le chene avec succes. La coupe a blanc commerciale, avec ousans epandage prealable d’herbicide sur la vegetation du sous-bois, et la plantation de chene sous couvert ont produit uneregeneration presque entierement dominee par une large gamme d’especes autres que le chene sur les deux stations. A l’in-verse, dans la coupe progressive jumelee a l’elimination de la vegetation de sous-bois, de 153 a 903 chenes a l’hectareprovenant de la plantation sous couvert ont atteint le statut de dominant ou de co-dominant a l’age de 19 ans. La maıtrisedes gaules de forte taille sous le couvert etait essentielle au succes de la regeneration en chene sur les deux stations. Surla station mesique, la plantation de chene sous couvert constituait un traitement complementaire necessaire alors que la re-generation naturelle etait adequate dans la placette de coupe progressive de la station mesique-seche. Cette etude indiqueque le chene peut etre regenere avec succes sur les stations productives de cette region apres une application unique detraitements sylvicoles moderement intenses meme si l’effort demande par la maıtrise de la vegetation de sous-bois peut de-meurer un obstacle a une application generalisee en l’absence d’incitations externes.

[Traduit par la Redaction]

Introduction

The decline of oak (Quercus spp.) regeneration on pro-ductive sites has been a chronic problem in a number ofphysiographic regions of eastern North America during thelast 60 years (Beck and Hooper 1986; Johnson et al. 2002).

Limited establishment, slow growth, and high mortality ofoak seedlings can leave these forests with an inadequatesupply of future recruitment, and as a result, mesophytichardwood species often dominate the canopies followinglogging or natural disturbance in oak stands on mesic sites(Beck and Hooper 1986; Abrams and Nowacki 1992). Theresponse of oaks to canopy removal is regionally variableand is influenced by stand and site characteristics (Weigel1999; Morrissey et al. 2008), but some long-term studies onmesic and dry–mesic sites found few oaks in the dominant,codominant, and intermediate crown classes 20–50 yearsafter logging (e.g., Beck and Hooper 1986; Hix and Lorimer1991).

The success of silvicultural treatments in fostering oakdominance can likewise vary with the intensity of treat-ments, site factors, and geographic region. The shelterwoodmethod, in conjunction with oak planting and competition

Received 7 April 2008. Accepted 12 August 2008. Published onthe NRC Research Press Web site at cjfr.nrc.ca on 30 October2008.

N.A. Povak,1,2 C.G. Lorimer, and R.P. Guries. Department ofForest and Wildlife Ecology, University of Wisconsin-Madison,120 Russell Labs, 1630 Linden Drive, Madison, WI 53706,USA.

1Corresponding author (e-mail: [email protected]).2Present address: Wenatchee Forestry Sciences Laboratory, 1133N. Western Avenue, Wenatchee, WA 98801, USA.

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control, has given some of the best results in the first 5–20 years after treatment (Johnson 1984; Spetich et al.2002). The more intensive variations on this approach haveincluded artificial regeneration with large-caliper nurserystock, repeated control of competing vegetation, one ormore prescribed burns, and control of herbivory (e.g.,Zaczek et al. 1993; Spetich et al. 2002). However, evenintensive treatments are not always successful on mesicsites. The best results have been obtained in the drier por-tions of the oak range, where competing species are notnumerous or fast growing (Schlesinger et al. 1993; Spetichet al. 2002). Intensive treatments have often been less effec-tive on mesic sites in more humid regions, where oaks mustcompete with a variety of shade-tolerant species and aggres-sive, fast-growing competitors such as yellow poplar (Lirio-dendron tulipifera L.) or black cherry (Prunus serotinaEhrh.) (e.g., Loftis 1983; Schuler and Miller 1995).

While the shelterwood method with competition controlprovides one of the better prospects for securing oak regen-eration, the labor and costs of such treatments are disincen-tives for private landowners, who own most of the oakforests in eastern North America. Most private forests in theregion are harvested periodically, but loggers often removeonly the larger merchantable trees because of limited mar-kets for small trees, and silvicultural treatments are infre-quent (Kittredge et al. 2003). This practice leaves theunderstory largely intact, which inhibits oak regenerationand contributes to the conversion of oak stands to stands ofless valuable species (Hix and Lorimer 1991; Abrams andNowacki 1992). Harvesting only the larger trees has effectson forest composition and structure that are somewhat simi-lar to those of certain natural disturbances such as wind-storms and insect outbreaks (Fajvan and Wood 1996;Arevalo et al. 2000), but effects are likely to be differentfrom those of fire, which discriminates against small treesand commonly leaves larger trees intact (Hutchinson et al.2005; Blankenship and Arthur 2006). The effects of heavypartial cutting of overstory trees probably vary among re-gions, but this common practice has important implicationsfor future forest composition over large areas.

Developing lower-intensity treatments that would be morefeasible for small private landowners (e.g., oak under-planting with mechanical or chemical understory controlapplied only once in the first 20 years) might help maintainoak-dominated stands. This paper reports the results of along-term experiment designed to evaluate the effectivenessof several low- and moderate-intensity silvicultural treat-ments on productive soils in a humid climate. Specific ob-jectives were to (1) identify the least intensive combinationof treatments needed to successfully regenerate oak on pro-ductive sites, (2) compare treatment effects on oak regenera-tion and forest composition with the results of typicalcommercial harvests, and (3) identify some of the more im-portant biological and site factors contributing to the long-term success or failure of individual oak saplings.

The experiment was designed to compare the growth andsurvival of both natural and planted oak regeneration undera gradient of treatment intensities, ranging from an untreatedcontrol to an understory and overstory treatment with anunderplanting of oak seedlings. For the purposes of thispaper, we consider commercial clear-cutting (with or with-

out oak underplanting and herbicide treatment) to be a low-intensity treatment, and approaches that include two-cutshelterwood with removal of understory trees (in some casesalso with underplanting and herbicide spray application) tobe moderate-intensity treatments. Early responses of oakseedlings beneath the shelterwood were reported in Pubanzand Lorimer (1992) and Lorimer et al. (1994). In 1992 theoverstory was removed on all shelterwood and untreatedcontrol plots. Nineteen growing seasons following planting,the stands formed a closed canopy of small pole-sized trees,allowing us to evaluate the interim success of each treatmentin promoting oak dominance.

Methods

Site descriptionTwo mature oak forests of average and above-average

productivity were selected for the study in the unglaciatedregion of southwestern Wisconsin, where oak regenerationdifficulties are a common occurrence (Martin et al. 1992;Hix et al. 2005). The climate in this region is classified ashot continental under Bailey’s ecoregions (Bailey 1995),with mean monthly temperatures ranging from –6 to 22 8C.Precipitation is well distributed throughout the year andaverages 81 cm annually; the mean length of the growingseason is 150 days. Topographically, the area is a deeplydissected upland (cuesta) with broad, rolling ridges andsteep, narrow valleys (Martin 1965). A loess cap of variablethickness overlies residual soil weathered from dolomite andsandstone (Hole 1976).

Before European settlement, about two-thirds of southernWisconsin was oak savanna and prairie, with closed-canopyoak forests making up an additional 9% of the landscape(Curtis 1959). Fire was a major factor in shaping the distri-bution of vegetation types. Closed forests of sugar maple(Acer saccharum Marsh.) and other mesophytic species oc-cupied about 22% of the landscape, but were largely re-stricted to steep north slopes or isolated larger tracts whererivers or streams served as firebreaks (Finley 1976; Leitneret al. 1991). The two study areas are located east of theKickapoo River and north of the Wisconsin River, which inpresettlement times apparently hindered the spread of firesdriven by the prevailing west and southwest winds. Sur-veyors noted an abrupt transition from oak savanna to oakor maple forest east and north of these rivers (Finley 1976;Hix et al. 2005). Records of the General Land Office Surveyfrom 1843 to 1845 for township lines within 200 m of thetwo study areas indicate that the locally prevailing vegeta-tion type was Quercus alba – Acer saccharum – Tilia amer-icana forest. The broad rolling ridges, especially west of theKickapoo River, were described as ‘‘thinly timbered,’’ oftenwith ‘‘small scattering oaks’’ (Hix et al. 2005).

The first study site (hereafter referred to as the mesic site)is located on public land administered by the Wisconsin De-partment of Natural Resources in Crawford County(43807’11@N, 90852’39@W). It is situated low on a protectedeast-facing slope with an elevation of *240 m and a meangradient of 28%. The soils are Dubuque silt loam (OrthicGray Brown Luvisol in the Canadian system or TypicHapludalf in the US system), a well-drained loess soil over-lying clayey residuum from dolomitic limestone bedrock.

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Solum thickness ranges from 45 to 90 cm and depth to bed-rock is >100 cm. Site index for northern red oak (Quercusrubra L.) is 21.3 m. The site is classified as landscape unit3 in Hix et al. (2005) and is one of the most productivelandscape units observed in the region.

The second site (hereafter, the dry–mesic site) is locatedwithin the Kickapoo Valley Reserve in Vernon County(43838’26@N, 90834’21@W), approximately 85 km NNE ofthe mesic site. It is located high on the midslope andshoulder (elevation *320 m) of a steep WNW-facing slope.The soil is a Dorerton sandy loam (Orthic Gray BrownLuvisol in the Canadian system or Typic Hapludalf in theUS system), which is well-drained colluvium overlyinglimestone residuum. It is somewhat less productive than themesic site, with a site index for northern red oak of 18.9 m.Solum thickness ranges from 60 to 115 cm, and depth tobedrock is >90 cm. This site is of average productivity forthe region and classified as landscape unit 9 by Hix et al.(2005).

Prior to treatment, northern red oak was the dominantspecies, accounting for 30% of the total canopy basal areaat the mesic site and 62% at the dry–mesic site (canopyconsists of dominant, codominant, and intermediate crownclasses). At the mesic site, sugar maple, white oak(Quercus alba L.), and bitternut hickory (Carya cordiformis(Wangenh.) K. Koch.) were also canopy dominants, eachrepresenting *15% of canopy basal area, while at the dry–mesic site, sugar maple and white oak made up 21% and 4%of the canopy basal area, respectively. Density of canopyoaks averaged 142 ha–1 at the mesic site and 322 ha–1 at thedry–mesic site. Overstory oaks ranged in age from 100 to140 years. Forests were well stocked at both sites, rangingfrom 80% to 88% (McGill et al. 1991). Sugar maple ac-counted for 45% and eastern hophornbeam (Ostrya virgini-ana (Mill.) K. Koch.) 33% of the understory trees (5–15 cmDBH) at the mesic site. Eastern hophornbeam accounted for80% of the understory trees at the dry–mesic site (see Lor-imer et al. (1994) for a complete listing of species). Basedon a 1985 pretreatment inventory of seventy-two 15 m2

quadrats on each site, there were no oak seedlings (<75 cmtall) on the mesic site, and 2685 red oak seedlings�ha–1 onthe dry–mesic site.

Experimental design and preharvest measurementsA split-plot experimental design was established at each

site, with six main plots (38 m � 45 m) aligned along thecontour, each containing two nested subplots (10 m �22 m) where oak regeneration was monitored. Because ofspace constraints at the dry–mesic site, two main plots weresituated higher on the slope immediately above the others,and a blocking term was used to account for the potentialdifferences in site productivity. This resulted in a completelyrandomized design at the mesic site and a randomized blockdesign at the dry–mesic site. Treatments at the latter sitewere randomly assigned within three blocks of paired mainplots.

A shelterwood treatment was applied randomly to half ofthe main plots in April 1986, in which all understory treestaller than 1.5 m, and smaller canopy trees of non-oak spe-cies, were removed and treated with a stump herbicide(Tordon) to hinder resprouting. The shelterwood was

marked using residual crown cover criteria rather than a spe-cific target basal area; the goal was to create small spacesbetween overstory trees but no large gaps that could providean early stimulus to herbaceous and woody competitors. Inthese sites, thickets of Rubus spp. form in any sizable open-ings or in stands with low stocking, thereby likely hinderingthe development of natural oak regeneration. The shelter-wood treatment resulted in 80%–85% crown cover at bothsites and created open, park-like stands (Fig. 1), reducingbasal area from 23.2 to 14.2 m2�ha–1 at the mesic site andfrom 26.2 to 20.7 m2�ha–1 at the dry–mesic site. The threeremaining main plots at each site served as experimentalcontrols and their canopies were left completely intact.

A foliar spray of glyphosate was assigned randomly tohalf of the nested subplots in both shelterwood and controlplots and applied to all herbaceous and woodyvegetation <1.5 m tall (this was done in August 1985 to pre-pare the site for subsequent spring planting). Each subplotwas further divided into a 10 m � 10 m section where natu-ral oak regeneration was monitored annually, and a 12 m �10 m section in which 30 oak seedlings were planted at2.2 m � 2.4 m spacing (2500 seedlings�ha–1) in May 1986.One-year-old northern red oak seedlings grown from localseed were lifted from the Wilson State Forest Nursery inBoscobel, Wisconsin. Prior to planting, all seedlings were

Fig. 1. (A) One of the shelterwood main plots at the dry–mesic sitein 1986, in the first year after treatment. Note the dense understoryon the untreated control plot in the background. (B) Nineteen-yearold pole stand on the dry–mesic site in 2004, 12 years followingoverstory removal.

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undercut in the nursery to a root length of 20–25 cm, butwere not shoot pruned. Only the largest available seedlingswere used, although they would be considered relativelysmall by current nursery standards. Basal diameters of theplanting stock averaged (±SD) 5.9 ± 0.95 mm and meanshoot length was 34.6 ± 7.8 cm. Numbered aluminum tagswere used to permanently mark each planted seedling.Ninety seedlings were planted for each of the four treatmentcombinations at each site, for a total of 720 planted seed-lings in the experiment.

Oak regeneration was assessed annually in late summerfrom 1986 to 1990 while the shelterwood and intact forestcanopies were present. Planted and natural oak seedlingswere measured for total height, height growth, basal diame-ter, number of leaves, and number of first-order branches.Between 1988 and 1990, an annual census of oak competi-tors was included on shelterwood plots. All non-oak stemswere measured for height and extension growth on a ran-domly assigned 5 m � 10 m area within each natural regen-eration plot. In 1990 the percentage of branches browsed oneach planted oak seedling was also recorded, and each seed-ling was categorized as either being overtopped by neigh-boring competitors or growing free from major competition.

Postharvest measurementsIn the winter of 1991–1992, 6 years after planting, mature

canopy trees were removed from all main plots in a com-mercial harvest of merchantable trees. Since most of thesmall and nonmerchantable trees had already been removedon the shelterwood plots, these plots resembled a silvi-cultural clearcut following the 1992 harvest. In some areas,mainly on the original control plots, loggers left some scat-tered mature trees and smaller poles. Postharvest treatmentof residuals was not conducted because we were interestedin comparing the shelterwood treatment with typical com-mercial logging on private lands and in assessing the effectsof partial overstory retention on the success of oak regener-ation. Effects of residual trees on oak regeneration were in-cluded along with other variables not under directinvestigator control (e.g., height and vigor of oak seedlings,damage from harvest operations, slope position) and ana-lyzed by regression methods. The original control plots thatdid not receive the shelterwood treatment and which wereharvested in 1992 will hereafter be referred to as the com-mercial clearcut.

In the midsummer following harvest, surviving plantedoak seedlings were measured for height and assessed forseedling vigor and degree of herbivory. Browsing intensitywas recorded as low (<33% branches browsed), moderate(33%–50%), or high (>50%). Damage to planted oak seed-lings from harvest slash piles and skidder trails was also re-corded. Oak seedlings were assigned to one of three broadvigor categories based on seedling condition, number ofleaves, and current-year extension growth. Vigor categorieswere poor (heavy damage from harvest operations, frost, de-foliation, or browsing; few leaves, usually <10; and slowshoot growth, usually <8 cm), moderate (fair condition, usu-ally with 10–20 leaves and 8–13 cm of shoot growth), orhigh (good condition with little damage from harvest opera-tions, defoliation, or browsing; usually with >20 leavesand >13 cm of shoot growth).

In the summer of 2004, 12 years following overstory re-moval, all surviving planted and natural oaks in each subplotwere relocated and measured. Heights of oak saplings andpoles were measured using a fiberglass measuring pole(trees <8 m tall) or clinometer (trees >8 m tall) and diameterat breast height (137 cm; DBH) was recorded. Oaks werealso assigned a crown class rating following Smith et al.(1997), and the percentage of each oak crown exposed toskylight was visually estimated.

Stand structure, species composition of trees, and compet-itive status of oaks under each treatment were evaluated inseveral ways. Species composition and DBH of alltrees >137 cm tall were recorded on (1) the 5 m � 10 mplots used to evaluate competition on natural regenerationplots from 1988 to 1990 and (2) on a randomly assigned6 m � 10 m area located within each planted subplot.Heights of 6–10 randomly selected dominant and codomi-nant trees on each subplot were measured in 2004. Afterother measurements were completed, a random subsampleof dominant–codominant competitors and natural oaks fromshelterwood main plots (located outside the subplot bounda-ries) were felled in 2004 and examined for recent heightgrowth rates. Height growth measurements could not be ob-tained for natural oaks on the mesic site because few werepresent and none were in a dominant–codominant position.Total height of all felled trees was measured by first record-ing the angle of tree lean, and then once felled, the entirelength of the tree was measured using a measuring tape. Ex-tension growth of the terminal shoot was analyzed in the lab-oratory by measuring the length between visible bud-scalescars over the last 3–6 years, depending on the specimen.

Residual trees left after the 1992 harvest were analyzedseparately from the cohort of 19-year-old trees. Residualswere defined as trees older than the 1986 cohort that werenot removed in the 1992 harvest and included smaller ad-vanced regeneration (>2 cm DBH) as well as some poleand mature trees. In the shelterwood plots, where all com-peting understory trees had been removed in 1985, the 19-year-old cohort trees did not typically exceed 17–19 cmDBH, and so residual trees were classified as those >20 cmDBH in 2004. A 5-factor angle gauge was used at the centerof each planted row (total of 120 sampling points) to quan-tify residual tree basal area in 2004. The horizontal distancewas measured from each sample point to the center of eachtallied tree, and the species and DBH were recorded. Treecores were extracted at breast height and analyzed in thelaboratory using a Velmex measuring system to determinethe diameter of each tree following overstory removal in1992. An estimate of residual tree stocking following har-vest was then made using limiting distance theory, in whichtrees were included in the plot if their DBH was >2.14 timesthe distance (metres) from the sampling point (Husch et al.2003). This measure was considered to be a conservativeestimate of postharvest residual basal area, as it did not ac-count for residual tree mortality between 1992 and 2004.

At the dry–mesic site, estimated residual basal areas afterthe final 1992 harvest were low (<0.8 m2�ha–1 on all mainplots). At the mesic site, residual basal areas averaged4.2 m2�ha–1, ranging from 0.2 to 10.8 m2�ha–1 on differentsubplots. Residual trees were mostly pole-sized maples andhickories 15–30 cm DBH, but also included some oaks.

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Data analysisAnalysis of variance (ANOVA) was used to compare den-

sities of planted and natural oak saplings under each treat-ment and to test the null hypothesis of no differencesamong treatment means. The general linear model (GLM)procedure was used in SAS version 9.1.2 (SAS InstituteInc. 2004), with separate error terms for main plots and sub-plots as appropriate for the split-plot design. Data weretransformed when necessary based on the scatter plots ofthe residuals and non-normal response variables (Shapiro–Wilk’s test; P < 0.05). Square-root transformation was usedon 2004 natural oak densities, arcsine square-root transfor-mation on planted oak survival (percentage) and groundcover (percentage) in 1990, and natural log transformationfor planted oak growth rates in 1990. All reported mean val-ues were calculated using the original untransformed data.

To evaluate the current competitive status of each plantedoak, minimum criteria were defined in which a ‘‘successful’’oak in 2004 had to be taller than 80% of the mean canopyheight on the subplot (as in Johnson 1984; Spetich et al.2002), with at least 40% of its crown exposed to direct sky-light. The latter criterion was added to eliminate ‘‘weakintermediate’’ canopy trees, which have a very low chanceof remaining in the canopy of young even-aged stands(Ward and Stephens 1993). In subplots with residual trees,successful oaks also had to occupy a gap >200 m2 betweenresidual trees and be growing outside the drip line of all re-sidual trees.

Logistic regression analysis was used to estimate theprobability of planted oak success from biological and envi-ronmental factors prior to overstory removal and to identifyfactors that ultimately limited or contributed to regenerationsuccess in the postharvest stand (as in Johnson 1984; Spe-tich et al. 2002). A planted oak received a score of either 1(successful) or 0 (unsuccessful or dead) in 2004. Biologi-cally meaningful independent variables (oak size and vigor,competitor size and density, residual basal area, and topo-graphic site factors) were evaluated by backwards elimina-tion and retained in the equation if the P value was <0.05based on a �2 distribution (Hosmer and Lemeshow 2000).All models converged after less than eight iterations. TheHosmer–Lemeshow test was used to verify model predic-tions. In this test, a desirable P value for the overall modelis >0.05, indicating that the model predictions are not signif-icantly different from the observed data. Logistic regressionanalyses were conducted using PROC CATMOD in SAS.

Results

Treatment effects on survival and competitive status ofoaks

Untreated overstory followed by commercial clearcutThe fall of 1985, at the beginning of the experiment, was

an excellent seed year for oaks. By the following summer,mean density of natural oak seedlings beneath the untreatedcanopy ranged from 1467 to 2133 ha–1 on the two sites.However, natural seedling densities steadily declined in thefollowing years, with a net reduction by 1990 of 70% onthe dry–mesic site and 94% on the mesic site.

The fall of 1991 (a few months before the overstory re-

moval treatments) was also an excellent seed year for oaks,and many new seedlings became established by the follow-ing spring on the harvest site. However, attrition in subse-quent years was high. By 2004, 12 years after thecommercial clearcut of the original control plots, 283 naturaloaks�ha–1 of all crown classes remained on the dry–mesicsite, and only 50 natural oaks�ha–1 remained on the mesicsite (Fig. 2). All natural oaks on the mesic site were over-topped beneath the main canopy.

Oak seedlings underplanted beneath the undisturbed can-opy 6 years prior to the commercial clearcut also had highrates of attrition. More than 70% died in the first 5 years,and most of the remainder were either killed during the har-vest operation or died in the 12 years following the harvest(Table 1; Fig. 3). Cumulative 19 year survival of oaks thathad originally been planted beneath an undisturbed standand subsequently subjected to a commercial clearcut was3% or less on both sites (Table 1).

Overall, the subplots were well-stocked with trees of 19species at the end of the experiment, with densities rangingfrom 500 to 3900 canopy trees�ha–1 (dominant, codominant,and intermediate crown classes; Table 2). Oaks were only aminor component of the canopy of the commercial clearcutplots at both sites. At the dry–mesic site, only 17 ha–1 of thenatural oaks (all codominants) and 14 ha–1 of the plantedoaks (0.6% of the number originally planted) met the suc-cess criteria (Fig. 2; Table 1). At the mesic site, none of thenatural or planted oaks met the success criteria.

Twelve years after commercial clear-cutting, the dry–mesic site was heavily dominated by eastern hophornbeam,with smaller amounts of bitternut hickory, red elm (Ulmusrubra Muhl.), and sugar maple. The canopy at the mesicsite was dominated by sugar maple, eastern hophornbeam,and red elm (Table 2).

Shelterwood with removal of tall understory trees

Natural oak regenerationThe shelterwood and understory removal treatment had a

substantial beneficial effect on natural seedling densitieswithin 2 years after the start of the experiment. Five yearsafter shelterwood creation, natural seedling densities ex-ceeded 3000 ha–1 on both sites; these densities were signifi-cantly higher than those in the undisturbed control plots atthe mesic site (P < 0.03) and marginally significant at thedry–mesic site (P = 0.06).

There was considerable attrition of surviving natural oaksin the 12 years following removal of the shelterwood. Netreduction in natural oak regeneration was about 60% on thedry–mesic site, but the mean density of surviving oaks at theend of the study was still 1700 ha–1, which is about sixtimes higher than that on the clearcut plots. Natural oaksfared poorly on the mesic site, with a net reduction of 97%and a mean density at the end of the study of 83 ha–1 in allcrown classes (Fig. 2). Because natural oak densities variedconsiderably among shelterwood subplots (0–6700 ha–1),means were not significantly higher than those of clearcutplots at either site (P > 0.21).

At the end of the experiment, sizes of natural oaks on thedry–mesic plots that had received the shelterwood treatmentwere somewhat skewed toward the smaller diameter and

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height classes compared with other species present (Table 3;Fig. 4). Nevertheless, there were 650 dominant–codominantoaks of natural origin per hectare, and 450 ha–1 met the suc-cess criteria (Fig. 2). Natural oaks made up 15% of the can-opy layer, with eastern hophornbeam, red elm, and blackcherry making up much of the remainder. The ratio of themean diameter of dominant–codominant natural oaks to themean diameter of other dominant–codominant trees was 1.1,suggesting that a subpopulation of oaks was in a good com-petitive position.

Recent height increment measurements for natural red oakat the dry–mesic site suggest that the competitive position ofthese oaks may be improving over time. A subsample of 13natural red oaks felled in 2004 outside of the monitored sub-plots had a mean annual extension growth of 69 cm�year–1

in the past 3–6 years, a rather fast rate of height growththat was 18% higher than the mean of the seven speciessampled (Table 3). Oaks in the intermediate crown classwere 31% shorter than dominant–codominant oaks, but their

rate of height growth (mean of 68 cm�year–1) was similar tothat of the dominant–codominant trees, suggesting that treesin the intermediate class may still be competitive.

Performance of natural oaks was much poorer on the me-sic site. The few natural oaks that remained by the end ofthe study were generally smaller trees in understory or sub-canopy positions, and natural oaks made up only 2% of thecanopy on this site. Only one oak of natural origin (17 ha–1)satisfied the success criteria, and the shelterwood treatmentat the mesic site did not significantly increase natural oakdensities above levels on the commercial clearcut (F[1,4] =0.03, P = 0.87).

Planted oaksStronger and more persistent treatment effects were found

for planted oaks. In the first 5 years beneath the shelter-wood, planted seedling mortality was only 6% on both sites,compared with >70% on undisturbed control plots (F[1,2] =11.79, P = 0.08 on the dry–mesic site; F[1,4] = 60.39,

Fig. 2. Density of surviving and successful oaks (planted and natural) at each site in 2004 under each treatment combination. Abbreviations:C, commercial clearcut; C + H, commercial clearcut with herbicide spray treatment of low vegetation; S, shelterwood – understory removaltreatment only; S + H, shelterwood – understory removal and herbicide spray treatment.

Table 1. History of mortality, survival, and canopy recruitment for planted oaks on shelterwood and commercial clearcut plots,1986–2004.

Planted oak mortality and success (%)

Shelterwood Untreated control – clearcut

Dry–mesic Mesic Dry–mesic MesicDied beneath mature canopy or shelterwood (1986–1990) 6.1 6.1 71.1 73.1Harvest-related mortality, 1992 5.5 14.9 3.3 9.3Other unknown mortality, 1992 1.1 7.2 6.0 9.3Died 1992–2004 among cohort competitors 33.9 32.1 17.8 1.2Died 1992–2004 beneath residual trees 0.0 9.4 0.0 3.8Alive in 2004, suppressed beneath residual trees (1992–2004) 0.0 5.5 0.0 3.3Alive in 2004, suppressed by cohort competitors 3.9 6.6 0.6 0.0Alive in 2004, suppressed by other oaks 5.0 5.0 0.0 0.0Alive in 2004, unsuccessful canopy trees* 7.8 (194 ha–1) 4.4 (111 ha–1) 0.6 (14 ha–1) 0 (0 ha–1)Planted oaks successful in 2004 36.7 (917 ha–1) 8.8 (222 ha–1) 0.6 (14 ha–1) 0 (0 ha–1)Total (%) 100.0 100.0 100.0 100.0Total surviving oaks (ha–1) 1333 764 42 83Total codominant–dominant oaks (ha–1) 903 153 14 0

*‘‘Unsuccessful canopy trees’’ are trees in the intermediate and codominant crown class that did not meet minimum relative height and crownexposure success criteria.

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P = 0.002 on the mesic site). However, competing non-oakspecies rebounded quickly following the understory removaltreatment. Even 3 to 5 years after planting, before shelter-wood overstory removal, competitors were already morethan twice as tall as planted oak seedlings and were growing

two to three times as fast. Competing species at that timewere also about six times as tall as natural oak seedlingsand were growing four to five times as fast (Table 4).

As with natural seedlings, substantial attrition of plantedoaks occurred during and after shelterwood removal. Some

Table 2. Species composition (percentage) of canopy trees at the two study sites for combinedplanted and natural sub-subplots, 12 years after overstory removal.

Dry–mesic site Mesic site

Species ShelterwoodCommercialclearcut Shelterwood

Commercialclearcut

Quercus rubra 23.2* (39.6{) 1.9 (5.7) 11.0 (11.8) 0.0 (0.0)Ostrya virginiana 32.5 (u{) 58.4 (u) 6.1 (u) 14.7 (u)Acer saccharum 6.1 (10.4) 5.0 (15.0) 6.1 (6.6) 35.3 (41.4)Ulmus rubra 8.2 (14.0) 7.3 (21.9) 31.7 (34.2) 8.8 (10.3)Prunus serotina 6.8 (11.6) 2.3 (6.9) 19.5 (21.1) 8.8 (10.3)Fraxinus americana 3.6 (6.1) 3.1 (9.2) 8.6 (9.2) 3.0 (3.5)Carya cordiformis 4.3 (7.3) 6.5 (19.6) 0.0 (0.0) 3.0 (3.5)Ulmus americana 2.1 (3.7) 1.5 (4.6) 7.3 (7.9) 5.9 (6.9)Rhus hirta 3.2 (u) 5.8 (u) 1.2 (u) 0.0 (u)Acer negundo 0.7 (1.2) 1.2 (3.4) 4.9 (5.3) 2.9 (3.5)Juglans cinerea 1.1 (1.8) 1.2 (3.4) 2.4 (2.6) 2.9 (3.4)Juglans nigra 0.0 (0.0) 0.0 (0.0) 1.2 (1.3) 5.9 (6.9)Carpinus caroliniana 5.7 (u) 0.8 (u) 0.0 (u) 0.0 (u)Populus spp. 0.0 (0.0) 0.8 (2.4) 0.0 (0.0) 5.9 (6.9)Tilia americana 1.1 (1.9) 1.2 (3.4) 0.0 (0.0) 0.0 (0.0)Fraxinus pennsylvanica 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 2.9 (3.4)Betula papyrifera 0.3 (0.6) 1.1 (3.4) 0.0 (0.0) 0.0 (0.0)Acer rubrum 1.1 (1.8) 0.4 (1.1) 0.0 (0.0) 0.0 (0.0)Cornus alternifolia 0.0 (u) 1.5 (u) 0.0 (u) 0.0 (u)Total 100 (100) 100 (100) 100 (100) 100 (100)No. of sample trees 280 (164) 260 (87) 82 (76) 34 (29)No. canopy trees (ha–1) 4242 (2484) 3939 (1318) 1242 (1151) 515 (440)

Note: Canopy trees defined as trees in the dominant, codominant, and intermediate crown classes. Residualtrees not included.*On natural subplots only, red oaks made up 15% of the canopy layer in the shelterwood treatment on the dry–

mesic site and 2% on the mesic site. Planted oaks on planted subplots accounted for 22% of the canopy at the dry–mesic site and 17% at the mesic site.

{Values in parentheses represent the percent composition in the absence of shrubby understory species that arenot likely to be important canopy species in the future mature forest.

{‘‘u’’ indicates understory species.

Fig. 3. Survivorship curves for planted oaks by treatment. The overstory was removed on all plots in 1992.

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planted seedlings were torn up by skidders or buried underslash piles from the harvest operation. At the dry–mesicsite, 140 planted oaks�ha–1 died as a result of the harvest,and an additional *600 ha–1 were injured (seedlings bentover by slash or top broken off). At the mesic site, harvest-related mortality was nearly three times as high as that onthe dry–mesic site, possibly because of greater difficultiesin logging and skidding on the more uneven terrain. Har-vest-related losses on the mesic site were as high as 30%–40% on two of the six shelterwood subplots and <10% onthe remaining four subplots, leading to high within-treatmentvariability in oak survival at this site.

In the 12 years following shelterwood removal, more than40% of the original planted oaks on both sites became sup-pressed or died among cohort competitors. By the end of theexperiment, an additional 15% of the planted oaks at themesic site were either overtopped by larger residual trees orhad recently died beneath them (Table 1).

In spite of these losses and competition with other spe-cies, >700 planted oaks�ha–1 survived to age 19 on bothsites, which is significantly greater than the <100 ha–1 onthe commercial clearcut plots (P < 0.04; Table 5). Plantedoaks fared well on plots that had received the shelterwoodtreatment, especially at the dry–mesic site. Planted oaksalone made up about 22% of the main canopy of plantedsubplots at that site, and the relative diameter of codomi-nant–dominant oaks was 1.2, indicating that oaks were, onaverage, slightly larger than other dominant–codominanttrees. More than 900 planted oaks�ha–1 met the success crite-ria, which is significantly higher than the value obtained onthe commercial clearcut plots (F[1,2] = 46.52, P = 0.02).

At the mesic site, mean canopy height was nearly 3 mgreater than that at the drier site. Oaks were less dense andgenerally occupied the lower and middle diameter classes.Planted oaks at the mesic site made up about 17% of thecanopy. The relative diameter of canopy oaks was 0.7 (0.5if residuals trees were included), indicating that they weregenerally smaller than other competitors in the canopy layer.However, a mean of 222 oaks�ha–1 met the success criteria.Although this was not significantly higher than the value onthe clearcut plots (F[1,4] = 1.51, P = 0.29), not a singleplanted oak in the commercial clearcut was in a dominant–codominant position or met the success criteria.

Nineteen years after the shelterwood treatment and12 years after shelterwood removal, the overall canopy layerat the dry–mesic site was dominated by eastern hophorn-beam and was similar to that on the clearcut plots exceptfor the substantial admixture of oaks. The canopy on themesic site was dominated by red elm and black cherry,along with a substantial admixture of red oak and white ash(Fraxinus americana L.). The nominal plot tallies suggestthat the shelterwood and stump herbicide treatment greatlyreduced eastern hophornbeam dominance on the dry–mesicsite and sugar maple dominance on the mesic site (Table 2),but neither difference was statistically significant (t test; P =0.28, t = –1.14, and P = 0.45, t = 0.78, respectively).

Herbicide control of low vegetationSpraying of woody and herbaceous vegetation <1.5 m tall

in the first year of the experiment significantly reduced theoverall ground cover percentage and the density of woodysaplings, a trend that persisted for several years. By year 5of the experiment, there were no longer differences inground cover percentage on herbicide spray plots at the me-sic site, but differences persisted at the dry–mesic site (P <0.05).

The herbicide spray treatment had a somewhat longerlasting effect on woody competitors. After five growing sea-sons, woody competitors were only one-fourth to one-half asdense on sprayed relative to unsprayed subplots (P < 0.03),but there was no significant difference in competitor height(P > 0.73). By 2004 there was no longer a difference incompetitor density (dry–mesic site, F[1,2] = 8.19, P = 0.10;mesic site F[1,4] = 0.94, P = 0.39) or competitor heights inthe canopy (dry–mesic site, F[1,2] = 0.06, P = 0.83; mesicsite, F[1,4] = 1.52, P = 0.29).

The herbicide spray treatment increased natural oak seed-ling abundance at the mesic site early in the study (P < 0.03in year 5), with highest densities occurring on subplots thatreceived both the shelterwood and spray treatments. Differ-ences in that year were not significant at the dry–mesic site(P = 0.65). By the end of the study, effects of herbicidespray on natural oak densities were negligible at both sites(dry–mesic site, F[1,4] = 1.43, P = 0.30; mesic site, F[1,4] =1.97, P = 0.23).

The spray treatment also had little detectable effect on

Table 3. Total heights and recent height growth rates (± standard deviation) of canopy trees on shelterwood plots at the twostudy sites 12 years after overstory removal.


SpeciesTotalheight (m) n

Height growth(cm�year–1)* n

Totalheight (m) n

Height growth(cm�year–1)* n

Quercus rubra (planted) 8.1±1.5 82 nd{ 0 9.2±1.2 26 nd 0Quercus rubra (natural) 5.8±2.1 61 69.4±19.3 13 7.1±1.3 3 nd 0Acer saccharum 7.1±1.2 15 63.3±12.4 14 9.8±3.2 22 56.9±14.8 15Carya cordiformis 8.5±1.2 14 62.8±10.0 12 13.6±6.2 4 nd 0Fraxinus americana 6.3±1.3 11 51.8±12.6 9 9.2±1.3 6 67.3±11.2 5Ostrya virginiana 6.9±1.1 38 39.9±9.5 15 9.3±2.5 12 45.6±11.7 5Prunus serotina 7.6±1.5 15 60.0±14.6 16 10.9±1.6 22 60.6±10.3 14Ulmus rubra 7.9±0.9 14 63.7±14.9 13 9.3±1.8 23 73.2±15.9 15Average 7.2±1.8 250 58.6±16.3 92 9.7±2.4 118 62.3±15.6 54

*Mean annual extension growth rates for terminal shoots, determined from bud-scale scars on sample trees felled in 2004.{‘‘nd’’ indicates that no height growth data were available (trees not felled) for that species and location.

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planted oak seedlings. In year 5 there was no significant dif-ference in survival (P > 0.70) or height growth (P > 0.08) ofplanted oak seedlings between herbicide spray treatments ateither site. By the end of the experiment, planted oak den-sities were nearly equal on herbicide spray and untreatedsubplots at both sites (dry–mesic site, F[1,4] = 0.13, P =0.74; mesic site, F[1,4] = 0.03, P = 0.88; Fig. 3), indicating

that the herbicide spray treatment had no long-term benefitfor planted oak survival or growth.

Predicting planted oak success under the shelterwoodtreatment

Planted oak seedling size prior to overstory removal wasthe most influential factor related to future success at both

Fig. 4. Diameter distributions of planted and natural oak regeneration 12 years following overstory removal on shelterwood plots only. Theinset graphs are an enlarged view of the oak distribution. ‘‘Canopy competitors’’ include all trees in the dominant, codominant, and inter-mediate crown classes, as well as residual trees (>20 cm DBH).

Table 4. Mean total height and previous year’s height growth of oak seedlings and competitor species beneath theshelterwood overstory, 1988–1990.


Year Height (cm)Height growth(cm�year–1) n Height (cm)

Height growth(cm�year–1) n

1988 Competitors 97.7±14.5 18.0±5.6 136 115.8±27.6 25.8±11.0 226Natural oaks 16.8±3.7 5.0±1.7 207 17.1±1.0 4.6±0.7 325Planted oaks 39.9±4.3 7.2±1.8 173 51.0±2.7 9.4±1.2 176



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sites. At the dry–mesic site, 1990 seedling height and slopeposition were the most significant predictors, with the prob-ability of success increasing with greater sapling height anddistance up the slope (Table 6, eq. 1; Fig. 5A). For a 40 cmseedling at a midslope position (50% of slope length), theprobability of still being successful at age 19 was 29%;however, a seedling of this size located higher on the slope(95% of slope length) had a 52% probability of success(Fig. 5A). At the mesic site, a seedling’s height in relationto that of its competitors and its basal diameter prior tooverstory removal were the best predictors of seedlingsuccess (Table 6, eq. 2; Fig. 5B). Several variables relatingto the vigor of the planted oaks, including height growthand number of leaves and branches, were significant at themesic site in bivariate logistic regression (Table 7). At thedry–mesic site, few variables were found to be significant;as long as a seedling was reasonably tall at the time ofoverstory removal (i.e., >60 cm), it had a fairly high prob-ability of success.

Evidence of browsing by deer (Odocoileus virginianus)was noted throughout the experiment, but browse class wasnot significantly correlated with planted oak success at ei-ther site (P = 0.09 for the mesic site and P = 0.90 for thedry–mesic site). Prior to overstory removal, 79% of theseedlings at the dry–mesic site and 90% of the seedlings atthe mesic site had no evidence of browsing or had sufferedrelatively light browsing (<33% of branches browsed at thetime of midsummer census). Only about 5% of the seedlingsat the mesic site and 8% at the dry–mesic site had sufferedheavy browsing (>50% of branches browsed). Followingharvest, deer browsing increased slightly in both incidenceand intensity, but browse class still had no significant rela-tion to oak success (Table 7).

Injury to seedlings during overstory removal had variableeffects on oak seedling success. Following the harvest, seed-ling damage associated with slash piles was a significant

negative factor at both sites (Table 6, eq. 4; Table 7). Thehigher postharvest basal areas on some subplots at the mesicsite also had a negative influence on oak success (Table 7;Fig. 5D). At the mesic site, residual basal area, harvest dam-age, and oak height were all significant variables in forecast-ing success of individual seedlings (Table 6; Fig. 5D) andcorrectly predicted 40% of the successful oaks.

Damage from slash appeared to substantially reduce theprobability that a tree would be successful at age 19. Forexample, the probability of success for a 120 cm seedlinggrowing beneath 2 m2�ha–1 of residual overstory basal areafell from 47% to 8% when injured by slash (Table 6, eq. 4).At the dry–mesic site, landscape position and height follow-ing overstory removal were still the most important factorsinfluencing oak success and correctly predicted 47% of thesuccessful oaks (Table 6; Fig. 5C).

Discussion

Successional trajectories under low-intensitymanagement

Oak regeneration problems after both commercial andsilvicultural clearcuts have often been reported, although inretrospective studies, stand and seed crop conditions beforeand immediately after the harvest are usually not known(e.g., Abrams and Nowacki 1992; Martin et al. 1992). Thisexperiment indicates that oak regeneration difficulties instands of the Lower Great Lakes region can occur evenwith abundant natural oak seedlings in the preharvest standand after heavy overstory removal in a year following abumper acorn crop, as has also been reported in oak forestsof the southeastern United States (Beck and Hooper 1986;Loftis 1990a). Regeneration failures after clear-cutting inthe present case occurred even though there were no com-petitors capable of unusually rapid height growth (like Lirio-dendron in the southern United States or Populus in regionsnorth of the study area). The experiment also illustrates thedifficulty in stimulating natural oak seedlings on mesic sitesto reach a competitive size, even over a span of 6 years in ashelterwood environment in which all woody competitorsand much of the herbaceous vegetation had been removed(see also Loftis 1983; Schuler and Miller 1995).

Some caution is necessary in interpreting levels of oak de-cline in the first few decades after clear-cutting. In somecases, oaks may initially form only a small percentage of re-generating stems but progressively increase their share ofcanopy space in the pole and mature stages because of lowmortality and fast growth (Oliver 1978), especially wherestump sprouting is prevalent (Heiligmann et al. 1985; Mor-rissey et al. 2008). Therefore, interpretations are best basedon absolute rather than relative numbers of oaks, and moreweight should be placed on evidence from closed-canopystands that have reached the pole stage. In the present study,current densities of canopy oaks in the commercial clearcutplots are too low (0–50 ha–1 or 0%–15% of the preharvestoak density) to form more than a minor component of thefuture mature forest even if they all survive, and the19 year trend has been a substantial decrease in bothplanted and natural oaks. Of the 50 canopy oaks�ha–1 onthe dry–mesic clearcut plots, only 17 ha–1 were codominanttrees; the remainder were classified in the field as ‘‘weak

Table 5. Analysis of variance of treatment effects onthe arcsine square-root proportion of surviving plantedoaks from 1986 to 2004.

Source df MS F P

Mesic siteMain plot

S 1 0.7193 9.53 0.0367Error 4 0.0755

SubplotH 1 0.0049 0.29 0.6213S � H 1 0.0357 2.08 0.2227Error 4 0.0172

Dry–mesic siteMain plot

S 1 1.6647 59.63 0.0164B 2 0.0503 1.80 0.3573Error (S x B) 2 0.0279

SubplotH 1 0.0000 0.00 0.9541S � H 1 0.0019 1.57 0.2789Error 4 0.0012

Note: S, shelterwood – stump herbicide treatment; H, her-bicide spray treatment; B, blocking term.

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intermediates’’. Similar findings have been reported else-where by Beck and Hooper (1986) and Abrams and Now-acki (1992).

The vegetation response on these sites 12 years after com-mercial clear-cutting is a canopy dominated by easternhophornbeam with a variety of other midtolerant and shade-tolerant hardwoods, similar to what was present in theunderstory of the previous oak stands (Lorimer et al. 1994).Dominance by eastern hophornbeam is likely to be short-lived, however, because of its slower growth rate (Table 3).Dominant–codominant trees of taller-statured species on thepresent study areas are sufficiently dense (303–1000 ha–1)that they will probably be able to overtop most or all of thecurrently abundant Ostrya and other shrubby species andform a closed canopy at maturity. The mature canopy on un-managed sites is therefore likely to be a mixture dominatedby sugar maple, bitternut hickory, black cherry, red elm,white ash, and basswood (Tilia americana L.). On currentlandscapes in the region, these species are often intermixedwith oaks in the canopy of mature forests (and alsodominate the understory) on 10 ecological landscape unitswith north- and east-facing slopes (Hix et al. 2005). Standsthat were clear-cut 20 to 50 years ago, even those on south-facing slopes, are now often heavily dominated by maplesand other mesophytic species (Hix and Lorimer 1991).Although oaks are likely to decline in future mesic anddry–mesic forests in this area under the prevailing naturaldisturbance and management regime, they may persist as amajor component on some of the drier habitat types withsandier soils (Kotar and Burger 1996).

Some early studies reporting a paucity of oak advance

regeneration in mature stands noted that oak saplings mightdevelop once the forest ages and canopy gaps begin to form(e.g., Peet and Loucks 1977). However, it now appears un-likely that gap dynamics or natural disturbances, which inthis region are caused primarily by windstorms, ice storms,and mortality from native and exotic pests, will substantiallyalter the successional trajectories described above. Old-growth oak stands on mesic sites are usually deficient invigorous advance regeneration of oak, and openings createdby overstory gap dynamics are captured quickly by tall ad-vance regeneration of non-oak species (Abrams and Copen-heaver 1999; Taylor and Lorimer 2003). The successionaltrajectory on most landscape units in the Kickapoo Valleyappears to be toward a northern hardwood (sugar maple do-minated) forest with an intermingling of species (Caryaspp., Ulmus rubra, Juglans spp.) from the central hardwoodregion (Hix and Lorimer 1991; Hix et al. 2005). Relativedominance of individual species, however, will likely shiftover time as a result of differential height growth, responseto crowding, and impacts of exotic insects and diseases suchas gypsy moth (Lymantria dispar), Dutch elm disease(caused by Ophiostoma ulmi), and emerald ash borer(Agrilus planipennis).

Least intensive treatments required for oak regenerationIn spite of the difficulties with oak regeneration under the

prevailing logging practices, the results of this experimentindicate that it is feasible to regenerate stands with a sub-stantial component of oak (150–900 ha–1) on mesic as wellas dry–mesic sites in this region following a single applica-tion of moderately intense silvicultural treatments. The rapid

Table 6. Coefficient estimates (B) for logistic regression models used to predict the probability of success forplanted oaks after 19 years, based on independent variables measured in year 5 (before overstory removal) andin year 7 (after overstory removal).

1990, before overstory removal 1992, after overstory removal

Variable entered Dry–mesic (eq. 1) Mesic (eq. 2) Dry–mesic (eq. 3) Mesic (eq. 4)X0: constant –4.0831** –7.6268*** –7.1761*** –6.6241***X1: sqrt(1990 height) 0.2762*X2: arcsin(sqrt(slope position))* 1.8058* 2.0723**X3: 1990 height (cm)X4: 1990 basal diameter (mm) 0.2133*X5: 1990 oak relative height{ 4.8968***X6: 1992 oak height (cm) 0.0675***X7: ln(1992 oak height) 1.2177**X8: sqrt(1992 residual BA) –1.1193*X9: 1992 harvest damage –2.3974*

ModelHosmer–Lemeshow (p value){ 0.446 0.616 0.427 0.975McFadden’s rho-squared§ 0.036 0.209 0.063 0.393Sample size 168 170 168 154% correct predicted responses 42.2 26.5 47.1 39.3

Note: Models are of the form: P = [1+exp(–(B0 + B1X1 +. . . + BnXn))]–1. Significance level of B: *, P < 0.05; **, P < 0.01;

***, P < 0.001.*Slope position is the distance of the planted oak from the bottom of the slope relative to the total slope length.{Oak relative height in 1990 was calculated as the planted oak height divided by the mean height of all competitors (>90 cm

tall) at each planted sub-subplot.{Hosmer–Lemeshow statistic has a �2 distribution with 8 df. P values >0.05 indicate model predictions not significantly

different from observed data.§Rho-squared is a transformation of the log-likelihood ratio statistic and is analogous to R2 in ordinary least-squares

regression.

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height growth of oak relative to that of competing species inrecent years at the study site (Table 3) suggests that the sta-tus of oaks will be maintained or possibly improve in thesestands in the near future.

Among the treatments investigated in this study, controlof tall understory trees prior to final harvest was essentialon both sites for oak regeneration success at age 19, andplanting was necessary on the mesic site. These long-termresults support the hypothesis that an important factor inoak regeneration failures following canopy disturbance iscompetitive inequities between existing tall advance regen-eration of other species and the smaller natural or plantedoak seedlings (Loftis 1983; Lorimer et al. 1994). The shel-terwood – understory removal treatment reduces these com-petitive inequities not only by removing competitors, butalso by stimulating growth of oaks, by providing sufficienttime for oaks to develop a stronger root system, and by re-taining sufficient canopy cover to prevent rapid growth ofcompetitors (Loftis 1990a; Johnson et al. 2002).

Loftis (1990a) has noted that the shelterwood methodmay stimulate the growth of oak seedlings already present,but it does not necessarily promote the establishment ofnew oak seedlings in stands lacking them. In the presentstudy, the survival and establishment of oak seedlings were

greater in the shelterwood cut than in the control plots, pos-sibly because the shelterwood cut took place during a goodseed year. However, the growth of natural oak seedlings wasvery slow compared with that of seedlings of competingspecies. Natural oak seedlings on both sites averaged only*15% of the mean height of sapling competitors prior tooverstory removal. The success of natural regeneration onthe dry–mesic site, in spite of these odds, appears to be re-lated to greater variation in growth rates and vigor. Prior tocanopy removal, 450 natural seedlings�ha–1 on the dry–mesicsite had more than 17 leaves per seedling, compared withonly 98 ha–1 on the mesic site. Canopy closure and crownclass differentiation also occurred at a slower pace on thedry–mesic site, probably allowing oaks more time to de-velop a more extensive root system.

The success of natural regeneration on the dry–mesic siteimplies that less time and expense will be required for silvi-cultural treatments, but the understory removal treatment isstill sufficiently labor intensive that it is likely to remain adisincentive for many private landowners. In principle, pre-scribed burning might be able to control understory vegeta-tion in a more cost-effective manner, but multiple burnsover a period of years may still be required, and prescribedburns are usually not very effective at causing top-kill of

Fig. 5. Estimated probabilities of planted oak success based on logistic regression analysis of conditions prior to overstory removal in 1990(A, B) and following overstory removal in 1992 (C, D). See Table 6 for corresponding equations. In Fig. 5B, the response surface shown iscalculated using the observed mean basal diameter of 11 mm for oak seedlings prior to overstory removal.

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large subcanopy trees (Hutchinson et al. 2005; Blankenshipand Arthur 2006).

The shelterwood treatment in this experiment included astump herbicide application to hinder resprouting but didnot include a separate treatment of shelterwood with nostump treatment. Two case studies in the region suggestthat shelterwoods without stump herbicide treatment cansucceed in regenerating oaks on good sites if understorycompetitors are cut (Johnson et al. 1989; Martin and Hix1988); this lower-cost approach may be worth further inves-tigation. In drier environments in Missouri, Johnson (1992)suggested that herbicides may not be necessary or cost ef-fective in reaching oak regeneration goals.

Underplanting of oaks by small landowners is often per-formed at rather low densities, far less than the 2500 ha–1 inthe present study. If we assume a 20 year success rate of35% for planted oaks (as was observed on the dry–mesicsite and probably obtainable on mesic sites with alterationsin harvest design), about 715 seedlings�ha–1 would need tobe planted under a shelterwood overstory to achieve250 dominant–codominant oaks�ha–1 at age 20. A low plant-ing density such as 60 ha–1 would result in oaks being a sig-nificant but minor component of the stand (21 ha–1),although the success rate might be improved by using larger

nursery stock and conducting a release treatment (Dey et al.2008).

Among the eight combinations of treatments at the sub-subplot level in this experiment, one of the least intensivetreatments involved oak underplanting beneath an otherwiseundisturbed mature forest several years prior to harvest.While intended primarily as an experimental comparison forthe shelterwood–underplanting subplots, underplanting vigo-rous oak nursery stock prior to harvest could, in principle,be a relatively low-cost means of compensating for thepaucity of vigorous natural oak seedlings in most maturestands. But, mortality of planted oak seedlings under theclosed-canopy forests was so high, and vigor of survivorswas so poor (see also Schuler and Miller 1995), that thistreatment cannot be recommended as an effective means ofcompensating for deficiencies in natural regeneration.

Oak success in relation to seedling and residual standcharacteristics

Long-term success rates of oak plantings generally im-prove with increasing size of nursery stock (e.g., Johnson1984; Spetich et al. 2002). When large 2–0 seedlings(11 mm basal diameter) were planted beneath shelterwoodoverstories in Arkansas, eleventh-year dominance probabil-ities on good sites increased more than fourfold when com-pared with those of 6 mm seedlings, which are equivalent tothe stock used in our experiment (model 4 in Spetich et al.2002). Although we selected the largest nursery plantingstock available when the study was initiated, these 1–0 seed-lings were much smaller than the stock commonly used to-day; recent hardwood nursery practice is to grow seedlingsfor 2 years in beds of lower density. Oak success probabil-ities might therefore be considerably higher than what weobserved if large 2-year-old seedlings are planted.

Minimum sizes of competitive oak seedlings at the timeof overstory removal probably vary with geographical andsite factors, species and initial sizes of competitors, and fre-quency and intensity of competition control measures. Earlystudies in Illinois and Missouri demonstrated that with natu-ral oak regeneration after clear-cutting and manual competi-tion control, only oak advance regeneration taller than about135 cm was capable of achieving dominance 10–12 yearsafter clear-cutting (Sander 1972; McQuilkin 1975). With theconditions in the present study, planted oaks that were ulti-mately deemed successful at the end of the experiment weregenerally smaller at the time of overstory removal, averag-ing 55 cm in height and 9.8 mm in basal diameter at thedry–mesic site and 84 cm in height and 12.8 mm in basaldiameter at the mesic site. At the dry–mesic site, oaks ofnatural origin made up 15% of the dominant–codominanttrees by age 19, even though few natural oak seedlingswere >60 cm in height at the time of final harvest. Other in-vestigators have also reported successful development of oakseedlings that were <100 cm at the time of release (Gott-schalk and Marquis 1982; Weigel 1999). Johnson et al.(1989) found that even new oak germinants established atthe time of clear-cutting were successful in a case wheretwo herbicide applications to understory vegetation hadbeen made 2 years prior to harvest. While larger seedlingsgenerally have a higher success rate and may be required instands containing vigorous competitors like yellow poplar

Table 7. Bivariate logistic regression analysis of individualpredictor variables of planted oak success on shelterwoodplots.

Model P values

VariableDry–mesicsite

Mesicsite

1990 free-to-grow status 0.043 0.0151990 total oak height (cm) 0.163 0.0011990 oak relative height* 0.143 <0.0011990 basal diameter (mm) 0.941 0.0021990 height growth rate (cm) 0.831 0.0111990 no. of 1st order branches 0.539 <0.0011990 no. of browsed branches 0.787 0.4651990 % browsed branches 0.895 0.0911990 no. of fully intact leaves 0.758 <0.0011990 no. of partial leaves (from

herbivory)0.080 0.004

1990 total no. of leaves 0.176 <0.0011990 competitor trees per hectare 0.970 0.2081990 mean competitor height (cm) 0.688 0.0061992 total oak height (cm) 0.021 <0.0011992 vigor class 0.143 <0.0011992 browse class 0.227 0.5891992 oak damage from slash piles 0.037 0.0041992 residual tree basal area 0.618 0.0071992 residual tree density 0.304 0.024Slope position (%){ 0.088 0.749Aspect of subplot (cosine(45-as-

pect)+1)0.447 <0.001

Note: Variables in 1990 were measured 2 years prior to overstoryremoval, and variables in 1992 were measured in the growing seasonafter overstory removal.*Oak relative height = oak height (cm) / mean competitor height

(cm).{Slope position is the distance of the planted oak from the bottom of

the slope relative to the total slope length.

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(Sander et al. 1984; Loftis 1990b), smaller oak seedlingsalso appear capable of contributing to future stocking in cer-tain species–site combinations and with effective competi-tion control.

Deer browsing is often cited as a major contributor to oakregeneration failures in some areas (Marquis et al. 1976;Collins and Carson 2003), and deer populations have risento record levels over much of their range in recent decades(Rooney and Waller 2003). Some degree of herbivory wasnoted on most seedlings during the first years of our experi-ment, but it was not heavy enough to be a significant factorin determining planted oak success at either site. Since thebeginning of the experiment in 1985, however, deer popula-tions in southwest Wisconsin have more than doubled(Rolley 2005) and have increased by more than 400% since1962 (Wisconsin Department of Natural Resources 2007).Current estimates (2005 population) for the two deer man-agement units surrounding the study sites are 10–14 deer�km–2 (Rolley 2005). Oak regeneration deficiencieshave been reported at similar deer densities (10–17 km–2) incentral Massachusetts (Healy 1997). If the study was to beconducted today with current deer population levels, brows-ing intensity might be much higher and result in substan-tially lower success rates than we observed, possiblymaking protection from browsing damage essential.

Most oak regeneration studies to date have focused exclu-sively on even-aged management, although stand-history re-constructions of old-growth stands indicate that they wereoften broadly uneven aged (e.g., Abrams and Copenheaver1999; Rentch et al. 2003). By mapping the release dates ofold oak trees in five old-growth forests, Rentch et al. (2003)obtained evidence that openings leading to successful oakcanopy recruitment were often relatively small (<200 m2).Some studies have also demonstrated satisfactory survivaland growth of oak regeneration after heavy partial cutting(Sander et al. 1976; Lorimer 1983). The historical and con-temporary evidence suggests that some type of multicohortmanagement should probably be feasible in oak forests,even though residual trees can have a detrimental effect ofoak survival and growth (Martin et al. 1992; Miller et al.2004). In the present study, success probabilities droppedto <20% where residual basal areas exceeded 5 m2�ha–1,even for seedlings >100 cm tall at the time of overstory re-moval (see also Schuler et al. 2005). The experiment never-theless did result in the successful coexistence of at leasttwo cohorts of oaks on each site as a whole.

The severity of detrimental effects from residual trees de-pended on their size and spatial dispersion. At the dry–mesicsite and some subplots at the mesic site, younger oaks weresuccessful where mature residuals were at low density or ag-gregated in tight clusters. The main failures occurred onsome subplots on the mesic site where numerous large sap-lings and poles of shade-tolerant species had been left un-treated. Over the next 12 years, crowns of these treesexpanded to form nearly a closed canopy, overtopping mostof the younger oaks. The detrimental effects of residual treesneed not discourage experimentation with multicohort man-agement of oak. However, as suggested by this study andthe experiment of Miller et al. (2004) on two-aged manage-ment, pole and large sapling competitors will need to becontrolled in the openings, and mature residual trees may

need to be spatially clustered rather than uniformly dis-persed. Further experiments with multicohort managementof oak forests may be desirable as a means of achieving aes-thetic goals and more closely emulating natural disturbanceprocesses.

AcknowledgmentsWe thank the staff of the Wisconsin Department of Natu-

ral Resources, the US Army Corps of Engineers, and theKickapoo Valley Reserve for their partnership and logisticalassistance over the past 20 years of this study. Specialthanks are due Randy Urich, Adrian Hagen, andGary Harden for coordinating the timber sales and to MarciWest for permission to work on the Kickapoo Valley Re-serve. Joe Schwantes and Frank Kirschling assisted in themore recent field measurements. This study was funded bythe Kickapoo Valley Reforestation Fund, a bequest ofRalph Nuzum to the University of Wisconsin Foundation,and by the College of Agricultural and Life Sciences, Uni-versity of Wisconsin-Madison.

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