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Dale L. NolteU.S. Department of AgricultureAnimal and Plant Health Inspection ServiceWildlife Services, National Wildlife Research Center

Kimberly K. WagnerU.S. Department of AgricultureAnimal and Plant Health Inspection ServiceWildlife Services

Andy TrentProject Leader

USDA Forest ServiceTechnology and Development ProgramMissoula, MT

3E32E84—Animal Damage Management

November 2003

United StatesDepartment ofAgriculture

Forest Service

Technology &DevelopmentProgram

2400 TimberNovember 20030324-2832-MTDC

In cooperation with

United StatesDepartment ofAgriculture

Animal andPlant HealthInspectionService

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Timber DamageTimber Damageby Black Bearsby Black BearsApproaches to ControlApproaches to Controlthe Problemthe Problem

Timber Damageby Black BearsApproaches to Controlthe Problem

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Introduction ___________________________ 1

Consequences of Bear Damage __________ 2

Damage Management ___________________ 3Bear Removal _________________________________ 3Silvicultural Practices____________________________ 3Alternative Food Sources ________________________ 4Alternative Approaches __________________________ 5

Summary _____________________________ 7

ContentsContents

1

IntroductionIntroduction

WW hen black bears (figure 1) emergefrom their winter dens, foods arerelatively scarce. Because trees

are already producing sugars (carbohy-drates) during the early spring, bearsstrip the bark and eat the newly formedwood underneath.

Bears have caused significant damage tosome stands of timber, particularly in thePacific Northwest. This report considerswhy bears cause such damage and whatmanagers can do to reduce the damage.Sugar concentrations vary, dependingon the tree species and the season. Forinstance, bears generally forage on hem-lock before Douglas-fir, which probablyreflects an earlier flush of nutrients in

the hemlocks. Similarly, bears may havesampled a tree early in the spring, onlyto return a few weeks later and strip it,presumably when sugar concentrationsare greater.

Bears use their claws to strip bark from atree, then feed on the sapwood (newly

Figure 1—Black bears strip the bark from trees to eat the sapwood.

formed outer wood) by scraping it fromthe heartwood (older central wood) withtheir teeth. Scattered remnants of barkstrewn at the base of a tree and verticaltooth marks indicate bear activity. Occa-sionally a tree will be “frilled,” with barkstrips loosened at the base and pulled upaway from the tree. These strips mayhang from more than 5 meters up onsome trees, such as western red cedar.

Bears usually forage on the lower trunk oftrees, girdling the bottom 1 to 1.5 meters.Some bears may climb the tree and siton branches to feed higher up. Bearshave been known to strip entire trees.Damage within a stand can be extensive.A single bear can strip bark from as manyas 70 trees per day.

Stripping trees for food is different thanmarking trees to stake out territory. “Beartrees” are rubbed and scent-marked byboth sexes, especially by adult malesbefore and during the mating season.Marks are usually made by biting or claw-ing conifer or deciduous trees about 1.5to 2 meters above the ground. Althoughmarked trees are common in most areaswhere black bears are found, damagecaused by marking is not severe.

Tree species stripped by bears vary de-pending on the location, probably reflectingthe species available. In the Pacific North-west, bears frequently girdle (strip barkall the way around the trunk) of Douglas-firs, primarily immature smooth-barkedtrees ranging from 15 to 30 years old.Girdled trees die because they cannottransport nutrients from the branches tothe roots. Trees of any age are vulnerableto bear damage. Although westernhemlocks are sometimes stripped, theyare stripped earlier in the year and arestripped less frequently once Douglas-fir trees break dormancy. Bears appearto prefer redwoods in northern California,western red cedar in British Columbia,and western larch in interior forests.Other species reported to have beenstripped by bears include silver fir, balsamfir, grand fir, subalpine fir, noble fir, bigleafmaple, red alder, western larch, PortOrford cedar, Engelmann spruce, whitespruce, red spruce, Sitka spruce, white-bark pine, lodgepole pine, western whitepine, aspen, black cottonwood, bittercherry, willow, and northern white cedar.

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WW hen bears strip trees (figures 2aand 2b), they can harm the healthof a stand or a particular species

within the stand. Complete girdling islethal. Partial girdling slows a tree’sgrowth and increases the likelihood ofinsect and disease infestations. Bearsoften strip the most vigorous trees.Damage frequently occurs after standimprovements, such as thinning.

While bear damage occurs nationally,damage occurs frequently in the PacificNorthwest. Whether the damage reflectsimproved stands with just one timberspecies, decreased alternative forage,behaviors that offspring have learnedfrom their mothers, or increased bearpopulations is largely unknown. All ofthese causes probably play some role.Stand improvements have increased thepalatability and nutritional content of thetrees. Cubs that eat the sapwood of treesstripped by their mother will probablycontinue stripping trees when the cubsare adults.

Consequences of Bear DamageConsequences of Bear Damage

Figures 2a and 2b—Douglas-fir trees peeled by a black bear feeding on sugars in the newlyformed wood beneath the bark.

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Damage ManagementDamage Management

TT hree primary options are available toreduce bear damage:

Eliminate or remove the problemanimals or reduce the overall bear pop-ulation.

Apply silvicultural practices thatminimize the attractiveness and vulner-ability of the trees.

Provide an artificial alternative foodsource during the spring.

The appropriate approach depends onmanagement objectives and site char-acteristics. A damage management planmay incorporate more than one of theapproaches. Other nonlethal approaches(such as devices to frighten bears, fences,or repellents) can be considered, butare generally impractical for large foreststands.

Bear Removal

Historically, bears were killed to protecttimber resources from bear damage.Animal damage control agents orprofessional hunters were hired to trapand hunt bears in areas where damagewas occurring. Although regulationsgoverning control measures havechanged, professional agents are stilleffective in removing problem animals.Aldrich foot snares permit agents totarget bears in specific areas. Trainedhounds also can be used to locateproblem bears. Depredation hunts maybe used to remove problem animals.

Recreational hunting can be used to sup-press bear populations. Special bear huntshave been authorized in some high-dam-age areas. Hunters can be encouragedto hunt in areas where tree stands aremost vulnerable to bear damage. In suchcases, maps of specific bear-damageareas can assist sport hunters. Masterhunter programs provide additionaltraining for hunters. Graduates of suchprograms provide a pool of ethical and

knowledgeable hunters for use in sensitivedamage-control situations such as these.

The general public would often preferthat bears be moved rather than killed.Moving bears is generally impractical.Few, if any, unoccupied favorable sitesexist where bears will not cause similarproblems. Furthermore, once bears havelearned to strip trees, moving the bearsmay move the problem. Even after bearshave been moved, they are likely to returnto their original capture site. Bears thatare relocated may transport diseases orparasites to other bear populations.

Silvicultural Practices

Bears are selective when choosing theareas where they forage and the treesthey strip. Several trees within a standmay be stripped while adjacent trees areignored or barely damaged. Bears selectthe healthiest, fastest growing trees.Damage occurs more frequently aftercertain silvicultural practices. Thinnedstands tend to be more vulnerable thandense stands. Depredation also appearsto increase after urea fertilizer has beenapplied.

A series of studies was conducted toassess whether bears selected trees tostrip based on chemical constituentswithin a tree’s vascular tissue and theimpact of forest management practices onthese constituents. Sugars and terpeneswere correlated with the extent of damageinflicted on a tree. Generally, sugars aredesirable to bears and terpenes are not.Sugars are concentrated in the vasculartissue of trees. Conifers have high con-centrations of terpenes, compoundsknown to discourage animals from feedingon other plants. A summary of the testresults include:

The amount of damage and theconcentrations of sugars and terpenesare correlated. The ratio of sugars toterpenes tended to be higher in preferred

trees than in trees that were not preferredor were rejected. Bears appear to foragein a way that maximizes sugars whileminimizing terpenes.

Thinning significantly increased theconcentration of sugars while having onlya minor impact on terpene concentration.The net effect of thinning was an increasein the sugar-to-terpene ratio in the vas-cular tissue. These results help explainwhy bears are more likely to strip treesin thinned stands than in dense stands.

Fertilizing trees with urea increasedtree diameter and sugars the year afterapplication, but did not alter the terpenes.The increase in tree diameter and sugarswas not apparent in later years. Thesedata suggest that fertilizing will probablynot increase the potential of bear damagefor more than 1 year.

Pruning treatments significantlydecreased the amount of vascular tissueand the sugar concentration without affect-ing the terpene concentration. Pruningdecreased the sugar-to-terpene ratio.This suggests that bears would tend toavoid pruned trees. Bear preference forunpruned trees was demonstrated in asurvey of bear damage on a 20-hectareOregon Department of Forestry pruningtest site. Statistical analysis revealed thatunpruned Douglas-fir trees were four timesmore likely to be damaged than prunedtrees. Similarly, unpruned western hem-lock trees were three times more likely tobe damaged than pruned trees.

The impact of selection of thefastest growing progeny on allocation ofsugars and terpenes in a tree’s vasculartissue also was investigated. Sampleswere collected from six known geneticfamilies of Douglas-fir at five differentprogeny test sites. Chemical assaysrevealed that terpene concentration wasnot necessarily correlated with growth.Allocation of sugars in vascular tissueswas subject to the interaction betweenthe environment and genetics. Thesedata suggest that it may be possible toselect trees that are less palatable tobears without sacrificing growth potential.

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Damage Management

Silvicultural practices affect chemicalconstituents found in trees. The extent towhich altering practices to reduce beardamage depends largely on alternativeforaging options. For example, a prunedstand located among unpruned standsmay be less vulnerable to bear damage,but if all stands are pruned, the bears’options become limited to ingestingfewer carbohydrates or going hungry.Given this option animals generally donot select starvation. Greatest value ofunderstanding the impacts of silviculturalpractices may be predicting where andwhen bear damage is likely to occur andappropriately implementing managementtechniques.

Alternative FoodSources

Private timber managers began investi-gating alternative damage control tech-niques during the mid-1980s. In 1985,bears were provided an alternative foodsource to reduce tree girdling. During thefirst year, about 2,250 kilograms of pelletswere fed to bears from 10 feeders. Sinceits inception, this program has continuedto grow. During 2001, about 300 metrictons of pellets were offered from about900 feeders across western Washington,and a few feeders in California andOregon.

Feeding Stations—Most forest mana-gers in western Washington use similarapproaches when feeding bears. Feedingstations (figure 3) are constructed from55-gallon (250-liter) metal or plasticdrums. An opening in the front providesaccess to pellets. A simple self-feedingdelivery system prevents bears from spil-ling pellets. A foam-insulated plywood roofkeeps pellets dry. A single feeder holdsabout 90 kilograms of pellets. Commercialpellets are about 0.6 centimeter in diam-eter and 1.3 centimeters long. Theyresemble dry commercial dog food, butare greenish. The sugar concentration inpellets is high (about 20 percent) and

provides at least four times the concen-tration of sugar found in the vasculartissue of Douglas-fir trees during thespring. The pellets include fats, proteins,vitamins, and minerals to provide thebears a nutritionally balanced diet. Feedersnormally are placed near a road so theycan be restocked easily, but away frompublic areas to avoid possible conflictswith people. All feeders are removedfrom the forest at the end of the feedingseason, sometime during the middle ofJuly. Bears normally wean themselvesfrom feeders once alternative foods (suchas berries) become available. Whenfeeding stations are no longer being used,they are removed to reduce vandalismand the perception that feeders couldserve as bait stations for persons huntingbears.

The supplemental feeding programappeared to be effective in reducing beardamage in particular timber stands. Bearsgenerally girdle fewer trees after theystart consuming pellets. However, limitedempirical evidence was available to doc-ument these observations. In addition, the

effect of supplemental feeding on bearbehavior is largely unknown. Severalstudies were conducted to learn moreabout supplemental feeding and its effects.Summaries of the results of those studiesfollow.

Efficacy—Overall, supplemental feedingreduced damage to Douglas-fir trees.Damage in stands with supplementalfeeders was just one-fifth the damage instands without feeders during the firstyear pellets were offered to bears (table1). In most feeding sites, damage waseven less severe during the second year(table 2). Bears probably require time tolocate feeders and begin using them.

Nutritional consequences—Duringthe spring, bears living in areas withfeeders gained more weight than bearsin areas without feeders. By the fall, bearsof the same age had the same weightwhether they lived in areas with feedersor not. Therefore, feeding probably doesnot improve the reproductive fitness ofbears. The study did not assess whetherthe program benefited lactating females.

Figure 3—A feeding station that dispenses pellets with a high concentration of sugar. Whenthey had the opportunity, bears tended to feed on pellets rather than strip bark from trees.

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Damage Management

Table 1—Douglas-fir trees damaged by black bears on seven 20-hectare timber stands withfeeders and seven 20-hectare timber stands without feeders.

Damage before feeders were installed

Damaged trees Undamaged trees Total

Stands with feeders 1,798 5,181 6,979Stands without feeders 1,647 5,104 6,751

Damage the first year after feeders were installed

Damaged trees Undamaged trees Total

Stands with feeders 35 5,002 5,037Stands without feeders 187 4,861 5,048

Table 2—Douglas-fir trees damaged by black bears on seven 20-hectare timber stands withfeeders and seven 20-hectare timber stands without feeders the second year after the feederswere installed.

Damage the second year after feeders were installed

Damaged trees in Damaged trees inStand stands with feeders stands without feeders

A 55 33B 2 33

C 2 22D 5 21

E 4 3

F 2 24G 2 15

Total 72 151

Table 3—Type and number of black bears monitored at feeders in western Washington during1999 and in western Oregon and Washington during 2000.

Black bears monitored at feeders during 1999 and 2000

Type of bear 1999 2000

Females 4 12

Females with cubs 2 2

Cubs (sets) 2 2

Adult males 5 32

Subadult males 6 2

Yearlings 1 7

Total 20 57

However, the high-energy diet in areaswith feeders may enhance a female bear’smilk production, improving the chanceher cubs will survive.

Behavioral consequences—Bearbehavior around the feeders was moni-tored. All classes of bears fed at thestations (table 3). There was no indicationthat one class of bears (such as females)avoided feeders during times of high usemore than another class of bears (suchas large males). Most bears visited morethan one feeder. Bears generally fed atfeeders every 2 or 3 days. No bears wereobserved protecting feeders from intrud-ers. Dominant bears may not restrictaccess to this resource because feedersprovide an unlimited amount of food.Radio telemetry studies showed that thepresence of feeders did not affect thesize of a bear’s home range.

The use of feeding stations requires along-term commitment and should becontinued until trees are old enough thatthe risk of damage is reduced. Somebears may not eat from the feedingstations and may continue to damagetrees. Lethal control may have to becombined with feeding stations beforedamage is reduced to acceptable levels.

Alternative Approaches

When bears are damaging valuable trees,a manager needs to develop a damagemanagement strategy that considers allfeasible approaches to resolve the prob-lem and selects the most appropriateapproach for each situation. Manyapproaches other than supplementalfeeding have been suggested by personswith little or no experience in protectingforest resources. Although most of thesesuggestions lack merit, they should beconsidered. Under some conditions, itmight be feasible to adapt some aspectsof them into a comprehensive manage-ment strategy.

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Damage Management

Conditioned avoidance—Condi-tioned food avoidance occurs when afood is altered to produce gastrointestinaldistress. Generally, animals are morelikely to form aversions to novel foodsthan to known “safe” foods. Traininganimals to avoid safe foods requiresrepeatedly feeding them altered foodthat produces gastrointestinal distress.When bears peel trees, they are eatinga known source of sugars when othersources are limited. Using conditionedaversion to train bears to avoid sapwoodis impractical.

Fencing—Excluding animals from asite prevents them from causing damage.However, constructing and maintaininga fence that will keep bears out of foreststands would be costly, particularly inareas with steep terrain. Regardless of thecost, a fence would impede movementsby other wildlife, pose a hazard to somespecies (such as deer), and require dis-turbing natural habitats during installation.Fencing can be used to protect smallresearch plots or valuable genetic sourcesfrom bears, but fencing is not a practicalapproach to prevent damage at most sites.

Birth control—Some persons haveproposed managing black bear popula-tions through contraceptives or chemicalsterilants. If conception were preventedin enough females, local populationswould decline over the long term. Theefficacy of this approach and the ethical

concerns associated with it remain contro-versial among biologists and the generalpublic. Although this approach is occa-sionally promoted by some individualsor groups, no chemical or biologicalcontraceptive agents are available foruse in bears.

Devices to frighten bears—Devicestraditionally used to frighten animals, suchas propane cannons, sirens, lights, andscarecrows, are generally ineffective, evenover the short term. The effect of thesedevices on bears has not been studied.Bears generally avoid human activity.Lights or noisemakers can deter bears,but bears will become accustomed(habituated) to these devices over time.

Devices activated by an animal’s pres-ence are generally more effective thanpermanent or routine displays. Thesedevices will probably deter bears longerthan devices that are active whether ornot a bear is present. Installing deviceson the scale needed to deter bears froma forest plantation would be costly be-cause the entire plantation would haveto be covered. Forest plantations providehabitats and resources for numerouswildlife species that would be harmed ifthe devices were truly effective. The eco-logical implications of treating thousandsof forest hectares with sirens and lightswould probably prevent their use. Thesedevices are generally impractical, exceptto protect small, carefully selected areas.

Repellents—Repellents reduce thedesirability of a food. Bears avoid applestreated with bittering agents. Severalcommercially available deer repellentscontain bittering agents. A study assessingwhether repellents could prevent bearsfrom stripping trees was inconclusive, butresults indicated that additional researchwas merited. If repellents are appliedacross a plantation, bears may have tochoose between going hungry in thespring or eating a nutritious, but bitter,food. Bears might move to untreatedareas. But the practicality and cost oftreating thousands of trees each springwill probably prevent repellents from beingused, except to protect small, carefullyselected stands.

Alternative natural plants—Bearsare believed to peel trees because otherforage is limited or of poor quality. Beardamage occurs during the spring whenmost plants are dormant or are too smallto provide much nutrition. Landownerscan encourage plants that are availableduring the spring, such as skunk cabbageand devil’s club. Salmon berry producesa berry crop earlier than most plants.Alternative plants should have sugarlevels that are higher than tree sap todeter bears from stripping trees. Theseplants are unlikely to provide enoughfood to maintain bears throughout thespring. In addition, if these plants wereencouraged near a vulnerable timberstand, they might attract bears to thestand.

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SummarySummary

MM anagers should consider all optionswhen developing a managementplan to reduce damage caused

by bears stripping trees in the spring. Acombination of methods will probably beneeded for animal damage managers tomeet their objectives while maintainingviable wildlife populations. Removingbears can stop immediate problems andreduce problems over the long term.Fewer bears will peel fewer trees. Lowerpopulations reduce competition foralternative foods.

Silvicultural practices can play a role inmanaging this problem. Delaying thinningor maintaining higher stocking rates mayreduce the number of trees that bearsstrip. Pruning trees also appears to reducedamage. Stand improvements, such asthinning or fertilizing, increase the potentialfor damage. Altering silvicultural practicesmay encourage bears to feed elsewhere,but will not stop them from stripping trees.

Providing bears an alternative food mayreduce damage to timber resources, but

will not eliminate it. Even with supple-mental feeding, extensive damage mayoccur on some sites. Other proposedapproaches to reduce the number oftrees stripped by bears are impracticalfor timber plantations.

Bear management that protects timberresources is often controversial. Managersneed to consider economics and theecological and social implications of anyaction before implementing managementplans to reduce the damage caused bybears.

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Notes

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Notes

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Notes

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Single copies of this document maybe ordered from:USDA Forest Service, MTDC5785 Hwy. 10 WestMissoula, MT 59808–9361Phone: 406–329–3978Fax: 406–329–3719E-mail: wo_mtdc_pubs@fs.fed.us

Dale L. Nolte is the project leader at theOlympia Field Station of the U.S. Depart-ment of Agriculture’s Animal and PlantHealth Inspection Service, WildlifeServices, National Wildlife ResearchCenter. He earned a Ph.D. in foragingecology from Utah State University.

Kimberly Wagner is a environmentalcoordinator for the U.S. Department ofAgriculture’s Animal and Plant Health In-spection Services, Wildlife Services. Sheearned a Ph.D. in wildlife damage man-agement from Utah State University’sDepartment of Fisheries and Wildlife.

Andy Trent is a project engineer at MTDC.He received his bachelor’s degree inmechanical engineering from MontanaState University in 1989. Before comingto the center in 1996, Andy worked as acivilian engineer for the U.S. Departmentof the Navy. Andy works on projects inthe reforestation and nurseries, foresthealth protection, and watershed, soil,and air programs.

About the Authors

Library Card

Nolte, Dale; Wagner, Kimberly K.; Trent,Andy. 2003. Timber damage by blackbears: approaches to control the problem.Tech. Rep. 0324–2832–MTDC. Missoula,MT: U.S. Depart-ment of Agriculture,Forest Service, Missoula Technology andDevelopment Center. 10 p.

Describes alternative approaches to con-trolling the damage black bears causeduring the spring when they strip bark toeat the newly formed wood underneath.One bear might strip as many as 70 treesin a day. The trees will be damaged andmay be killed if the bark is stripped all the

way around the tree, girdling it. Bearsappear to strip the most vigorous trees,preferring stands that have been thinned,or those where urea fertilizer has beenapplied. Bears also appear to prefer treeswith a high concentration of sugars relativeto the concentration of terpenes. Pruningdecreases the sugar-to-terpene ratio,reducing the likelihood that trees will bestripped by bears. Bears generally quitstripping trees once other foods becomeavailable during the late spring or earlysummer. One approach to reducing dam-age has been to provide supplementalfeed (pellets resembling dog food) in

stands being damaged by bears. In onestudy, damage was just one-fifth as muchin stands with feeders as in standswithout feeders. Killing bears in areaswhere trees are being stripped can alsoreduce damage. Other approaches, suchas relocation, contraception or sterilization,or repellents, are not generally practicalfor protecting forest plantations.

Keywords: animal damage control, carbo-hydrates, fertilization, girdling, plantations,reforestation, repellents, silviculture,sugars, terpenes, thinning, Ursusamericanus

For additional information about thisresearch, contact Dale or Kimberly:

Dale NolteUSDA APHIS WS, National Wildlife Research Center9730–B Lathrop Industrial Dr.Olympia, WA 98512Phone: 360–956–3793E-mail: dale.l.nolte@aphis.usda.gov

Kimberly K. WagnerUSDA APHIS, Wildlife Services6135 NE. 80th Ave., Suite A8Portland, OR 97218Phone: 503–326–2346E-mail: kimberly.k.wagner@aphis.usda.gov

For additional information about thisT&D project, contact Andy Trent atMTDC:Phone: 406–329–3912Fax: 406–329–3719E-mail: atrent@fs.fed.us

Electronic copies of MTDC’s docu-ments are available on the Internet at:http://www.fs.fed.us/eng/t-d.php?link=pubs