The art and science oftree risk assessmentcontinues to evolve as we increase our knowl-edge base and with field experience. Treesdo not fail at random. Tree failures are pre-dictable over a broad time range. And treeassessment is not an exact science but ascience nonetheless and one that is constantlyadvancing.

An arborist performing tree risk assess-ments must be well trained in biology, treestructure, and tree mechanics. The arboristshould possess a high level of comfort andexperience with the inspection process. Ittakes a trained eye to recognize the subtlesigns of impending mechanical failure. Thepossible result of over-reading these signs isoverreacting. Any existing signs of failuremust be thoroughly evaluated to determinecause. Because every tree is different, perform-ing tree risk assessments is a learning process.This article briefly reviews structural defectsand basic tree mechanics.

The process for evaluating the risk oftree failure begins with visual inspection fordefects (visual tree assessment, VTA), followedby sounding for suspected decay and prob-ing, if necessary, with a portable drill, incre-ment borer, or an advanced decay detectiondevice. Formulating a decision involvesconsidering several factors, including multi-ple defects, species characteristics, locationand extent of decay, characteristics of decayorganisms, crown size, crown ratio, stemtaper, exposure, target considerations, treevalue, and owner attitude.

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Components of RiskAssessment

The three components of tree risk assessmentproposed by Matheny and Clark (1994) arethe tree’s failure potential, an environmentconducive to tree failure, and a target.

Failure PotentialAlthough some trees without defects fail inmajor storms, the presence of any defect willincrease the chances of failure. Each specieshas its own profile of defects. Some factorsthat must be considered include the species’growth habit, tree condition, branch attach-ments, resis-tance todecay, condi-tion ofanchoringroots, cul-tural ormaintenancehistory, andprevious

damage. In addition, the severity of anydefects found should be considered. Otherfactors related to the site such as intensity ofuse, soil condition, and prevailing windsmust be considered in conjunction with thedefects present when assessing the potentialfor failure. Any individual factor can directlyimpact tree safety (or, more often, multiplefactors impact the tree’s failure potential). Thesize of the tree or tree part that may fail is alsoimportant. Usually, the tallest, most exposedtree and tree parts are of greatest concern.

Environment Conducive to FailureA weakened tree that is exposed to additionalloads from wind, ice, or other factors obviouslywill have an increased likelihood of failure,especially if the load is unusual in directionor magnitude. Most tree failures occur duringor as the result of storms, and exposure torain, snow and ice loading, and lightningincreases risk of branch and tree failures.Many site factors and past history can influ-ence tree condition and the types and sever-ity of the defects present.

Some examples of stress factors and theinjuries or defects they can cause are listedin Table 1.

Learning objectives—The arborist will be able to

h explain the components of tree riskassessment.

h describe categories of structuraldefects.

h apply guidelines for evaluating decayabove and below ground.

h discuss some basic principles of treemechanics.

Tree Risk Assessment&Tree Mechanics

000000

000000

By Ed Hayes

Table 1. Stress factors and the injuries or defects they can cause.

Stress Factor Resulting Injury or DefectSoil compaction, paving, and grade changes Dieback and deadwoodConstruction injury to stem and roots Cankers, decay, cracks, leaning,

and windthrowWounds, flush cuts, cavity fillings, and Cankers, decay, and cracksother mistreatments Planting too deeply Dieback, deadwood, stem-girdling

roots, and windthrow

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TargetTheoretically, without a target there is nohazard. However, in urban settings, we rarelycan completely discount target potential.Targets include facilities, people, and personalproperty. In various tree-rating approaches,targets may be prioritized by intensity ofuse or exposure to people.

Inspections andDocumentation

Inspecting trees for defects must be a carefuland systematic process. The entire tree mustbe inspected. Inspections should be doneonce a year and following storm events.Inspections are best conducted during theleaf-off season for temperate climate hard-wood species to facilitate observation andinspection. Always document the evalua-tions, recommended actions, and actionstaken. Keep permanent records.

Structural DefectsIdentification and correction of structuraldefects such as weak branch attachments,leaning, cracks, wounds, deadwood, anddecay may reduce the failure potential (and,therefore, reduce risk to property and injuryto people).

Branch Attachments andBranch FailuresWeak branch attachments include unionswith included bark and branches formedfrom epicormic buds. A weak union withincluded bark has bark present inside thebranch union. There is little wood tissueattachment between the codominant stemsor branch union with included bark. Weakunions are common in species with an oppo-site bud set. Examples include maple (Acer)and ash (Fraxinus) species. Weak unions areoften easy to evaluate. Weak unions withopen cracks or decay are obvious hazards.The propagating rib on opposite sides ofthe weak union can be an indicator of aninternal crack and, in some cases, decay.

When trees are topped, overpruned, orstressed, they produce epicormic buds.Branches from epicormic buds are weaklyattached, especially if there is internal decaybelow the attachment, which is often the

case below old topping cuts. Weak unionsare one of the most common causes of branchand stem failures during storm events andone of the easiest defects to prevent. Guide-lines call for subordination or removal of allcompeting leaders beginning the secondyear after planting. This practice shouldcontinue over several years to produce onecentral leader.

Leaning TreesMost trees lean to some extent. Phototropism(growing toward light) can cause a naturallean, which may or may not be a problemin later years. The key question is, is the leannatural or is the tree failing? When leaningtrees have preliminary signs of failure, furtherinspection is essential because failure maybe imminent (Figure 1). Arborists shouldbe familiar with these signs.

• At ground level, look for soil lifting,movement, or mounding associatedwith root or root plate disruption, orcracks in the soil near the base. Lean-ing trees with recent soil lifting, move-ment, or mounding couldindicate the tree is failing.

• At the base, look for com-pressed or buckling fibers onthe lower or compression sideand horizontal tension cracks(perpendicular to the stem)on the upper or tension side.Tension cracks are rare andrequire immediate action—tree removal.

• When roots are severed, therecan be a significant loss of rootanchoring support, especiallyfor leaning trees. It is the small-diameter, lateral roots undertension that provide the great-est anchoring support for thetree. The tensile (tension-pulling strength) of a 2-inch-diameter root is dramatic andequivalent to as much as 4tons of holding power.

CracksThe most common cracks observedin trees are radial cracks. Radial

cracks are wood fiber separations along therays in the axial plane (up and down). Opencracks are physical separations of the woodfibers. Open cracks indicate that part of thetree has failed or is failing. All cracks arisefrom load imbalances (mechanical stress)and are in most instances predisposed bynatural flaws in tree anatomy (natural weakpoints). Internal cracks may or may not bevisible from the outside. Cracks are evaluatedfor the extent of compromise to the branchor stem cross section, as well as their locationand their association with other defects.

Wounds and Cankers Wounds can lead to cankers, which arelocalized diseased areas on stem tissue thatmay be shrunken and discolored. Woundsassociated with canker tissue usually fail tocompartmentalize completely and oftenbecome more serious defects. Wounds mustbe evaluated for the extent to which theycompromise the strength and integrity of abranch or stem. If decay is present, theseverity of the defect is greater.

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Figure 1. This sugar maple (Acer saccharum) has a leanthat is not natural, and the tree is failing. Internal decayhas reduced the remaining wall to under 20 percent.Note the root flare delamination. There are opposingshear cracks perpendicular to the lean and an obvioustension crack. Immediate removal was necessary.

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Deadwood Dead trees and dead branches can fail atany time. Dead and decayed wood fail atdifferent rates depending on species, mate-rial size and weight, and resistance to decay.Dead branches or dead tops that have alreadybroken off and lodged (hangers) are especiallyhigh risk.

Decay Understanding the process of tree decay isvital to evaluating trees for decay. The symp-toms and signs of decay include cavities, holes,cankers, branch stubs, fruiting structures(mushrooms and conks) of decay organisms,stem bulge, and stem swelling (Figure 2). Treeroots typically decay from the bottom up.

The key to understanding compartmen-talization is that decay is confined to thewood present at the time of wounding unlessit breaks through the barrier zone. The extentof the internal decay is not the primary issue.Most important is the amount of soundwood present in the stem or remaining wall.(Periodic reinspection is required becausesome decay organisms can cross the barrierzone into wood tissue that was laid downafter initial wounding.)

The amount of sound wood is determinedbased on stem and branch diameters orcross-sections. Trees can have decay and

still be stable; in fact,most large trees docontain decay. Gener-ally, a full-crown treewithout a lean can beup to 70 percent hol-low—a level of risk thatcan be tolerated (in theabsence of cavities orother factors). Treessuch as this may bewithin safety marginsbut must be fully eval-uated for possiblemitigation.

Anything above thisamount of decay (70percent), or defects inaddition to the decay,increase risk factors(Figure 3). Current guidelines in the UnitedStates recommend a minimum 30 percentof the stem and branch wood diameters besound. The amount of sound wood remain-ing is calculated as the ratio of remaining wall-to-stem radius (t/R), or 15 percent of stemor branch diameter (remaining wall on eachside of the stem or branch), excluding thebark. The guidelines apply to full-crown trees.

Where large cavity openings exist, guide-lines call for increasingthe sound woodrequirement (Figure 4).The size, location, andextent of the cavity mustbe considered in assess-ing the risk potential. Ifa tree has less than 20percent residual wood,the Bartlett Tree ResearchLaboratory (Fraedrichand Smiley 1999)recommends that it beremoved withoutclimbing. Instead, itshould be removedusing an aerial lift orcrane.

Another way toapproach estimatingwhether sufficient soundwood remains is using

a guideline that calls for 1 inch of soundwood to be present in the remaining wallfor each 6 inches of stem or branch diame-ter. This guideline is a starting point, a wayto quickly estimate the sound wood require-ment. The actual remaining wall thicknessmust be calculated as previously described,based on the results of the inspection at thedrilling site, as well as the cavity openingpercentage. For asymmetrical decaycolumns, the risk of failure increases whenthe areas of decay are large and the remain-ing wall on the decayed side is thin. Ulti-mately it may be possible to use wind loadanalysis to predict tree failure factoring instem form, geometry, and average materialwood fiber strength. More research isneeded to provide base information thatcan be applied to this form of analysis.

A guideline suggested by the Bartlett TreeResearch Laboratory (Fraedrich and Smiley1999) for major buttress root flares is a soundwood requirement of 15 percent of dbh forevery two out of three roots present. TheBartlett Tree Research Laboratory recommendsthat trees with more than a third of thebuttress roots significantly decayed, miss-ing, or severed be recommended for removal.They define “significantly decayed” as havingless sound wood on the top of the buttressroot than dbh times 0.15. For a 30-inch tree,if the buttress root has less than 4.5 inches

Figure 2. This bur oak (Quercus macrocarpa) has a sign of decay,yet there is no symptom of swelling. Further examination is needed,first by sounding, then by invasive or advanced techniques. Thestem may have an asymmetrical column of decay, the decay maybe extensive, or both. Check through the opening first.

Figure 3. This failed bur oak had a 15 percent remaining wall formost of 75 percent of the stem circumference. It was in the openwith a full crown, which is why it failed in an average storm event.

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of sound wood, it would be consideredsignificantly decayed.

Diagnostic Tools toEvaluate Tree Defects

Fortunately, arborists now have a numberof tools at their disposal for evaluating treedefects. The basic equipment includes, butis not limited to, a mallet for sounding; diam-eter tape; logger’s tape (or suitable distancetape); an increment borer; a portable drill(preferably 18 volt or higher); drill bits (bradpoint), 1/8-inch by 8 to 12; clinometer; calcu-lator; soil excavation tools; and any of theadvanced decay detection devices such asthe Resistograph. Invasive techniques (drillingor boring) are used where needed to con-firm or alleviate suspicions of decay. Thewounds made by the drills may or may notimpact the ultimate extent of decay depend-ing on the decay organism involved and thetree’s ability to compartmentalize the wounds.One noninvasive piece of equipment avail-able is the PICUS sonic tomograph, which

produces a graphic representationof a tree’s cross section based onthe speed of sound among multi-ple points around the circumfer-ence of the tree (Figure 5). In thefuture, ground-penetrating radaradapted to use for decay detectionmay be possible.

Basic Tree MechanicsTension is pulling or lengthening;compression is pushing or short-ening. A tree is loaded by tensionon the outside and by compressionon the inside. Shear is slippage.Claus Mattheck lectures abouthow trees “react” over time tomechanical stress by adding morewood where the loading is greaterand less wood where the load forcesare lesser (adaptive growth). Adap-tive growth is uniform (mechani-cal) stress distribution. Leaningtrees, for example, can put downwood tissue to adapt to the addedload of the lean. Conifers (gym-nosperms) “push themselves up”on the lower compression side with

compression wood (reaction wood). Hard-woods (angiosperms) “pull themselves up”on the upper tension side with tension wood.

Stems and branches must be supportedwith adequate taper (slenderness ratio, alsocalled the height-to-diameter ratio). Taperserves as defense against stem and branchfailure. Typically, mature hardwoods in theupper midwestern United States have slen-derness ratios of 20 while conifers are 30.Lack of taper is a risk factor for solitary trees,trees from stands that become edge trees,and for branches with taper problems (liontailing).

Trees evolved with excess capacity bothbiologically and mechanically. In engineering,this is referred to as the safety factor or designfactor. That is, if a structure is designed tohandle loads that are double the expectedworking loads, they are said to have a safetyfactor of two. The safety factor for trees hasbeen estimated to be about four times theirworking load. The concepts of safety and reli-ability are linked. A safe structure performs

reliably under the normal working condi-tions it experiences. The connection here isthat most trees perform reliably when struc-turally sound. As tree defects take their toll,the safety margin is reduced. A tree with astructural defect can fail at less than normalworking conditions (average storm events).

The goal of risk assessment is to predictpotential failure in trees. Typically, normalweather conditions are factored in. In mostgeographic areas, wind forces up to 50 mphare considered normal. At wind forces inexcess of 70 mph, even structurally soundtrees can be overpowered.

Treatment and PreventionMitigation is the process of reducing treefailure potential. A level of zero risk doesnot exist unless the tree is removed. Some-times the risk of failure is unacceptable andcannot be mitigated. Once a tree is con-demned, its removal should take place assoon as possible or the target area should beclosed. Short of removal, treatment optionsinclude moving the target, removing danger-ous branches, cabling and bracing, prop-ping, reducing the crown, and closing thearea. Treatment options for cavities are limited.No scientific or experimental evidence existsto support cavity treatments; more often,cavities are filled for aesthetic or mainte-nance reasons.

Prevention of tree defects involves focus-ing on all of the best management practices:selecting quality planting stock; matchingspecies to site; planting properly; pruningto establish a strong structure; and avoidingwounds, injuries, and construction damage.

Managing Veteran TreesOld trees are sometimes called veteran trees.Arborists in Europe have been focusing ontechniques to preserve veteran trees fordecades or even centuries. The steps toconsider include

• predicting the failure pattern that is tobe expected.

• outlining the options that would needto be taken to prevent the failure.

• evaluating the outcomes of thosetreatments over time.

Figure 4. This red maple (Acer rubrum) has a 20 percentcavity opening from mid-opening to the ground linerelative to the increasing circumference. A consistent,20 percent remaining wall exists around the cone-shapedbase. Although the maple is 80 feet tall, it is protectedby its downslope position by some large white pinesabove. There is no immediate target.

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If mechanical means are to be used, howwill they be dimensioned? Static cablingover time can suppress secondary thicken-ing or diminish the positive effects of adap-tive growth. Should more dynamic cablingtechniques be considered? Sometimes cableinstallation is accompanied by crownreduction. Many European arborists advo-cate progressive crown reduction over aperiod of many years to reduce failurepotential. While this practice certainly canbe effective in reducing the likelihood offailure, other arborists would argue that theaesthetic value of the tree is also reduced,perhaps unacceptably.

The great Wye Oak failed in June 2002after a long, distinguished life in Maryland.Its fate may have been sealed many yearsago when the decision to cable was followedby the decision to add more cable. It stilltook an enormous force to topple the WyeOak, but, judging from the pictures of theamount of sound wood in the remainingwall, the tree had few safety reserves left.The wind force merely had to challenge theaboveground mass. Members of the Societyof Commercial Arboriculture visited theWye Oak in 2000 during ISA’s Baltimoreconference. At that time, a small debatearose between some European and someAmerican arborists. The Americans advo-cated preserving the tree in its full majesty

by employing judicious pruning to mini-mize removal of live tissue, installing cables,and taking measures to improve and maintaintree vitality. The Europeans suggested agradual and systematic reduction of thecrown to reduce failure potential. The lattermeasures likely would have saved the WyeOak, yet many arborists would questionwhat would be left to save.

Managing RiskTwo important strategies should be used formanaging the potential liability associatedwith performing risk assessments. The firstis expert tree assessment (training), and thesecond is limiting liability. Consultingarborists should consider using a disclosurestatement. A disclosure statement does notremove the arborist from all potential liabil-ity, but it can limit liability. Ultimately, thedecision of how much risk to accept lieswith the client. The goal of the assessmentis to determine the risk and convey it to theclient in an easy-to-understand way and inwriting. The arborist may make recommen-dations and suggestions, but the clientdecides the best course to take.

All trees have a risk of failure. As treesincrease in size, mass, and maturity, the riskof failure increases. Eventual failure isinevitable. Arborists must learn from tree

inspections andtree failures. Themost importantand reliable fieldinformation isobtained from atree or branchfailure that doesnot strike animportant target.Perform the dis-sections, take themeasurements,and do the math.A tree failure with-out a loss is avaluable piece ofinformation. Edu-cation in treebiology and struc-

tural mechanics is important, and trainingin risk assessment is essential. Even withthorough education and training, however,nothing can substitute for experience andgood judgment.

References Fraedrich, B.R., and E.T. Smiley. 1999.

Guidelines for Qualifying and EvaluatingWood Decay in Stems and Branches. BartlettResearch Technical Report, Bartlett TreeResearch Laboratory, Charlotte, NC.

Matheny, N.P., and J.R. Clark. 1994. APhotographic Guide to the Evaluation ofHazard Trees in Urban Areas. InternationalSociety of Arboriculture, Champaign, IL.

Figure 5. A tomograph taken of a cross section of a tulip poplar (Liriodendron tulipifera) (left) with the dissectedcomparison (right).

Ed Hayes is the author of the field guideEvaluating Tree Defects, now in itssecond edition. His address is Safetrees,532 22nd Street NE, Rochester, MN55906; e-mail ehayes@safetrees.com.