British Columbia’s Dangerous Tree . - US Forest Service
British Columbia’s Dangerous TreeAssessment Process1Todd Manning,2 Peter Bradford,3 Cary White,4 David Rowe,5 NancyDensmore,6 and Stewart Guy7AbstractNew dangerous tree assessment guidelines have recently been developed by the Wildlife TreeCommittee of British Columbia. They are intended to provide information and technicalprocedures for assessing and safely retaining trees in various situations, ranging from parksand urban/municipal settings to industrial forestry activities and wildland fire fighting. Thedecision to retain a wildlife tree must include worker and/or public safety, the type of activityoccurring around the tree, site factors and tree condition, and wildlife habitat values. Theseguidelines will be of interest to persons involved in any activity where the management oftrees for wildlife habitat or other purposes is desirable.IntroductionIn British Columbia, the historical definition of a standing dead tree or “snag”has been: “Any standing dead or dying tree over 3 meters in height.” New Workers’Compensation Board Occupational Health and Safety Regulations (WCB 1998) wereadopted into law effective April 15, 1998. With these new regulations, the term snagwas replaced with “dangerous tree.” According to section 26.1 of these regulations,a dangerous tree is now defined as:“Any tree that is hazardous to workers because oflocation or lean, physical damage, overhead hazards, deterioration of the limbs, stemor root system, or a combination of these.”Recently, new dangerous tree assessment guidelines and technical criteria weredeveloped by the Wildlife Tree Committee of British Columbia in conjunction withthe changes to the Workers’ Compensation Board regulations. The guidelines werefirst pilot tested throughout British Columbia with a range of individuals experienced1An abbreviated version of this paper was presented at the Symposium on the Ecology and Managementof Dead Wood in Western Forests, November 2-4, 1999, Reno, Nevada.2Coordinator of the Wildlife Tree Committee of British Columbia, and Habitat Forester for Manning,Cooper & Associates, 5148 William Head Road, Victoria, B.C., Canada, V9C 4H5 (e-mail:tmanning@islandnet.com)3Forest Practices Forester, British Columbia Forest Service, Forest Practices Branch, 1450 GovernmentSt., Victoria, B.C., Canada, V8W 3E7 (e-mail: peter.Bradford@gems1.gov.bc.ca)4Occupational Safety Officer, Workers’ Compensation Board of British Columbia, Prevention Division,4980 Wills Rd., Nanaimo, B.C., Canada, V9T 6C6 (e-mail: cwhite@wcb.bc.ca)5Program Manager (Forestry), Workers’ Compensation Board of British Columbia, PreventionDivision, 1066 Vancouver St., Prince George, B.C., Canada, V2L 5M4 (e-mail: drowe@wcb.bc.ca)6Emerging Issues Forester, British Columbia Forest Service, Forest Practices Branch, 1450 GovernmentSt., Victoria, B.C., Canada, V8W 3E7 (e-mail: nancy.densmore@ gems2.gov.bc.ca)7Identified Wildlife Biologist, British Columbia Ministry of Water, Land and Air Protection,Biodiversity Branch, P.O. Box 9338, Stn. Prov. Govt., Victoria, B.C., Canada, V8W 9M1 (e-mail:stewart.guy@gems9.gov.bc.ca)USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.863
Dangerous Tree Assessment Process—Manning, Bradford, White, Rowe, Densmore, and Guyin occupational health and safety, logging, forest pathology, and forest and wildlifeecology. This information was subsequently incorporated into the provinciallysponsored “Wildlife/Danger Tree Assessor’s Course” (see WTC 2001), whichprovides technical information and practical field skills to persons who wish to assesstrees for hazards and wildlife habitat value.Determining Tree DangerThe determination of a tree’s failure potential (i.e., the likelihood that all or aportion of a tree will break), and ultimately whether or not it is dangerous, involves afour step process:Step 1―Determine the level of ground or tree disturbance around the treeStep 2―Conduct a site assessment overviewStep 3―Conduct a visual tree inspectionStep 4―Make the appropriate safety decision.Step 1―Level of DisturbanceVarious activities are associated with differing levels of ground or treedisturbance. Activities rated as low disturbance, such as surveys, tree pruning, or useof back country hiking trails, involve negligible ground or tree disturbance, and as aresult expose people to very little danger. However, as the level of disturbance orexposure increases, such as with timber harvesting, heavy machinery use, or in highuse park facilities, the potential danger and risk of injury also increases.Consequently, potentially dangerous trees considered for retention in these situationsmust be carefully assessed for any hazards in order to determine tree failure potentialand reach an appropriate safety decision.Step 2―Site Assessment OverviewThe determination of a tree’s failure potential begins with a site assessmentoverview, which involves assessing forest health factors (e.g., root rots, insectdamage), stand condition (e.g., age, tree species, presence of heart rots), soil profileand condition, and other site variables (e.g., windthrow hazard, slope, fire damage).The site overview provides a context for subsequent assessment of individual trees(i.e., it identifies overall site problems, such as damaged stands or root rot, which caninfluence tree failure).Step 3―Visual Tree InspectionThe third step in the dangerous tree assessment process is a visual treeinspection, which results in a failure potential rating (low, medium, or high) for agiven tree defect. Failure potential is rated according to tree species groupings, andconsiders the following factors: hazardous tops, large dead limbs, witches’ broom,split trunk, stem damage (scarring, butt rot), thick sloughing bark, fungal fruitingbodies (conks, mushrooms), butt and stem cankers, tree lean, and root condition.864USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
Dangerous Tree Assessment Process—Manning, Bradford, White, Rowe, Densmore, and GuyFour native tree species groupings have been identified: Douglas-fir-larch-pinesspruces; western redcedar and yellow cedar; hemlocks and true firs; and deciduoustrees (“soft” hardwoods such as aspen, cottonwood, paper birch, maple, alder).Failure potential ratings for a particular defect, such as stem damage, can bedifferent based on the tree species grouping (e.g., cedars have a larger permissiblestem scarring threshold than the other tree species groupings).With adequate experience and training, the visual tree inspection can be anefficient process that usually requires only a few minutes per tree. If the visualinspection is inconclusive as to the condition of the tree (e.g., root rot is suspectedbased on the site assessment), a detailed tree assessment can be conducted in order togather more information on the condition of the bole (stemwood shell thickness) androots. This involves exploratory tests in the lower bole (e.g., increment boring,drilling, or sounding) and excavating and probing around the root collar.Step 4―Making a Safety DecisionOnce a failure potential rating has been determined from the visual treeinspection, a tree can be rated as either safe (S) or dangerous (D), depending on thelevel of disturbance or type of activity around the tree. This procedure is illustrated intable 1. A classification for coniferous trees is shown in figure 1.Table 1—Overall Tree Danger Rating.Level ofdisturbance1 (Low)2 (Medium)3 (High)4 (Very High)1Detection failure potentialLowS1SSS (for class 1 trees)S (for class 2 cedars withlow failure potential)S (class 2 and 3 trees withno defects) 3D (all other trees)MediumS1SS (for veteran anddominant conifers) 2D (for all other trees)DHighS1DDDFor Low level (1) disturbance activities, any trees that have one or more of the following significanttree hazards will become D (dangerous): Insecurely lodged trees, or hung-up limbs or tops Highly decadent or unstable trees (e.g., 50 percent cross-section area stem damage, or 50percent lateral roots damaged/unsound) Trees with recent high lean ( 15 percent toward work area or target) and damaged rootsystem/anchoring soil layer.2Employ safe falling and yarding practices around trees assessed as SAFE with Medium failurepotential; do not hit or disturb the trees, and fall and yard or skid away.3Trees with no defects will not have any defect indicators other than small dead limbs or minor stemscrapes that are not associated with decay or loss of structural strength. Class 2 trees with no defects willusually be wind- or snow-snapped green trees. Class 3 trees with no defects will usually be insect kill,climate kill, or a recent light intensity fire where no structural tree damage occurred.USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.865
Dangerous Tree Assessment Process—Manning, Bradford, White, Rowe, Densmore, and GuyFigure 1—British Columbia’s Tree Classification System (adapted from Maser andothers 1979).If a tree is determined to be dangerous for a particular type of work activity, thenappropriate safety procedures must be implemented. These include removing the treeor any hazardous parts (e.g., top, limbs) or establishing an appropriately sized safebuffer area (called a no-work or hazard zone) around the tree to eliminate exposure tothe hazard. However, if a tree is assessed as safe for a given type of activity, workcan proceed up to the tree regardless of whether it is dead or live.Inherent to the implementation of any safety decision is an understanding of theconcept of “risk.” RISK HAZARD x EXPOSURE. For example, if there is exposureof workers to a dangerous tree or a “target” exists (e.g., buildings or equipmentwithin striking distance of a dangerous tree), then an inherent risk of injury orproperty damage also exists. On the other hand, if there is no hazard (i.e., the tree isnot dangerous) or there is no target exposure, then there is no or very minimal risk.ApplicationsWildlife tree patches and other stand-level reserves (e.g., riparian managementareas) are generally the safest and most operationally efficient means of retainingwildlife trees and other biodiversity attributes at the stand level in forestry operations.However, the new dangerous tree assessment process now facilitates retention ofSAFE standing dead and SAFE live defective trees in forest harvesting operationsoutside of patches and reserves. This is particularly relevant to partial-cuttingsilvicultural systems, where the retention of individual standing dead and livedefective trees is desirable in order to provide additional stand structure, wildlifehabitat, and biodiversity values over time.866USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
Dangerous Tree Assessment Process—Manning, Bradford, White, Rowe, Densmore, and GuyThe dangerous tree assessment process can be applied to a variety of situations,including: Forest harvesting Silviculture operations such as tree planting and stand tending All silvicultural systems, but especially useful for partial-cutting systemssuch as variable retention, group selection, shelterwood, and single treeselection Wildland fire fighting Road sides Utility corridors Parks and recreation sites (e.g., trails, campgrounds and other developedfacilities) Recreational wooded areas such as golf courses and ski hills.With these “dangerous tree assessment tools” now available to forest managers,there can be more flexibility in the type, condition, density and location of retainedtrees in harvest blocks and other applications where the retention of trees is amanagement objective.Dangerous Tree Assessment Research and TrainingThe description and quantification of tree conditions and defects, and thecorrelation to the likelihood of tree failure, has received relatively little research.There have been a few pioneering studies in this area, however, such as Wagener’sassessment of tree hazards in California recreation sites (Wagener 1963). Thefindings of Wagener’s research continue to be broadly applied in North Americanhazard tree and arboriculture management. Some researchers in the southeasternUnited States (Smiley and Fraedrich 1992) and in southern California (Matheny andClark 1991) are relatively active in this field, as are Mattheck and Breloer (1997) inGermany.In Canada, there is a general lack of research in the field of hazard treepathology and applied management. However, in British Columbia, a researchproject has recently been completed (Manning 2001). This project collectedquantitative data on visible external tree defects and corresponding internal treeconditions. Correlations were tested between the occurrence of external tree defectsand the incidence of internal decay and the resultant loss of stem shell thickness.Highly significant differences (p 0.001) were found between the actual/expectedstem shell thickness ratios for the three tree defect failure potential ratings (low,medium, high). This suggests a strong positive relationship between the actual shellthickness measured at the corresponding defect positions and the assigned failurepotential rating for that defect. For example, where a high failure potential rating wasrecorded for a defect, such as a stem scar, the corresponding shell thickness at thatposition was less than the theoretical required shell thickness at the same position.Based on these correlations and related interpretations, the tree failure potentialUSDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.867
Dangerous Tree Assessment Process—Manning, Bradford, White, Rowe, Densmore, and Guyratings and associated safety procedures used in the “Wildlife/Danger TreeAssessor’s Course” (WDTAC) appear to be justifiable and reliable.The results of this research will ultimately be used to improve dangerous treeassessment procedures and related safety training methods in the WDTAC. Threecourse modules currently exist for the WDTAC: forest harvesting and silviculture;wildland fire fighting; and parks and recreation sites.The WDTAC is jointly sponsored by the British Columbia Ministry of Forests,the Workers’ Compensation Board of B.C., and the B.C. Ministry of Water, Land andAir Protection. Specific details (e.g., level of disturbance/work activity ratings) andtechnical procedures associated with the four-step dangerous tree assessment process,as well as additional information about WDTAC modules, can be found on theWildlife Tree Committee of British Columbia web site: www.for.gov.bc.ca/hfp/wlt/.AcknowledgmentsThe authors would like to thank the members of the Wildlife Tree Committee ofBritish Columbia and the WDTAC course instructors for their support in thedevelopment of the new dangerous tree assessment process. We would also like tothank Dr. Eric Allen (Canadian Forest Service), Dr. Duncan Morrison (CanadianForest Service), and Alan Lundgren (TimberWest Forest Ltd.) for their help with treepathology and development of defect indicators.ReferencesManning, E. T. 2001. British Columbia’s dangerous tree assessment process―Implications for worker safety (destructive sampling field project). Final reportprep. for IWA Canada-Forest Industry SAFER Council, Weyerhaeuser B.C. CoastalGroup, and BC Ministry of Forests. June 2001; 15 p.Maser, C.; Anderson, R. G.; Cromack, K.; Williams, J. T.; Martin, R. E. 1979. Dead anddown woody material. In: Thomas, J. W., tech. ed. Wildlife habitats in managedforests–the Blue Mountains of Oregon and Washington. Agric. Handb. 553.Washington, DC: Forest Service, U.S. Department of Agriculture; 78-95.Matheny, N.; Clark, J. 1991. Evaluation of hazard trees in urban areas. Urbana, IL:International Society Arboriculture; 72 p.Mattheck, C.; Breloer, H. 1997. The body language of trees—a handbook for failureanalysis.London, UK: The Stationery Office; 239 p.Smiley, E. T.; Fraedrich, B. R. 1992. Determining strength loss from decay. Journal ofArboriculture 18(4): 201-204.Wagener, W. 1963. Judging hazard from native trees in California recreational areas: aguide for professional foresters. Res. Paper PSW-P1. Berkeley, CA: Forest Service,U.S. Department of Agriculture; 29 p.Wildlife Tree Committee of British Columbia (WTC). 2001. Wildlife/danger tree assessor’scourse workbook―forest harvesting and silviculture module. Victoria BC: BritishColumbia Ministry of Forests, Workers’ Compensation Board, Minerals Water, Landand Air Protection, June 2001; 139 p.Workers’ Compensation Board of British Columbia (WCB). 1998. Occupational Health &Safety regulation, industry/activity specific requirements, parts 20-33. Richmond,BC: Workers’ Compensation Board of British Columbia, April 1998; 151 p.868USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002. 863 British Columbia’s Dangerous Tree Assessment Process1 Todd Manning,2 Peter Bradford,3 Cary White,4 David Rowe,5 Nancy Densmore,6 and Stewart Guy7 Abstract New dangerous tree assessment guidelines have recently been developed by the Wildlife Tree Committee of British Columbia.
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