|Tree hazard evaluation is best accomplished in
three steps; identification, documentation, and corrective action.
Recreation sites should be stratified into tree hazard risk zones before beginning a tree inspection (Figure 19). Plan your route through the area to include evaluation of all trees within areas of intensive public use. Trees 8 inches or greater in diameter at breast height should be carefully evaluated since 62 percent of reported failures occurred in these sizes of trees.
|Inspection intensity should vary directly with
the risk level. All trees within falling distance of targets (structures,
vehicles, or recreationists) should be inspected. The height of hazardous trees
projected to the ground determines the width of the hazard zone. Inspections
should be concentrated in High Risk Zones (1) because people and most
structures are concentrated in these areas. This zone includes areas around
designated camp or picnic units and along major roads and trails. Medium Risk
Zones (11) should be inspected commensurate with amount and type of use. Low
Risk Zones (I 11) have a reduced potential for damage and therefore regular
inspections have low priority.
Trees must be evaluated individually. Due to differences in site, micro-climate, developmental history, and inherent genetic characteristics, trees vary in hazard potential. Defects must be considered in relation to external factors such as prevailing winds, snow loads, location with respect to other trees, relative vigor, and distance and direction from a target.
Recreation site managers should be cognizant of the need to manage all vegetation in developed sites. Proper management will extend the useful life of such sites and perhaps avoid costly renovation.
The purpose of a hazard tree evaluation is NOT to remove every tree that exhibits defects; rather, the goal is to preserve the greatest number of trees in recreation areas consistent with safety requirements. Removal of too many trees in an area can destroy the aesthetic qualities for which the site was selected. Also, stand stability may be affected and the probability of wind-throw increased.
Tree inspection should be systematic. Tools necessary for this activity include binoculars, hand axe, hand saw, increment borer, diameter tape, compass, 50-foot retractable loggers tape, camera, and Tree Hazard Evaluation Forms R2-2300-11a (Appendix). When inspecting a tree begin at the base of the tree and work upwards toward the crown noting all defects. Examine all sides of the tree for hazard indicators and take increment cores of suspect trees. Look carefully at the tree base and exposed roots. After noting any structural defects, step back and consider aspects of the environment that may influence the hazard and note the proximity to any targets. In completing the Tree Hazard Evaluation form, assign a risk rating to each tree (high, medium, or low) and decide on the type and priority of corrective action. For high risk trees, remove the tree, the defective portion, or the target, or note on form why no action is to be taken; for medium risk trees monitor the tree for another year or remove the tree or the target; for low risk trees, monitor, do not remove. Trees of high risk would include those with substantial rot defect due to basal wounds and root rot as indicated by fruiting bodies. Weigh the benefits that a tree is providing against the hazard that it poses, then ask yourself; are the benefits worth the risk?
It is extremely important to document hazardous trees. In order to reduce liability, a record is required. Documentation insures that the land manager has systematically inspected the area for hazards. Heavily-used areas should be inspected annually prior to the recreation season. Summer and fall use areas should be inspected in the spring; whereas, winter use areas should be inspected in the fall. Additional inspections are warranted any time following severe storm activity. In fact, for more than 87% of reported failures, wind was listed as a contributing factor.
The Tree Hazard Evaluation Form is designed to aid the evaluator in several ways: (1) to aid in deciding the risk rating of each tree; (2) to ensure all basic hazard information is gathered; (3) to provide program continuity despite personnel changes; and (4) to provide a permanent record and case history for all evaluated sites.
The hazard rating of a tree is determined by three major factors (tree species, potential targets, and defects present). These factors are listed on the Tree Hazard Evaluation Form and are further subdivided into risk values based on past experience and research data provided by Dr. Lee A. Paine (Forest Pathologist, retired, Pacific Southwest Forest and Range Experiment Station). When rating a tree an evaluator checks all situations which are applicable to the factor being considered (i.e., (A) species, (B) target, or (C) defect). After all factors have been considered, the evaluator adds the risk values under each separate factor (total not to exceed 3) and multiplies the sums together, the product being the overall risk rating. In the example on the Tree Hazard Evaluation Form (Appendix) tree 1 is a 12 inch d.b.h. ponderosa pine (Risk Value=2) near Unit # 10 and parking pad (Risk Value=3) with butt rot (Risk Value=3). The risk rating of this tree is Medium (2x3x3=18); however, the amount of sound wood observed on the increment borings is sufficient to keep the tree for now (Figure 7), so the action is to observe the tree until the next intensive inspection (3-5 years). At the next inspection the current form will be used to identify those trees which need special attention. A new form will be completed at that time and will replace the old form in the files.
The back of the Tree Hazard Evaluation Form is designed for the evaluator to map all trees evaluated. This will provide a permanent record of tree location for successive visits to the site.
Corrective action is the final step in the evaluation process. It is also the most expensive and time-consuming but greatly reduces the probability of serious damage, costly cleanup action, or tort claims.
Action should be taken as soon as possible after an evaluation. In some situations hazard reduction can often be accomplished by means other than tree removal. For example, pruning dangerous limbs or stimulating tree vigor may alleviate the hazard.
High-value scenic trees can be reinforced or the defective portion removed. In some instances the target, if portable, (i.e. picnic table) can be moved to a safe distance. Marginally hazardous trees should be recorded on the form, observed over a period of time, and if the risk of failure increases, corrective action should be taken. Closing the recreation site should be considered as a viable option. If corrective action needs to be taken when sites are occupied, the situation should be explained to the public.
|TREE HAZARDS IN
WINTER SPORTS AREAS
Ski areas provide unique problems in hazardous tree management. Considerable alteration of stand composition occurs when ski areas are developed. Formerly protected trees become exposed when stands are opened by construction of ski trails and lift lines. Exposure often leads to greater tree failure due to windfall than occurred before the site was disturbed (Figure 20). The seasonal peak of visitor use in winter sports areas often corresponds to the time of year when many tree failures naturally occur.
Trees adjacent to permanent structures, such as buildings, ski lifts, and along ski trails should be inspected annually for possible hazards prior to the use season. Procedures for examination are the same as those described for other recreation areas. Of special importance in ski areas are trees leaning over structures such as lifts.
Trees with high risk ratings should be removed. Again, the goal of hazard analysis should be to locate and evaluate potential tree hazards and yet retain desirable tree cover consistent with safety requirements.
Figure 20 Windthrow of lodgepole pine along a chair lift at a ski area.