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Utility Vegetation Management in the Urban Forest
As a result of the Northeast blackout of August 2003 where the lack of vegetation maintenance allowed a tree to knock out power for over 50 million people the Federal Energy Policy Act (EPAct) was passed in June of 2005. The Federal rules require “vegetation clearance to prevent flashover between vegetation and supply conductors.” This mandate for utility companies requires that utilities keep electrical transmission lines operational. These regulatory mandates requires that electrical utilities eliminate all power outages caused by vegetation interference with power transmission lines. Companies failing to meet the requirements face penalties and fines of up to $1 million per occurrence per day. This is quite an incentive for power companies to remove vegetation, which may be hazardous to their lines. However, this incentive is not a mandate to improperly prune trees, prune trees that are not required to be pruned or remove trees that fall below clearance standards. The EPAct only applies to transmission lines operated at 200kV or higher and any lower voltage line designated by the utility as critical to the reliability of the electrical system. For reference, 200kV lines are multi-line, large tower transmission systems not general residential or 3-phase commercial lines. Therefore, there is no Federal mandate to remove all vegetation from or near all utility lines. This is left as a company decision on vegetation management.
The EPAct requires the utility to prepare and keep current a formal Transmission Vegetation Management Program (TVMP). The TVMP is required to include the utility company’s objectives, practices, approval procedures and work specifications. It also must include a work schedule based on the anticipated growth of vegetation and other factors that could affect the relationship of vegetation to transmission lines. Utility companies have routinely used the National Electrical Safety Code (NESC) (ANSI C2 – 2002) to determine line clearance distances instead of the Federal Guidelines (FAC-003-1), which uses IEEE Standard 516-2007 for determining the correct clearance and allows for consideration of local conditions.
Beyond natural disasters and land development, utility pruning and removal can be a significant impact on the urban forest. In general trees are caught between utilities trimming from the top down and municipal arboriculture practices to provide clearance for sidewalks and vehicles by trimming from the bottom up. This creates serious problems for the trees.
While not universally practiced, many utility companies or their contractors continue the practice of "topping" or "round-over" pruning. This technique describes a method of pruning where cuts are made at arbitrary points on a branch along a uniform plane within the crown. The resulting crown form is artificially uniform following pruning. Unfortunately, topping causes more problems than it solves. Because the tree sprouts grow rapidly, the tree must be pruned frequently.
Two commonly accepted pruning alternatives are "lateral drop crotch pruning," also known as "natural pruning," and "side-pruning." Lateral or natural pruning requires the tree trimmer to pick out the branches growing toward the lines and remove them where they attach to the next lateral limb. Properly placed cuts produce few sprouts and natural growth rates. The next cycle of trimming requires fewer cuts than the topping approach, because the problem branches were removed during the previous cycle and sprout growth has been slower. This method is used both on trees growing beneath or to the side of utility lines
Problems between trees and utility lines generally occur because the wrong species has been planted directly underneath or adjacent to power lines. Under certain circumstances when this occurs there may be no other alternative short of removal and replanting.
Increasingly, utilities install underground distribution lines that require trenches. These trenches frequently damage tree roots, which slows tree growth, and leads to decline of the crown and root system. Branches may die, increasing the chances of wind damage and invasion of wood decaying fungi or insects. Augering, tunneling, or boring through the root zone of the tree will cause less damage to the root system.
Safety and liability are issues of concern to everyone. Dead or dying trees in a park or along a street will eventually fall, possibly causing injuries or fatalities. If such trees are close to overhead power lines, their failure could tear down lines and poles, causing outages and other damage. Vehicular and pedestrian traffic is another concern. Street trees must be managed to ensure visibility and clearance for streetlights and signs.
Below are types of utility pruning that may save a tree under or adjacent to a utility right-of-way from being removed .