A panel of 23 experts from the Internation Society of Arboriculture (ISA) is crafting a best management practive (BMP) for integrated vegetation management (IVM) to serve as a field guide for front-line supervision, as well as an aid for managers to help facilitate planning.
The need for a BMP for utility rights of way (ROW) became evident in the Federal Energy Regulatory Commission's (FERC) 2004 Utility Vegetation Management Final Report. Researchers concluded that current industry requirements and standards are inadequate to require utility companies to achieve the level of utility vegetation management necessary to improve reliability by reducing tree-caused transmission outages.
The BMP will equip practitioners with techniques that they can apply to electric ROW projects in order to exceed the requirements of the North American Electric Reliability Council (NERC). The NERC 2006 Standard FAC 003-1 Transmission Vegetation Management Program is designed to protect the transmission electric grid from tree-caused outages.
BEST MANAGEMENT PRACTICES
The ISA developed BMPs as companions to the American National Standards Institute (ANSI) standards. ANSI A300 is the standard for tree-care operations and tree, shrub and other woody plant maintenance. It also provides minimum performance standards for use in arboricultural specifications. ANSI A300 provides direction on what to do, while the ISA BMPs offer information on how to do it. The IVM BMP is designed to supplement ANSI A300 Part 7, which deals with IVM. It can also be used to fulfill other objectives, such as vegetation control on gas pipeline ROW and activities outside the scope of utility ROW management, including restoring ecosystems, controlling invasive weeds and other actions. The BMP will contain sections on safety, communication, planning and implementation, and application.
PLANNING AND IMPLEMENTATION
BMPs call for a systematic way of planning and implementing a vegetation- management program that complies with the NERC standard. It is applicable to distribution as well as transmission projects and consists of six elements:
- Set objectives
- Evaluate the site
- Define action thresholds
- Evaluate and select control methods
- Implement IVM
- Monitor treatment and quality assurance.
Decisions are required in setting objectives, defining action thresholds, and evaluating and selecting control methods. The process is cyclical and ongoing because vegetation is dynamic, and managers must have the flexibility to adjust their plans at each stage as new information becomes available.
Objectives should be clearly defined and documented. Examples of objectives include promoting safety, preventing outages caused by vegetation growing into electric facilities and minimizing outages from trees growing outside the ROW. Other sample objectives are maintaining regulatory compliance, restoring electric service during emergencies, protecting structures and security, maintaining access and clear lines of sight, protecting the environment and facilitating cost effectiveness.
Objectives should be based on specific site factors such as workload and vegetation type, in addition to human, equipment and financial resources. The overriding focus should be on the environmentally sound, cost-effective control of species that potentially conflict with electric facilities, while promoting compatible, early successional, sustainable plant communities.
Workload evaluations are inventories of vegetation that could have a bearing on management objectives. Workload assessments can draw data on an array of vegetation characteristics, such as location, height, density, species, size and condition, hazard status and clearance from conductors. Assessments should be conducted with voltage, conductor sag from ambient temperatures and loading, and the potential influence of wind-on-line sway in mind.
Comprehensive program-level evaluations can be made of all target vegetation on a system, while project-level evaluations focus on vegetation relevant to a specific job. On one hand, comprehensive evaluations provide the advantage of supplying a complete set of data upon which to base management decisions. On the other hand, the surveys can be impractical for utilities with large numbers of trees, limited human and financial resources, or both.
Point sampling offers an alternative for utilities for which comprehensive inventories are impractical. It involves dividing a management area (a system or project) into equal-sized units and selecting a random sample sufficient to statistically represent the total work quantity. Every plant or plant community of interest within each selected area is inventoried, with collected data used to forecast the total workload.
Managers have a variety of control methods from which to choose, including manual, mechanical, herbicide and tree-growth regulators, biological and cultural options.
Manual methods employ workers with hand-carried tools, including chain saws, handsaws, pruning shears and other devices to control incompatible vegetation. The advantage of manual techniques is that they are selective and can be used where others may not be. However, manual techniques can be inefficient and expensive compared to other methods.
Mechanical controls are done with machines. Although mechanical control methods are efficient and cost effective — particularly for clearing dense vegetation during initial establishment or reclaiming neglected or overgrown ROW — they can be nonselective and disturb sensitive sites.
Tree-growth regulators and herbicides are essential for effective vegetation management. Tree-growth regulators are designed to reduce growth rates by interfering with natural plant processes. They can be helpful by reducing the growth rates of some fast-growing species where removals are prohibited or impractical.
Herbicides control plants by interfering with specific botanical biochemical pathways. Herbicide use can control individual plants that are prone to resprout or sucker after removal. When trees that resprout or sucker are removed without herbicide treatment, dense thickets develop, impeding access, swelling workloads, increasing costs, blocking lines of site and deteriorating wildlife habitat. Treating suckering plants allows early successional, compatible species to dominate the ROW and out-compete incompatible species, ultimately reducing work.
Herbicides can be selective or nonselective depending on their type. Selective herbicides only control specific kinds of plants, when applied according to the label. By contrast, non-selective herbicides work against most plants. Nonselective herbicides can be effective where a wide variety of target plant species are present, such as during initial clearing or reclaiming dense stands of invasive or other undesirable plants.
Application techniques also can be either selective or nonselective. Selective applications are used against specific plants or pockets of plants. Nonselective techniques target areas rather than individual plants. Nonselective use of nonselective herbicides eliminates all plants in the application area. Nonselective use of a selective herbicide controls treated plants that are sensitive to the herbicide without differentiating between compatible or incompatible species. Selective use of either would only control targeted vegetation. Selective use is preferable unless target vegetation density is high.
HERBICIDE APPLICATION METHODS
Herbicide application methods are categorized by the quantity of herbicide used, the character of the target, vegetation density and site parameters. Treatments include individual stem, broadcast and aerial treatments.
Individual stem treatments are selective applications. They include stump, basal, injection, frill, selective foliar and side-pruning applications. Due to their specific nature, proper individual stem applications work well to avoid damage to sensitive or off-target plants. However, they are impractical against broad areas or for sites dominated by undesirable species.
Broadcast treatments are nonselective because they control all plants sensitive to a particular herbicide in a treatment area. They can provide a degree of selectivity with proper herbicides. Broadcasting is particularly useful to control large infestations of incompatible vegetation (including invasive species) in ROW or along access roads.
Aerial control methods are also nonselective but can provide a level of selectivity with proper herbicides. Aerial applications are useful in remote or difficult-to-access sites, and can be cost effective and quick, especially if large areas need to be treated. They also can be used where incompatible vegetation dominates a ROW. The primary disadvantage of an aerial application is that it carries the threat of off-target drift, so it must be performed under low-wind conditions with low-toxicity herbicides.
CULTURAL CONTROL METHODS
Cultural methods modify habitat to discourage incompatible vegetation and establish and manage desirable, early successional plant communities. Cultural methods take advantage of seed banks of native, compatible species lying dormant on site. In the long run, cultural control is the most desirable method where it is applicable.
Cultural control, also known as cover-type conversion, provides a competitive advantage to short-growing early successional plants, allowing them to thrive and eventually out-compete unwanted tree species for sunlight, essential elements and water. The early successional plant community is relatively stable, tree-resistant and reduces the amount of work, including herbicide application, with each successive treatment.
The wire-border zone technique is a management philosophy that can be applied through cultural control.
The wire zone is the section of a utility transmission ROW directly under the wires and extending outward about 10 ft on each side. The wire zone is managed to promote a low-growing plant community dominated by grasses, herbs and small shrubs (under 3 ft [1 m] in height at maturity). The border zone is the remainder of the ROW. It is managed to establish small trees and tall shrubs (under 25 ft [7.6 m] in height at maturity). When properly managed, diverse, tree-resistant plant communities develop in wire and border zones. The communities not only protect the electric facility and reduce long-term maintenance, but also enhance wildlife habitat, forest ecology and aesthetic values.
The wire-border zone is a best practice in many instances but is not suitable in all situations. For example, standard wire-border zone prescriptions may be unnecessary where lines are high off the ground, such as across low valleys or canyons. One way to accommodate variances in topography is to establish different regions based on wire height.
For example, over canyon bottoms or other areas where conductors are 100 ft (30 m) or more above the ground, only a few trees are likely to be tall enough to conflict with the lines. In those cases, trees that potentially interfere with the transmission lines can be removed selectively on a case-by-case basis. In areas where the wire is lower, perhaps between 50 ft to 100 ft (15 m to 30 m) from the ground, a border zone community can be developed throughout the ROW. Where the line is less than 50 ft off the ground, managers could apply a full wire-border zone prescription.
Stream protection is an environmental advantage of this type of modification. Streams often course through the valleys and canyons where lines are likely to be elevated. Leaving timber or border-zone communities in canyon bottoms helps shelter this valuable habitat, enabling managers to achieve environmentally sensitive objectives.
All laws and regulations governing IVM practices, and specifications written by qualified vegetation managers must be followed. IVM control methods should be implemented on regular work schedules, which are based on established objectives and completed assessments. Work should progress systematically, using control measures determined to be best for varying conditions at specific locations along a ROW. Some considerations used in developing schedules include the importance and type of line, vegetation clearances, workloads, growth rate of predominant vegetation, geography, accessibility and in some cases, time lapsed since the last scheduled work.
CLEARANCES FOLLOWING WORK
The transmission owner should establish and document appropriate minimum-clearance distances to be achieved at the time of work. These clearances are defined and required as clearance 1 in the NERC transmission vegetation-management program standard. A qualified vegetation manager should determine appropriate clearances for the vegetation type on a system. Clearances following work should be sufficient to meet management objectives, including reducing electric safety risks, service-reliability threats and cost. At the very least, post-work clearances should be sufficient to maintain minimum NERC-mandated clearances until the next systematic work.
MONITOR TREATMENT AND QUALITY ASSURANCE
Vegetation-management programs should have systems and procedures in place for documenting and verifying that the vegetation-management work was completed to specifications. The NERC Transmission Vegetation Management Program requires transmission owners to follow these systems and procedures. Post-control reviews can be comprehensive or based on a statistically representative sample. This final review points back to the first step and the planning process begins again.
Managers should select control options to best promote management objectives. Tree-resistant plant communities can help reduce long-term workloads and costs, because once established, they out-compete incompatible plants. When effectively implemented, IVM is a systematic, preventive strategy that results in site-specific treatments to meet management objectives. A sound program includes documented processes to evaluate results, which should involve both monitoring for quality assurance while work is underway and after it is completed. However, the overriding focus should be on environmentally sound, cost-effective control of species that potentially conflict with the electric facility while promoting compatible, early successional, sustainable plant communities where appropriate.
Randall H. Miller is vice president of the Utility Arborist Association, past chair of the Edison Electric Institute Vegetation Management Task Force and an editorial board member of the Journal of Arboriculture and Urban Forestry. He has served on the International Society of Arboriculture's (ISA) Certification Test Committee and is past president of the Oregon Urban and Community Forest Council. Miller joined PacifiCorp in 1993 and has been system forester with them since March 1999. Miller earned his bachelor's degree in horticulture and master's degree in urban forestry. He is an ISA-certified arborist and certified utility specialist. Miller has more than 40 arboriculturally related writing credits and speaks widely on arboricultural, urban forestry and utility forestry-related topics. firstname.lastname@example.org