In Defense of Utility Pruning

The work of utility arborists is out there for all to see, and most people have an opinion about it. There is certainly plenty of discussion, both pro and con, about the amount of pruning and the appearance of trees following the work. What many do not understand is why the work is necessary and the potential danger posed by high-voltage primary lines – often 100 or more times greater than household current.

It is also true that people generally love trees near power lines on beautiful days, but are less charitable when a tree failure takes out their electric service.

This article explains how utilities walk the line between these interests, how some utilities fall short and what we should all know about how to apply the science of tree care when working near utility facilities. It also discusses the importance of green infrastructure in the form of a healthy, well-maintained urban forest, which makes life better for all of us.

Photo 1: The failure of large trees and branches causes physical damage to utility facilities. Unless otherwise noted, all photo courtesy to the author.

The value of electric-service reliability

Placing value on electric-service reliability can be difficult. From the utility side, when the power is out, revenue is lost and costs are incurred to repair damaged infrastructure, though these costs are generally recovered in the rates paid by utility customers.

The real value of reliability is downstream, as measured by lost business when the power is out – factories idled, sales lost, data unprocessed. Estimates vary, but government analysis suggests that power interruptions cost our economy more than $100 billion annually. One measure suggests that end users value reliability at anywhere from 10 to 1,000 times the retail price for electricity. In short, this tells us that utility customers, whether homeowners or businesspeople, really hate it when the power goes out.

Clearance, or risk reduction?

Most utilities cite trees as the most frequent cause of electric-service interruptions. Furthermore, large branches and whole trees falling from outside of utility easements are most often the culprits. An outage occurs when the weight or momentum of falling trees is great enough to cause physical damage (e.g., broken poles, lines pulled to the ground) (Photo 1), or when a branch or tree leans against energized conductors or crosses the phases, leading to a fault. In this second case, the tree or branch must be large enough, have a certain amount of moisture and be in contact with the conductors long enough for a fault path to develop. (Photos 2 and 3)

Photo 2: In this high-voltage demonstration at a Bonneville Power Administration facility, a 3-inch diameter, 5-foot-long section of green tree branch was placed phase-to-ground. As the voltage increased, it took only a few seconds for the branch to heat up- and for water to be forced out of the branch as steam – before a carbon path formed to create a short.

Small-diameter sprout growth is far less likely to cause an interruption. A 2001 study of a utility in upstate New York found that 86% of the interruptions on the system were caused by trees and branches falling in from off the ROW, and that small-diameter growth posed relatively little risk and did not cause significant loss of power due to incidental contact. Similar findings have been noted by other utilities.

Since the greatest risk to utility facilities is from failure of larger-diameter trees and branches, it follows that utilities should direct more resources toward mitigating this threat. So why is so much time and money spent on clearing small-diameter sprouts and branches that pose relatively little risk?

Utilities focus on clearance pruning for several reasons. First, given enough time, trees growing into high-voltage power lines will cause outages and other public-safety concerns, including the risk of wildfire. Clearance pruning also opens critical sightlines for utility personnel to access and inspect their facilities. (Utility easements allow access without permission from the property owner.) And, perhaps most important, clearance is relatively easy to specify, price, perform and inspect.

Photo 3: The branch from Photo 2, showing the blackened carbon path where the fault developed.

To be most effective, clearance pruning should be performed before growth reaches power lines and at intervals such that relatively small amounts of biomass are removed. Longer intervals result in the need to prune more, which often causes fast-growing sprouts to quickly overtake the line. And, of course, the dramatic changes in the appearance of infrequently pruned trees leads to unhappy tree owners and more community complaints.

If not done properly, clearance pruning may actually increase the likelihood of failure to utility facilities. In some cases, clearance pruning simply amounts to “lion tailing,” which ANSI A300 standards describe as the removal of “… interior lateral branches, resulting in a concentration of growth at branch ends.” (Photo 4) A300 labels lion tailing “an unacceptable pruning practice.”

Photo 4: Focusing solely on clearance may actually increase risk of failure. In this case, the overextended branch above the conductors was untouched, while small, interior lateral branches below were removed.

Preventing tree failure and impact to utility facilities may seem like an impossible task, considering the vast number of line miles that are within the target zones of trees. One utility forester described the problem this way: “Attempting to determine which tree along a ROW will fail is like looking at a pot of beans and trying to figure out which one will make me fart.” Furthermore, many trees are not on the utility easement. Securing permission to prune or remove such trees requires negotiation with sometimes reluctant property owners. All of this requires time and effort and, of course, money.

The alternative to clearance pruning – systematically identifying trees with a higher risk of failure and impact to utility facilities – requires professional expertise and a prescriptive, more costly approach. Fortunately, research into tree biomechanics has increased our understanding of the problem. Standards and best-practices guides have been written. Thousands of arborists are now ISA Tree Risk Assessment Qualified (TRAQ) and have been trained to find concerns before failures occur. Many utilities have piloted hazard-tree-mitigation programs, some with demonstrated positive results. But considering the cost of repairs following tree failures and the value of reliability to end users, it is surprising that the utility industry has not invested more resources in tree-risk reduction.

Storm-risk preparedness

There are many factors besides trees that utilities must consider when preparing for storms, starting with understanding what kinds of storms are most likely to strike. The age, condition and design of utility infrastructure also has much to do with how well a utility fares. But, like responsible homeowners, utilities must recognize that systematic tree maintenance will reduce tree damage when storms strike.

Extreme events that are considered outside of “normal” weather – a Category 5 hurricane, a tornado or an extraordinary accumulation of ice – will cause catastrophic damage to trees and utility infrastructure no matter what. Fortunately, such events are rare in any given location, although climate change appears to be increasing both the frequency and intensity of storms.

How systems perform in more routine, expected weather events – and how utilities respond when damage occurs – tests the preparedness of utilities and communities. Maintaining appropriate clearance is a factor, as both trees and power lines move considerably with increased loads caused by wind and precipitation. But to reduce major damage from large-tree and -branch failure, risk evaluation and appropriate mitigation are necessary.

Photo 5: Utility infrastructure is seldom celebrated for its aesthetics.

Defining the appropriate tree

Nobody admires utility lines for their appearance. (Photo 5) People have complained about them and the treatment of adjacent trees since the first telegraph lines were strung in the 1840s. Early on, lines were often attached to living trees. In 1918, Alfred Gaskill, New Jersey’s state forester, promoted this idea in a publication that decried the “butchering” of trees by power companies. (Figure 1) While rare, such attachments are still occasionally in use today.

In the preface to his 1940 textbook, Tree Clearance for Overhead Lines, utility forester George Blair emphasized that, “… trees must be vigorous and beautiful; overhead-line service must be continuous and dependable. In this measure of quality, each is essential to the happiness of civilized people.” Clearly, the value of trees in and around where people live has been understood by all parties for generations.

For decades, the term “right tree, right place” has been the mantra of utilities when informing the public about tree planting. But this phrase means different things to different people. We can all agree that planting trees with the potential to grow to 100 feet or more is a bad idea under any power line. But urban foresters also are eager to increase beneficial canopy cover in their communities, and view streetside utility easements as fertile ground for medium-sized-canopy trees that would require minimal utility pruning.

Figure 1: In an official 1918 publication, the New Jersey state forester suggested that “…crossarms and insulators be fastened to strong trees, rather than set poles.” Image courtesy of New Jersey State Forest Service.

Common ground

Clearly, utilities, municipalities and property owners have an interest in promoting good tree care. Well-maintained, healthy trees provide a range of benefits, are safer and are less likely to cause service interruptions.

Some of the benefits provided by urban forests accrue directly to utility interests. Streets, parking lots and buildings absorb heat from the sun, creating “urban heat islands” that are significantly warmer than surrounding areas. This increases the energy required to cool homes and businesses. Meeting the summer peak demand is costly for utilities and further drives up climate-warming greenhouse gas (GHG) emissions.

Electric utilities are under intense pressure to reduce their net GHG emissions. Two ways to do this are reducing demand and capturing and storing excess carbon. Urban forests do both; they keep cities cooler, which significantly reduces peak demand for electricity, and, as trees grow and add biomass, they capture and store carbon. The larger the tree, the more cooling shade and the more carbon captured.

Research has clearly shown that the value of the benefits provided by trees far outweighs the cost of maintenance. This is where utilities can work with urban foresters to plant medium-sized, decurrent-canopy trees that mature at or below power-line height and require minimal pruning. (Photo 6) This, along with updated utility distribution systems designed to be compatible with such trees, can be a win for everyone.

Photo 6: These live oaks with a natural spreading habit pose relatively low risk while providing valuable cooling shade.

Going green

Utilities have long viewed trees as a problem to be solved, rather than a solution to problems. At a conference for upper-level utility managers several years ago, one utility executive suggested that “tree trimming is a necessary evil.” He was right that tree pruning is necessary, but wrong about “evil” – neither the trees nor the people who maintain them are evil. Another session was titled, “Tree Trimming – Will it Ever End?” The answer, of course, is “no” – at least not as long as civilization persists. Of course, when necessary, trees should be pruned properly to reduce risk.

Across their operations, utilities are taking a different approach to environmental concerns. With climate change, green infrastructure is a critical component of our urban environment that requires ongoing maintenance and occasional replacement. Utilities are a natural partner in creating and maintaining a healthy, vibrant urban forest. We should encourage them in every way we can.

Geoffrey Kempter is a technical services manager with Asplundh Tree Expert, LLC, a 47-year TCIA member company based in Willow Grove, Pennsylvania. He was the chair of the 2017 ASC A300 Pruning Revision Subgroup. He has served as the Asplundh representative on the ASC A300 Committee since 1996, and was the recipient of TCIA’s 2018 Pat Felix Volunteer of the Year Award.

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