Vegetation management practices on power line rights-of-way (ROW) matter to utilities and power consumers, primarily in terms of reliability of power delivery and the number of outages experienced. The ROW steps a utility takes have an impact on its customers’ perception of the utility as well as on the dynamics between the local wildlife that live on the ROW and the plant communities that sustain them.
For plants and animals, the stakes are quite a bit different. Results of 60 years of ecological research on Pennsylvania transmission ROW demonstrate plant communities can be managed selectively to support both reliable electric service and a diverse plant community for wildlife habitat.
State Game Lands 33
The Pennsylvania State Game Lands 33 (SGL33) research project in central Pennsylvania (U.S.) began in 1953 in response to public concern, particularly from hunters, about the impact of vegetation management practices on wildlife habitat within electric transmission ROW. Today, SGL33 is the site of the longest continuous study measuring the effects of herbicides and mechanical vegetation management practices on plant diversity, wildlife habitat and wildlife use within a ROW. Similar studies have been conducted since 1987 at a companion site, Green Lane Research and Demonstration Area (GLR&D), in southeastern Pennsylvania. Both projects provide valuable information for understanding the response of plants and animals to vegetation management on ROW.
The three original research objectives of the SGL33 project were as follows:
• To compare the effectiveness of commonly used vegetation management practices on controlling trees incompatible with management objectives for ROW function
• To develop tree-resistant plant cover types
• To determine the effect of vegetation management on wildlife habitat and wildlife species of high public interest.
The practices studied were hand-cutting, herbicides and mowing alone or in combination with herbicides applied across the entire width of the ROW. Although herbicides and application methods varied over the years as new products and techniques became available, specific treatments (whether mowing, hand-cutting or herbicides) have remained consistent since 1953.
In general, integrated vegetation management (IVM) on transmission ROW is a two-step process. First, vegetation managers use herbicides to control undesirable trees incompatible with management objectives for ROW function. Second, they must use appropriate management practices to help develop tree-resistant plant cover that will reduce reinvasion of tall-statured trees such as white oak or red maple.
Researchers began documenting game species such as white-tailed deer and eastern cottontails on treated SGL33 sites in the 1950s, and continue to monitor and measure plant and animal biodiversity within both study areas. From 1982 to the present, a concerted effort has been made to examine wildlife ROW usage through a series of studies focusing on songbirds, large and small mammals, butterflies, amphibians and reptiles.
The studies show plant communities can be changed with the use of appropriate herbicides and application methods, and vegetation management practices can result in diverse vegetation that provides forage and habitat for wildlife on ROW. Thus, plant communities can be created to inhibit tree establishment, thereby reducing maintenance costs for utilities and mitigating the potential for power outages.
When a transmission ROW is cleared initially, a short-term decrease in total vegetative cover occurs. Following tree canopy removal, plants that tolerate high levels of sunlight increase in dominance, and tree seeds present in the soil germinate and grow. Thus, follow-up management is necessary for maintaining a low-growing plant community to optimize the safe and reliable transmission of electricity.
Data collected from the SGL33 and GLR&D sites indicates that herbicide treatments to remove incompatible species such as tall-statured trees produce a distinct change in the plant community. Post-treatment vegetative cover ranges from grasses to herbicide-tolerant wildflowers, shrubs and small trees. These new plant communities are relatively stable and have diversity that equals or exceeds nontreated areas.
The data also shows ROW vegetation managers can predict cover types and develop the kind of vegetation desired in a particular situation by prescribing appropriate maintenance. Management units treated with herbicides alone or in combination with mowing had fewer incompatible trees per acre within the wire zone compared to units with mowing alone or hand-cutting treatments. The diverse plant community created within the ROW as a result of vegetation management practices produces a variety of native species important for wildlife food and cover.
Bird populations have been studied extensively on the SGL33 ROW since 1982. More than 40 bird species have been noted on the ROW, with the most common being those that nest in brushy or grassy vegetation created by IVM practices. Bird populations proved to be more plentiful in the treated ROW than an adjacent forest, including herbicide-treated units, especially those with basal and foliar methods of application. In areas treated with herbicides, 712 birds were observed per day per 100 acres (40 hectares) compared to 552 birds on areas mechanically maintained. There were 44 different species of birds counted in 2000 to 2001 on the ROW compared to 39 in 1987 to 1988.
Properly maintained vegetation within a ROW benefits many bird species, especially those adapted to brushy, early successional habitats. In the northeastern U.S., populations of bird species using early successional vegetation as a group are declining faster than other groups such as forest or wetland birds. Therefore, a properly managed ROW is key to the conservation of birds such as the chestnut-sided warbler or eastern towhee that require early successional habitat.
The nesting ecology study conducted from 1991 to 1992 showed an increased survival for birds raising young in a well-managed ROW. Overall, the nesting success along the ROW was 66%, which is higher than the approximately 50% success reported in other studies of songbirds, including those conducted in managed forest stands (clear-cuts) in central Pennsylvania. Six different native plant species —blackberry, witch hazel, mountain laurel, hay-scented fern, blueberry and poverty grass — provided a variety of nest sites within the ROW for different bird species that depend on this linear, early successional habitat for breeding.
Reptiles and Amphibians
Forest-management practices such as clear-cutting can negatively impact some species of amphibians and reptiles. A two-year research study of amphibian and reptile populations on the SGL33 and GLR&D sites concluded the ROW contained a diverse assemblage of these species. Depending on the location, eight to nine different species were recorded on the ROW versus only two recorded in the adjacent forest. The most common species were red-backed salamander, Jefferson salamander, northern redbelly snake and northern ringneck snake.
Border zones were valuable habitat to salamanders, whereas wire zones were used most often by snakes. The ROW contains a much more diverse community of reptiles and amphibians than the adjacent forest and provides an acceptable habitat for these important species of wildlife.
Small mammals are important components of any ecosystem, including ROW. From an ecological perspective, small mammals serve as prey for predators and are major links in the food chain.
A two-year study was conducted on SGL33 to determine the relative abundance and species richness of small mammals on the ROW compared to the adjacent forest. Results of the study showed eight species of small mammals — compared to only two in the adjacent forest — were noted on the ROW:
•Five species of mice: the whitefooted mouse, meadow vole, red-backed vole, woodland jumping mouse and meadow jumping mouse
•Two shrew species: short-tailed and masked
•One short-tailed weasel species.
From the findings of this study and a companion study, it can be concluded specific treatments on the ROW produce cover types that benefit small mammals compared to the adjacent forest cover type. In addition, small mammals use a diversity of cover types, from grass to shrub, found on the ROW. Evidently, the ROW serves as a large forest clearing, which provides habitat for forest species (for example, a white-footed mouse and woodland jumping mouse) in border zones and habitat for early successional species (for example, meadow vole and meadow jumping mouse) in wire zones.
Butterflies are important indicators of environmental changes and barometers of a healthy ecosystem. They are valuable pollinators to many wildflowers and a food source for songbirds, small mammals and other wildlife. Habitat loss has caused some butterfly populations to decline nationally.
A two-year study on the SGL33 and a companion study on GLR&D sites compared butterfly populations on hand-cutting units versus herbicide-treated units. Results show the same or slightly more butterfly species occurred on the ROW than in the adjacent forest, and were more common in herbicide-treated units than on hand-cutting units. Common native butterfly species included aphrodite fritillary, little wood satyr, monarch, spicebush swallowtail, eastern tiger swallowtail and the exotic European skipper.
A major factor affecting the abundance and diversity of butterflies on the ROW was the presence and use of flowering plants as nectar (food) sources during the growing season. The use of herbicides as part of IVM practices promoted a rich wildflower community and habitat that supports a diverse butterfly community on the ROW.
White-tailed deer habitat and its use were evaluated on the SGL33 ROW before and after vegetation management treatments, and then compared to the adjoining forest. Studies found IVM treatments within the ROW caused a shift in vegetation, but suitability of the habitat for deer remained high. Deer use of woody plants also was found to be greater in the adjoining forest compared to the ROW, where more herbaceous vegetation was browsed. Deer use in the ROW was 48% greater than in the adjacent forest, and deer can have a positive impact on a ROW by browsing on incompatible trees in wire and border zones as well as by providing aesthetic value to a ROW.
Benefits in Practice
To sum up, long-term studies conducted on the SGL33 and GLR&D sites have shown economic, aesthetic and wildlife habitat benefits associated with IVM practices on transmission line ROW. This information is critical to helping ROW managers implement proper vegetation management practices that meet the needs of their industry, the public and wildlife. Future research will be shaped based on the needs of the utility industry to address conservation issues, new vegetation management techniques and concerns generated by the public and scientific community.
The author would like to thank her partners on this project: university researchers, Dow AgroSciences, Asplundh Tree Expert Co., First Energy, PECO Energy Co. and the Pennsylvania Game Commission.
Celestine Duncan ([email protected]) is the owner of Weed Management Services, which was started in 1988. Duncan holds a BS degree in agronomy/horticulture from New Mexico State University and MS degree in agronomy (weed science) from Montana State University. She conducts field research, environmental assessments and training programs throughout the Northwest on invasive plants. She is the editor of Techline, a suite of print and online resources the provides invasive plant professionals access to new, innovative and proven science-based information.
Editor’s note: References that support key findings in this article are available online at http://brambleandbyrnes.com/overview-of-studies/.