Integrated Vegetation Management on an Electric Transmission Right-of-Way in Southeastern Pennsylvania, U.S.

Vegetation in 1999 Prior to Treatments in June 1999

Excellent to moderate control of target trees was observed in herbicidal units. Excellent control of target trees was noted in wire zones of mowing plus herbicide units (618–642 trees/ha [250–260 trees/ac]), even though 12 years had elapsed since the 1987 treatment (Yahner et al. 1999). Moderate tree density (741–1,976 trees/ha [300–800 trees/ac]) was typical in wire zones of stem–foliage and foliage spray units. In contrast, tree control was poor in wire zones of mechanical units (mowing: 4,446–5,311 trees/ha [1,800–2,150 trees/ac]; handcut: 4,570 trees/ha [1,850 trees/ac]). Tree density in border zones of all units, except handcut, averaged 2,668 trees/ha (1,080 trees/ac); in handcut, density was 3,458 trees/ha (1,400 trees/ac).

Maximum tree height on the Green Lane ROW averaged 4.3 m (14 ft) and 4.9 m (16 ft) in wire and border zones, respectively (Yahner et al. 1999). Hence, trees were dangerously tall and in need of cutting to prevent possible power outages.

Immediately prior to the 1999 treatment of the Green Lane ROW, cover types in wire zones of herbicidal units (mowing plus herbicide, stem–foliage spray, and foliage spray) were classified as shrub–forb–grass–tree, whereas types in border zones of these units were tree–shrub–forb (Bramble et al. 1998). Both wire and border zones of mechanical units (mowing and handcut) were considered tree–shrub–forb cover type.

Vegetation in 2003 and 2004

The overall density of target trees in 2004 in all treatment units combined averaged 1,635 trees/ha [662 trees/ac] in wire zones and 1,894 trees/ha [767 trees/ac] in border zones (Table 1) (Yahner et al. 2003a; Yahner and Hutnik 2004a). Compared to densities noted in 2003, this represented only a 7.3% increase from the average overall density in wire zones (1,524 trees/ha [617 trees/ac] in 2003) but a 26.4% increase from the density in border zones (1,499 trees/ha [607 trees/ac] in 2003).

The overall densities of target trees, especially in wire zones, were influenced greatly by combining mechanical and herbicidal units (Table 1). For instance, if only wire zones of herbicidal units are considered, there were only 687 trees/ha (278 trees/ac) in 2003 and 728 trees/ha (295 trees/ac) in 2004. Yet, these densities in 2003–2004 were higher than those noted in 1999 prior to the last treatment. Hence, subsequent to a treatment, annual increases in target tree densities were expected as trees gradually invaded shrub, forb, and grass cover types, but these low densities in herbicidal units attest, in part, to the effectiveness of IVM for maintenance of electric utility transmission ROW (Bramble and Byrnes 1996).

Densities of target trees in border zones differed somewhat from those in wire zones in 2003 and 2004 (Table 1) (Yahner and Hutnik 2004a). Within most treatment units, there was no relationship between tree densities in wire zones versus adjacent border zones. In part, this was because border zones of each treatment unit were treated similarly in 1987, 1993, and 1999, and we should not expect any particular trends in tree densities among border zones of various treatment units. In fact, densities of trees actually declined in mechanical units between 2003 and 2004. From 1999–2004, the most common target tree species on the Green Lane ROW was white ash (Fraxinus americana); other relatively common target tree species were sassafras (Sassafras albidum), black cherry (Prunus serotina), eastern red cedar (Juniperus virginiana), and red maple (Acer rubrum).

By 2004, the maximum tree height averaged 4.8 m (15.7 ft) in wire zones and 6.7 m (22.0 ft) in border zones on the Green Lane Research and Demonstration Area (Yahner and Hutnik 2004a). As a consequence of these heights of target trees, which were greater than those recorded in 1999 prior to the last treatment, the Green Lane ROW was treated again in July–August 2004.

In both 2003 and 2004, shrubs and forbs were the most important cover types in wire zones of all treatment units, but grass cover type was important in wire zones of five of the ten units (Table 2) (Yahner and Hutnik 2004a). These results were similar to those observed in 1999 on the Green Lane ROW. Trees were a major cover type in handcut units. If vegetation were sampled in late July rather than earlier in the year, forbs perhaps would be better represented in cover types. The major shrub in wire zones of all units was Rubus, whereas the principal shrub in border zones was Japanese honeysuckle (Lonicera japonica). Goldenrod (Solidago spp.) was the dominant forb in most treatment units.

IVM of a ROW is a “tree resistant” but not a “tree proof” means of reducing tree invasion (Bramble et al. 1996). Competition with existing plants and wildlife predation on tree seeds on a ROW managed via the wire–border zone method keep tree invasion to a minimum. Thus, over the years since 1987, IVM and the wire–border zone method of ROW maintenance have increased the time between treatment cycles, thereby reducing labor and chemical costs for maintenance (Bramble et al. 1996; Yahner and Hutnik 2004b).

In summary, the plant community present on the Green Lane Research and Demonstration Area has remained relatively stable over the years since the onset of IVM and the wire–border zone method of maintenance in 1987. The result has been an excellent demonstration of how ROW maintenance can have multiple benefits to utility companies and wildlife (e.g., Yahner 2004). Recently, electric utility companies have recommended that IVM and the wire–border zone method be used on both new and existing ROW to mitigate undesirable tree invasion and possible power outages in the future (Burns 2004).