Managing Power-Line Corridor Vegetation

Methods

Twenty different sites were chosen for study. These sites supported vegetation where tree stems were at least five years old. At each site, woody plant communities were assessed in the corridor (typically 15 x 25 m plot), in the adjacent forest edge (10 m wide strip), and in the adjacent forest interior (10 m from the corridor). Tree stems were divided into two broad size classes for analysis: < 10 cm diameter but > 20 cm in height for seedlings and saplings, > 10 cm diameter for adult trees. A full description of sampling methods is provided in another publication (9).

Results

Tree Regeneration

Regeneration of woody plant populations in corridors under the pressure of repeated cutting will depend on the size and makeup of the propagule bank (i.e., buried roots, stems, and seeds), the movement of seeds from adjacent forests, the responses of these propagules to vegetation removal, and other habitat conditions such as slope, aspect, and competition (14). Mechanical mowing creates a myriad of microenvironments for seedling establishment and sprout growth. For example, the creation of bare spots where the blades or tires disturb the soil favors seed germination and seedling establishment of some species (14); the removal of plant parts eliminates competition and releases dormant buds of other species (6).

Changes in Species Composition

Repeated cutting of the vegetation in these power-line corridors has affected plant communities in three major ways: those species capable of rapid stem growth from root or stem buds are favored, e.g., black locust and white ash); those species that require high light and soil disturbance for seed germination and seedling establishment are favored, e.g., black locust, staghorn sumac, redbud, yellow poplar (Liriodendron tulipifera), and tree of heaven; and, shade tolerant species are not favored, e.g., sugar maple (Acer saccharum)and hackberry (Celtis occidentalis). These results contrast with previous research showing that deciduous forests of Kentucky regenerate largely unchanged in community composition when cut once (11), but are consistent with other studies showing an increase in black locust (1), white ash (12) and black cherry (15) when cutting or human disturbance is frequent. In short, as repeated human-generated disturbance replaces natural disturbance (i.e., fire, wind, and pathogens), tree species are favored that can persist under this new disturbance regimen (13).

Management Recommendations

If the long-term goal of vegetation management on power-line corridors is to gradually reduce woody plant populations, then management techniques must be chosen carefully to achieve this goal. A number of researchers have suggested that selective herbicide application, where tree density is low to medium, is one solution to the problem of tree regeneration (2,5,6). However, I suggest several other approaches that electric utilities might consider before, during, and after mechanical mowing and manual cutting are used as management techniques.

1. If new corridors are planned or are under construction in patchy landscapes, site these so that they avoid forest patches (10). Higher costs during construction may be recovered quickly in the future as the task of plant-community management is borne by landowners, farmers, and livestock.

2. Assess the wildlife-habitat and plant-habitat suitability of plant communities created by frequent cutting (2,3,4). The abundance of accessible browse, the incidence of fruit- and seed-bearing plants, and the use of corridors as animal migration routes may suggest that frequent cutting is actually more suitable for creating wildlife habitat than for inhibiting tree populations. Moreover, frequent cutting may create suitable habitats for the growth and maintenance of rare or endangered plant species. In a multiple-goal plan for corridors, benefits (e.g., public relations, greater wildlife populations, mitigation of habitat loss) must be balanced against costs of frequent cutting.

3. Experimentally examine the response of plant communities when cutting time, height, and frequency are varied in combination with herbicide use (7). To efficiently kill entire plant systems, cutting should be timed so that the maximum amount of carbohydrate manufactured during the current growing season is removed before translocation to belowground structures. For many trees the optimum time to cut for carbohydrate removal is late summer. Follow this with herbicide application in the following spring when physiological activity of trees is maximum.

4. Explore biological control of tree populations. On some corridors in northern Kentucky, cottontail rabbits appear to kill many black locust stems each year due to bark girdling. Further data are required to determine size classes of stems most vulnerable to girdling. Cutting could be used tohold stems in the most vulnerable stage of development. Moreover, in some areas domestic animals have been used to inhibit or hold in-check woody plant populations (8).

Conclusions

Although frequent cutting does not inhibit tree regeneration, it does have profound effects on the structure and composition of woody plant communities in power-line corridors. If a management goal calls for the maintenance of vegetation with high stem density and high species richness, then repeated cutting is well-suited to this goal. On the other hand, if line managers seek to develop tree-free vegetation then frequent cutting alone should not be used. As an alternative management approach, frequent cutting might be used to decrease the vigor of plant systems, to allow more efficient use of herbicides, and to make trees more accessible to browsing animals.