Small Mammals in Plant Cover Types on an Electric Transmission Right-of-way

ROW treatments

Each ROW research segment was divided into “treatment units” that were about 500 feet in length and sufficient to permit use of a commercial line clearance crew. Treatments were replicated four times.

A method referred to as the “wire zone-border zone method” was used for all treatment units (Figure 1). The wire zone included the ROW area lying under the transmission wires plus 10 feet on both sides. The border zones were the remainder of the ROW on both sides of the wire zone.

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Figure 1.

Diagram of a 230 kV line to show (1) a wire zone that included the ROW lying under the wires plus about 10 feet on each side, and (2) border zones that included the remainder of the ROW lying on each side of the wire zone. A low shrub-forb-grass cover type occupied the wire zone with a tall shrub-forb-grass cover type on border zones.

ROW maintenance treatments were applied in July 1987 as follows:

1. Handcutting - All trees and tall shrubs were cut to a stump height of about 4 inches on the wire zone. Tall-growing tree species only were cut on border zones.

2. Mowing - The entire ROW wire zone was mowed to a height of about 6 inches. Trees only were selectively treated on the border zones with a low volume basal spray of Access (12.5%) plus Garlon 4 (12.5%) in oil (75%).

3. Mowing plus herbicide - The entire wire zone was mowed to a height of about 6 inches, and the cut stubble of woody plants was sprayed immediately with a mixture of Tordon K (0.75%) plus Garlon 3A (0.75%) in water (98.5%). The border zones were treated as for “mowing” above.

4. Stem-foliage spray - All trees and tall shrubs on the wire zone were sprayed to cover stems and foliage with a mixture of Tordon K (0.5%) and Garlon 3A (0.5%) in water (99%). The border zones were treated as for “mowing” above.

5. Foliage spray - All trees and tall shrubs on the wire zone were sprayed to wet their foliage with Accord (1%) in water (99%). Border zones were treated as under “mowing” above.

6. High volume selective basal spray - Trees and tall shrubs on the wire zone were sprayed to cover the lower 18 inches of stems to a point of runoff to the root collar with Access (1.5%) in fuel oil (98.5%). Trees only were selectively sprayed on border zones.

7. Low volume basal spray - All trees and tall shrubs were sprayed to wet the lower 18 inches of stem and root collar with Access (12.5%) plus Garlon 4 (12.5%) in Arborchem basal oil (75%). Trees only were similarly treated on the border zones.

Cover types

The common cover types produced by the various treatments were first identified by their dominant life forms (tree, shrub, forb, grass), and secondly by their characteristic and dominant species (1).

1. Forest cover type (chestnut oak-red oak). The adjoining forest was classified as the chestnut oak-red oak forest type that is common on the ridge tops of the Appalachians (2).

2. Tree sprout cover type (oak, maple, and cherry stump sprouts). The characteristic and dominant species were chestnut oak (Quercus prinus), red oak (Quercus rubra), white oak (Quercus alba), black oak (Quercus velutina), black cherry (Prunus serotina), and sassafras (Sassafras albidum). Low shrubs, forbs, and grasses occupied small openings. This cover type developed on the ROW after handcutting of trees in 1987.

3. Grass-forb cover type (poverty grass-goldenrod). The characteristic and dominant species were poverty grass (Danthonia spicata) and goldenrod (Solidago rugosa and S. gramini folia). This type typically developed after either a foliage spray (Accord, 1 % in 99% water), or after a stemfoliage spray (Tordon K, (0.5%) plus Garlon 3A, (0.5%) in water (99%)) that was applied on wire zones in 1987.

4. Low shrub cover type (blueberry-sweetfern). The dominant species were blueberry (Vaccinium vacillans and V. angustifolium) and huckleberry (Gaylussacia baccata) with scattered sweetfern (Comptonia peregrina). Bracken (Pteridium aquilinum) and grasses were present in openings. This type was produced by a selective basal spray (Access, 1.5% fuel oil, 98.5%), on both wire and border zones.

5. Grass cover type (tall meadow fescue) Tall meadow fescue (Festuca elatior) was the dominant species growing in dense stands. This cover type developed after treatment of the wire zone where fescue was already present in large patches.

6. Cane thicket cover type (blackberry) This type consisted of blackberry (Rubus allegheniensis) in pure stands with a sparse ground cover of grasses and forbs. The cover type was produced by a selective basal spray (Weedone CB, 87.5% plus Garlon 4,12.5% undiluted) on the border zones.

7. Tall shrub cover type (witchhazel) The dominant species was witchhazel (Hamamelis virginiana), which occurred in pure stands on selectively sprayed border zones (Weedone CB, 87.5% plus Garlon 4, 12.5%, undiluted).

Small mammal census method

Two small mammal censuses were taken both on the ROW and in the adjoining forest. The first census was taken in August 1989 on two replicates of each of the seven plant cover types. A second census was taken in August 1990 on three replicates of each cover type. Seven metal live traps were equally spaced in a 3x3x3 grid on a 50 x 60 foot area within each replication.

Traps were examined in the morning of five consecutive days. Species and number of individuals trapped were recorded as well as sex, age, and reproductive condition of each animal. Each individual was ear-tagged to identify recaptures. Trap nights per cover type totaled 70 in 1989 and 105 in 1990.

Effect on small mammals

A diverse small mammal population composed of seven species in 1989 and eight species in 1990 was captured on the ROW (Table 1). The additional species in 1990 was an ermine (Mustela ermina) that was hunting on the ROW. This diversity was in sharp contrast to the total of two species captured in the adjoining undisturbed forest. Evidently, the ROW acted as a large forest opening which not only provided habitat conditions suitable for the forest species but also for numerous other nonforest species. Distinct differences in distribution of small mammal species among the cover types on the ROW were observed in both 1989 and 1990 (Table 1). It is of interest to note, however, that the occurrence of species in the ROW cover types closely followed that expected from descriptions in the literature (Table 2). This was exemplified by the three most common species on the ROW. The white-footed mouse was found in all cover types, except the fescue grass type. The meadow vole was found primarily in the fescue and poverty grass-goldenrod types. And the southern red-backed vole was found in shrub and tree sprout types, but not in the two grass cover types.

An obvious unevenness was observed in the distribution of numbers of individuals among species (Table 1). This was due to the relatively high abundance of the white-footed mouse in all cover types, except fescue grass where meadow vole was the predominant species. This unevenness was confirmed by a low Simpson index (a measure of the proportions of individuals in the various species) of 0.516 that was obtained for all cover types combined; with a maximum index possible of 0.882.

Relationship of small mammal populations with cover types

Although all ROW cover types and the adjacent forest type contained small mammals in appreciable numbers in both 1989 and 1990 (Figure 2), there were distinct differences between cover types. For example, the witchhazel cover type population was significantly larger (ANOVA - Duncan-Bonner test p=0.05) than any of the other five types. Also, populations in the blackberry, blueberry-sweetfern, and poverty grass-goldenrod covertypes were significantly larger than in the tree sprout and fescue types.

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Figure 2.

Number of individual small mammals captured per acre and per 100 trap nights in six ROW cover types and the adjoining forest in August 1989 (bar 1) and 1990 (bar 2). Plant cover type symbols are: FOR = forest, WIH = witchhazel, BLA = blackberry, BLU = blueberry-sweetfern, POG = poverty grass-goldenrod, TRS = tree sprout, FES = fescue.

The characteristic populations of cover types may be summarized as follows:

Forest cover type

The chestnut oak-red oak forest type, which was cleared to produce the ROW, was used as a baseline for comparison of the ROW population with the uncut forest.The outstanding features of the forest small mammal population was its relatively high population density, which averaged 101.6 individuals per acre (20.0/100 trap nights) for the two census years (Table 1); and its lack of species diversity. Only two species were captured in the forest in 1989 (white-footed mouse and masked shrew) and one species in 1990 (white-footed mouse).

ROW cover types

The six cover types that were commonly present on the ROW each contained at least three small mammal species, and as many as four to five (Table 1). A total of seven species was found on the ROW, and the individuals in the types ranged from 32.7 to 100.4 per acre (6.4 to 19.8/100 trap nights) (Figure 2).

Reproductive condition

In 1990, 31 small mammals were necropsied in the laboratory. Of these, four individuals contained embryos. A meadow mouse, captured in poverty grass-goldenrod, had four well-developed embryos; a red-backed vole, captured in the tree sprout type, had four small embryos; a white -footed mouse, captured in poverty grass-goldenrod, had four small embryos; and a masked shrew, captured in blueberry-sweetfern, had five embryos. Therefore, it was evident that small mammals were reproducing on the ROW. They also were raising young as evidenced by the one percent of the total captured on the ROW that were young of the year.

Several other characteristics of the small mammal population were also of interest in this connection. For example, 74.1 percent of the population of white-footed mice were breeding adults. Also, the sex ratio for white-footed mouse was 1 male:0.6 females, and for all species it was 1 male:0.8 females, which is similar to the sex ratios of 1:0.9 in the Moshannon State Forest and 1:0.5 in the Rothrock State Forest both located in central Pennsylvania (4).