Resistance of Plant Cover Types to Tree Seedling Invasion on an Electric Transmission Right-Of-Way

Methods

Tree seedling counts were made on belt transects in late summer of 1984, 1985, and 1986 on ROW areas that had received maintenance treatment in 1982. Cover values of plant species present on treatment areas were estimated in six broad classes following a method developed for ROW use (2). One belt transect (6.6 x 165 feet) was located on the ROW area under the wires in the dominant cover type of each 2.5-acre ROW treatment area. Two tree height classes, less than 1 foot and 1 to 3 feet, were recognized to distinguish recently established tree seedlings from those that were well established. Tree seedlings present were red maple (Acer rubrum), black cherry (Prunus serotina), red oak (Quercus rubra), white oak (Quercus alba), black oak (Quercus velutina), chestnut oak (Quercus prinus), and sassafras (Sassifras albidum). The ROW treatments applied in four replications in 1982 were:

1. Handcutting in which all trees and tall shrubs were cut.

2. Selective summer basal spray consisting of a mixture of Garlon 4 (2 gal.), Cidekick (0.5 gal.) and fuel oil (25 gal.) in 72.5 gal. of water applied to trees and tall shrubs.

3. Selective stem-foliage spray consisting of a mixture of Weedone 2, 4-DP (0.5 gal.), Amdon 101 (0.5 gal.) and Surfel (0.25 gal.) in 98.75 gal. of water applied to trees and tall shrubs.

4. Broadcast pellet application of Tordon 10K applied over the entire ROW wire zone area to remove trees, tall shrubs, and blackberry.

5. Selective frill and squirt application of Tordon RTU applied to trees only over the entire ROW.

Results

A high degree of resistance to tree invasion was shown by six plant cover types (Table 1). These cover types were dominated by dense patches of resistant species of either grasses and herbs (meadow fescue, poverty grass, panic grass, hayscented fern) or shrubs (blueberry and bear oak) (Figures 1–4). Although tree seedlings were not found in any of these types in 1986, a few were present in three types in 1984 and 1985. Such changes from year to year may be expected due to elimination of seedlings by adverse weather conditions, plant competition, allelopathy, and destruction by wildlife (deer, small mammals, etc.)

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Figure 1.

Grass cover type (tall meadow fescue) highly resistant to tree invasion.

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Figure 2.

Shrub-grass cover type (blueberry-fescue) highly resistant to tree invasion.

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Figure 3.

Herb-grass cover type (goldenrod-fescue) highly resistant to tree invasion.

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Figure 4.

Herb-grass cover type (hayscented fernblueberry) resistant to tree invasion.

A low degree of resistance to tree invasion was shown by six cover types in which numerous seedlings were found in 1985 and 1986 (Table 1). These shrub-herb and shrub-grass covers were dominated by shrubs of low or variable resistance to tree invasion, primarily blackberry, dewberry and witchhazel, (Figure 5). A tree sprout-shrub cover found on handcut ROW areas (Figure 6) was of very low resistance to tree invasion (Table 1). This type was dominated by clumps of tree sprouts below which shrubs and herbs grew sparsely in broken patches that were open to tree invasion.

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Figure 5.

Shrub-herb cover type (blackberry-goldenrod) of low resistance to tree Invasion.

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Figure 6.

Tree-shrub-herb cover type (oak sprouts-blueberry-goldenrod) of low resistance to tree invasion.

Although designation of cover types by their life forms has been used for initial recognition in the field, identification as a grass-herb cover or shrubherb cover, etc., must be accompanied by their dominant species composition which actually determines resistance to tree invasion (Table 1). For example, a shrub-grass cover dominated by blueberry and fescue grass was found to be highly resistant to tree invasion, while a shrubgrass cover dominated by blackberry and poverty grass was not resistant. In general, cover types dominated by blackberry were not resistant to tree invasion.

Resistance of individual plant species to tree invasion has been grouped for convenient reference into three classes: 1) species that were highly resistant, 2) species that were usually resistant but under certain conditions were of less resistance, and 3) species of low resistance (Table 2). Variable resistance was typified by poverty grass which was highly resistant under favorable growth conditions, but it often grew in tufts or clumps on poor sites where it was of low resistance to tree invasion. Hayscented fern was a special case as it usually was of high resistance and formed large patches; however, it was occasionally killed by late frosts which opened the patches to invasion over large ROW areas. Dewberry and sheep sorrel were resistant when in dense patches, but often grew in broken patches intermingled with species of lesser resistance which resulted in moderate resistance.

The typical cover type that may be expected from applications of common maintenance techniques can usually be predicted with a high degree of certainty (Figure 7). However, the presence of certain cover types were often determined by their presence on the ROW before treatments were applied. This often resulted in a cover not typical of the treatment prescribed. For example, where large patches of meadow fescue were dominant on a ROW area, a selective basal maintenance spray retained these fescue patches, and the result was a selective basal-sprayed ROW area dominated by grass rather than the shrub-herb-grass type normally expected.

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Figure 7.

Model for prediction of plant cover types that will develop after specific ROW maintenance treatments on mesophytic upland sites.

Another important feature of ROW which affects the kind of cover type that will be produced by a ROW maintenance treatment is the presence of viable seeds in the upper layers of soil. Hutnik et al (6) found that most of the viable dormant seed in the ROW seedbank were grasses, whereas seed of shrub and tree species were rare, even in the forest adjoining the ROW. As a result, any treatment which kills large patches of vegetation favors the establishment of grasses rather than shrubs and trees in spite of the nature of the cover type before treatment. This helps explain why applications of foliage spray, stem-foliage spray, mowing, and mowing with herbicide spray all result in a herb-grass cover in the years immediately after treatment (Figure 7).

Discussion and Summary

The degree of resistance of various types of plant cover to tree invasion is an important factor in planning long-term maintenance of a transmission line. Each year a supply of tree seeds enters a ROW as a result of wind, water, gravity, and animal transportation. After a ROW maintenance treatment, conditions are often especially favorable for germination of these seeds. However, large numbers of grass and herbaceous seed may also germinate, and soon competition becomes the dominant factor in determining the number and vigor of tree seedlings in the posttreatment vegetative cover.

A resistant grass-herb cover dominated by fescue grass, poverty grass, panic grass, hayscented fern and resistant shrubs may be encouraged by planned use of herbicide treatment techniques. This requires knowledge of the plant species present at the time of application and an understanding of their development by vegetative spread and their reproduction potential by seed both from plants currently present and in the soil seedbank.

With the knowledge that has been accumulated on the effects of various common ROW maintenance techniques on plant cover, it is now possible to predict the type of ROW cover that will develop following application of those techniques (3). A cover type dominated by tree sprouts and shrubs has developed after handcutting; selective basal spray has produced a shrub-herb-grass cover type; and stem-foliage and foliage sprays have produced a herb-grass cover type. The species composition of these cover types rather than life form has proven to be the determining factor in their resistance to tree invasion.