B. SIMONSSON, Swedish State Power Board
Few people give thought to the problems presented by the ever-spreading march of electric power-lines across woodlands and forests. The matter is largely taken for granted. Yet it raises a number of interesting points for the forester and, at the request of FAO, the writer here describes what has been done in Sweden to lessen the damage caused to the forest by the erection of power-lines.
The Swedish state power-line grid now extends to about 35,000 kilometers and the area covered by forest strips under the lines or otherwise affected by them is estimated to add up to about 120,000 hectares. For some 20 years studies have been made on how to secure safe electrical transmission at the least cost and with the least damage to the forest. The particular methods adopted as a result in Sweden may not be of general application but the underlying principles should be valid.
The original way of creating a forest strip for a power-line was to cut down all trees of a height that would cause damage to the wires or pylons if they fell towards it, such trees being technically called danger trees. The area under the line was clearcut and no vegetation higher than low brush was afterwards allowed to grow up. This method, of course, provides a high degree of security but inevitably the damage to the forest is considerable and the consequent compensatory costs to he paid to forest owners are high.
A first step towards lowering costs was to classify power-lines into two broad groups: lines of national importance whose functioning could not be allowed to be interrupted even for brief intervals; and secondary lines for local needs. The standard of maintenance of the forest strips under the latter group was relaxed and it was conceded that the risk of short interruptions, caused for instance by falling trees during storms, must be accepted.
Maintaining a forest strip under a power-line of essential importance still means, therefore, that no danger trees can be tolerated, All trees when they have grown to a critical size, must be cut. But it is no longer necessary that they be entirely felled.
Trials have been made with lopping the tops of danger trees, especially in scenic areas to avoid creating unsightly gaps. The experience with this top-cutting has been favorable, and it has gradually become evident that top-cutting can be used as a general practice. In fact it is now accepted that top-cutting under Swedish conditions has many advantages.
When trees have to he felled to maintain the security of a power-line, the forest owner suffers because the timber is cut before reaching maturity and full economic value, and because disposing of the logs often requires a disproportionate amount of extra work. These direct losses can be costed and compensation claimed. But an owner may often be caused also indirect losses. The danger trees are often the dominant stems that are wind-firm. If they are removed, the neighboring trees may suffer heavily from wind damage or exposure until they have developed sufficient power of resistance.
FIGURE 1. - The vegetation under this 200,000-volt power-line gives shelter to wild animals and protects the soil.
FIGURE: 2. - Top-cut trees under a 200,000-volt power-line
When, however, a tree is correctly top-cut, its timber increment and the protective functions which it exercises are not unduly affected. It can be left standing for a long time without risk and the owner can cut it down whenever he pleases. The loss of potential timber or pulpwood production is decreased and compensatory payments to owners are reduced proportionately.
It has been found that at least Pinus sylvestris on not too productive soils can be permanently stopped in height growth even at an early stage by a correct use of top-cutting methods. The procedures should, as far as Swedish experience goes, be as follows. The trees on a strip which is often quite wide (up to 40 meters for one single power-line) should be subject to a first thinning, giving the individual stems and crowns plenty of space to develop without, however, encouraging develop without, however, encouraging rapidly growing hardwoods. Often the thinning should be repeated and then the stems allowed to develop as high as is permissible. At that stage the tops should be cut off. As few as possible of the upper branches (which possess the ability of replacing the top of the tree) should be retained, and it has often proved advantageous to cut off also the outer parts of the upper branches of the remaining crown.
An operation of this kind can result in vigorous young stands that even after removal of the upper branches seem to be capable of producing peeler logs under a power-line where the permissible height growth is only 8 meters. In any case, even if high-quality assortments are not possible, a good production of pulpwood can be obtained even under rather difficult growing conditions.
Such crops have not only a market value. The stands provide cover for wild animals and game, and afford protection against erosion and soil degradation. But it is, of course, the economic aspect which makes this practice attractive. It has been found as a rule that a tree can be top-cut at no bigger cost than is required for felling it: in many cases, top-cutting is the cheaper operation. This means that top-cutting with the subsequent production of a usable crop of wood has the advantage over clear felling of unusable stems. This is particularly so in the case of small forest owners who are able to undertake some or most of the topping work themselves.
It must be pointed out, however, that any operations involving danger from electricity cannot be allowed to be undertaken by private landowners on their own initiative. This work must be done by or under the supervision of specially trained people. The production of usable wood under electric power-lines, therefore, necessitates co-operation between the owner of the line and the forest owner.
For heights up to 8 to 10 meters, top-cutting is done with special tools fastened to the end of insulated rods. A small saw is the most useful tool. For greater heights the operation must be carried out by climbing the tree. Special climbing-shoes of an American type have been in use in Sweden for some years. They allow easy climbing of bare stems as well as within the crowns, and permit walking on the ground.
A point to be noted about strips on which trees are to be allowed to grow is that the stands must never be so dense that the power-line cannot easily be inspected, and repairs must be possible. However, repairs are not often required and, if necessary, a repair-gang can with very little waste of time make the fellings necessary for their work. The off-chance of repairs being needed should not be permitted as a reason for the total prohibition of stands capable of producing usable wood. If on this pretext conifers are in fact not allowed, the strips will often become covered with a far more dense cover of hardwoods which require expensive cutting and chemical sprayings to be kept under control.
Top-cutting has been criticized to some extent on aesthetic grounds. The results on some occasions have indeed been eyesores but this has generally been because the work was carried out with less care and knowledge than was desirable. Top-cutting, if carefully performed by experts, is not scenically objectionable. On the contrary, it often results in stands that give an almost normal impression, and few observers, professionals or otherwise, can without careful examination tell whether a forest near a power-line has been subjected to top-cutting Some years after an operation, the crowns often give the impression of belonging to much older trees.
Of course, there are other possibilities for putting to use forest strips, other than growing timber. In valleys the soil is often better than average and the height to the electric wires considerable. That can of course improve the possibilities for making nurseries or growing Christmas trees but, especially on moist soils, a very good production of grazing and browsing for animals is possible. This can afford sanctuary for game animals which can be especially useful in countries where intensive forestry precludes other areas being available. In this connection, however, it is to be noted that in many countries chemicals are increasingly being used to control fast-growing hardwoods, particularly under power-lines, and if any forage production for wild animals is intended, the use of such chemicals must be carefully planned.
Forest strips under power-lines must not of course be used for storing anything inflammable. They cannot for instance be used as log landings during summer time. Winter use for log storage can be allowed if simple precautions for workers operating in the strips are observed.
As has been said, the risk of interruptions to secondary power-lines by falling trees can be permitted to a limited degree. This means that there is a possibility of narrowing the forest strips under them. Any narrowing of the forest strips under power-lines must of course inevitably involve some dangers. Even during normal fellings, a tree can by accident fall towards the power-line and act as a conductor for the electric current. For this reason alone all forest owners are required to notify when trees are to be cut near a power-line and to state what assistance is required. This assistance is rendered free by the power companies and to any extent necessary.
The first step taken in Sweden to narrow the strips was not very astute. The strips were narrowed to give about 5 meters space on each side of the line, resulting in a total width of 10 to 12 meters. In such funnels storms caused considerable damage both to trees and power-lines. To avoid storm damage, therefore, the next step was to narrow the forest strips up to the limit of electrical security. This finally ended up in cutting practically no strip at all, leaving only a very narrow tunnel for the lines.
Strangely enough, this method has been a success. The tunnels, so far as experience goes, result in much better security than the semiwide strips and less damage is caused to the forest.
A certain technique has had to be developed to ensure that the tunnels can be maintained even in vigorous young stands. The critical moment is when the tops of the trees are going to pass the wires. By removing the branches on the power-line side of the crowns, the tops of the trees are caused to bend away from the line: this is especially noticeable when the crowns are loaded with snow. Some years later the trees are again allowed to continue to grow unhampered end the result is something very much like a narrow tunnel.
FIGURE 3. - Under this power-line (130,000 volts) an 8-meter high vegetation can be allowed. Now, at an age of 20 years, the top-cut trees have produced pulpwood. In a not too distant future even peeler logs will be available.
FIGURE 4. - Vegetation round this 20.000-volt power-line being prepared to form a tunnel type forest strip. The top branches have been removed on the power-line side of the trees.
FIGURE 5. - Established tunnel type forest strips along a 30,000-volt power-line.
Of course, it is essential to remove weak stems or trees with weakened root system; and thinnings, if done at the right time, will have a favorable effect on the stability and development of the stems. Beyond that the tunnels need very little attention once they have been established, and it seems probable that they could function satisfactorily for many years with only light maintenance costs.
Experience with this method has now extended over more than a decade and beyond doubt it is satisfactory. For instance, in 1956 a storm passed over the western parts of south Sweden, causing very considerable damage. Thousands of trees were blown down in just those districts where the tunnel practice has been established, but a rapid survey of the grid system showed that only two trees had fallen over power-lines, causing slight trouble, whereas on either side of the lines the forests had suffered heavy windfalls. This surely affords evidence that by using the correct technique to establish power-line strips of tunnel type, the stems are, if anything, strengthened.