J. B. STOCKS
Forestry Commission of Great Britain
Until recently the re-establishment of new woodland on poor hardwood sites was almost invariably preceded by clear felling. Poor hardwoods can serve a useful purpose in the establishment of the newly planted crop. This shelterwood system is now being used very largely in the reforestation of derelict hardwood sites of many types.
The types of poor woodland under consideration are those which are unlikely to develop into marketable high forest or coppice. They are generally hardwood or broadleaf crops and can be divided broadly into-species which provide a high cover and those which are only capable of producing a low cover.
The most commonly found species in the former or high cover class are Betula verrucosa and B. pubescens, Fraxinus excelsior, Quercus robur, Q. petraea, Acer pseudoplatanus, Ulmus glabra, Tilia platyphyllos and Castanea sativa.
The low cover species are mainly of coppice origin such as Corylus avellana, Quercus, Castanea and Carpinus betulus or of shrub-like species among which the most common are Acer campestre, Salix capraea, Crataegus monogyna, Prunus spinosa, Cornus sanguinea, Sambucus nigra, Ilex aquifolium and Ligustrum vulgare.
Those species which develop the higher cover are capable of growing to tree heights but the lower cover species are in general less than 20 feet (6 meters) in height. Many different species are usually found on the same area except when the previous crop has been worked on a coppice rotation in which case the coppice species predominate.
There is often a great variation in the height and density of the cover crop. Under these circumstances it is advisable, if possible on management grounds, to leave the crop untreated for a few years until the taller, clean-growing species have suppressed much of the lower cover and thus made it easier to penetrate the area with men and tools and to select the desirable shade trees.
Density of canopy to be opened out prior to planting
The taller cover has been found to be more beneficial to the new crop. It is therefore desirable during the selection of the cover crop to retain those trees with a light foliaged crown which is high above the ground; the stems should be sturdy to obviate failing over. It is however desirable to remove initially all trees larger than 8 inches (20 centimeters) diameter at 5 feet (1.5 meters) from the ground and also trees with heavy spreading crowns. Such trees should always be removed before planting takes place as, except by girdling, it is not possible to remove them subsequently without incurring damage to the new crop.
It is also desirable to remove all dense shade-bearing low shrubs, prior to planting, except on the most shallow soils overlying chalk or limestone or where frost is severe. In such cases any form of controlled cover in the early stages of the new crop is beneficial. It is also advisable to tend the cover to form a single canopy rather than to allow the overshade to remain at different levels. The amount of overhead cover to be retained at the time of planting the new crop should not form a complete canopy, especially where it is low. Recent experience of planting under too dense a canopy has proved this by heavy mortality. During the wet Summer of 1958 losses were due to the new crop having insufficient light and remaining permanently wet and cold, particularly on the clay soils. Heavy losses during the extremely dry summer of 1959 were also encountered and this was attributed not only to insufficient light but to the excessive removal of soil moisture by the overcrop down to a considerable depth and the consequent complete drying out of the topsoil late in the season. This has been particularly disastrous in cases where an overwood of ash was used in too dense or even a complete canopy.
The amount of canopy to be retained at the time of planting varies with the species, the height of the cover crop and the light intensity of the locality. There is need for more drastic opening up where the light intensity is low, especially in industrial sites with polluted atmosphere. The ideal canopy to be retained would appear to be approximately 60 to 70 percent when the cover is tall but, as the height of the cover decreases so the opening of the canopy should be greater, so that with a low type of such species as hazel coppice (Corylus avellana) the cover should provide approximately a 40 to 50 percent canopy.
The species most preferred are Betula, Quercus and Ulmus. These species have the type of crown which permits a certain penetration of light which is favorable to the newly planted crop. Ash (Fraxinus) provides a similar type of cover but it removes very large quantities of moisture and nutrients from the soil. Acer, Tilia, Castanea and Carpinus cast considerable shade and the large leaves of Acer and Castanea which provide such an excellent leaf mold free from weeds prior to planting can cover up, at the time of leaf fall, those plants which are too small. If pure, these last species must have the canopy reduced to about 40 percent if the cover is tall, and if the cover is low it should be reduced to no more than 25 to 30 percent. Where mixtures occur, the overcrop should be treated on its merits according to the species present.
New crop species
This system is essentially most suited to the use of the more shade-tolerant trees and those species which would suffer from frost and exposure if planted without cover. It should be emphasized that the use of overhead cover is a means to the establishment of the most desirable species in relation to the permanent site factors and the cover should be manipulated accordingly. The species which have been successfully used are beech (Fagus sylvatica), western hemlock (Tsuga heterophylla), western red cedar (Thuya plicata), Lawson cypress (Chameacyparis lawsoniana), Douglas fir (Pseudotsuga menziesii), Norway spruce (Picea abies), grand fir (Abies grandis) and occasionally Abies procera, A. veitchii and Cryptomeria japonica. All these species appear to thrive under similar light conditions initially.
These facts are based on the assumption that the choice of species for the site is correct for it is quite clear that the shade-tolerant quality of any species is dependent on all other conditions for growth being suitable and that if a species is planted on an unsuitable soil or with inadequate rainfall or in a polluted atmosphere it is far less tolerant of shade than the same species growing in its ideal environment.
The type of plant to be used is preferably a well-grown transplant of 15 to 18 inches (38 to 46 centimeters). Use of seedlings is, on evidence, quite unjustified. There are also certain weeds such as Mercurialis perennis and Rubus spp. which grow profusely under denser shade than many trees can ever tolerate and transplants are essential if such dense weed growth is to be overcome.
Rate of opening out of the overwood
The rate at which the removal of the overwood should take place after planting varies according to the species which have been planted and to the weed and new coppice growth which is encountered and particularly to the rate at which the original crop has closed up. Where frosts, especially late spring frosts, are likely to occur, the removal of the shelterwood will be governed accordingly. Thus no hard and fast rules can be given as to the rate of removal of the overstory: one of the major considerations is that it shall be done in such a way that while the new crop never becomes weakened by too much shade the weed growth of the more light-demanding weed species is kept in control.
One of the major advantages of the system is that, while accepting some weed growth, it is possible to prevent undesirable weeds, such as the coarse grasses, from dominating as they would if the area were clear felled. Signs of the planted crop becoming suppressed by excess shade are the small bud formation, loss of color, drooping of the leaves or needles, weak shoots and flattening of the crowns.
The usual procedure for the removal of the overhead cover is for the work to be completed in three operations, in the first two of which a half of the canopy which remains is removed. The first removal takes place when the new crop is about 3 feet (I meter) in height which is usually three years after planting. The second removal is when the new crop is 6 to 8 feet (1.8 to 2.5 meters) in height or six years after planting, and the third and final removal takes place when the new crop is 10 to 12 feet (3 to 3.5 meters) in height which is usually nine to ten years after planting. It is however always advisable to remove the cover completely before the new crop has entered its crown, otherwise the branches of the overcrop will cause mechanical damage.
Courtesy: Forestry Commission of Great Britain
There is no appreciable difference in the rate of removal of the cover for the species which are most commonly used, except earlier removal for Douglas fir and Norway spruce for which the first removal is usually after two years rather than after three years which is safe for the more shade-tolerant species. Where hardwoods are planted it is now the accepted procedure to plant a conifer crop with the hardwood either in alternate lines or three rows of each alternating with the other. Douglas fir and beech have been mixed in this way and where such line mixtures have been planted under cover it is sometimes the practice to thin the cover off the more light-demanding Douglas fir before taking any cover from over the more shade-tolerant beech. This provides overhead light and side shelter, which is very important to the more light-demanding species, and overhead cover and side light to the more shade-tolerant species.
These times at which the removal of the overcrop should take place are general guides only and much will depend upon the rate at which the canopy of the overcrop closes and on the amount of weed growth which appears. The timely removal of the overhead cover is important not only for the reasons already mentioned but it also enables an even crop to be established and prevents the development of large coarse trees in the new crop. The final cutting of the shade trees is often combined with a "wolf thinning" of the new crop.
Financial considerations
Apart from the silvicultural and management advantages of the shelterwood system over complete clearance, the total costs of the two methods are very similar, generally with a slight advantage in favor of the shelterwood system. There is, however, no spectacular monetary saving, as is sometimes claimed, in using overhead cover rather than clear felling. Weeding costs are often much less under the overhead cover system but this may be offset by the extra care which has to be taken later in the removal of the overhead cover to avoid causing damage to the young crop.
In general, very little revenue is obtained from produce derived from the type of cover available. If there is a market then both systems benefit accordingly and if anything the clear-felling system is likely to benefit more due to the difficulty and extra expense of removing any material left as a cover crop. Apart from a very small saving in the number of plants used under cover, the costs to be considered and compared under the two systems are the felling of the overwood and the weeding until the new crop is established.
Of the many examples of costs which have actually been incurred, typical averages where the poor hardwood yields no revenue are as follows:
|
System |
Labor charges, £ per acre1 |
|
Clear-telling |
|
|
Complete clearance and burning lop and top |
35 |
|
Total weedings during 5 or 6 years |
24 |
|
TOTAL |
59 |
|
Overhead cover |
|
|
Preparation of overhead cover and stacking in drifts |
17 |
|
Total weedings during 4 or 5 years |
9 |
|
Removal of overhead cover in 3 stages together with the stacking and final clearing |
26 |
|
TOTAL |
52 |
1 £1 sterling = U.S. $2.80.
The costs of cover removal can be reduced considerably in those forests where local markets are available and in such areas the total operation has been carried out at a profit; but such markets are only to be found in a few localities. Nevertheless by a careful study of available markets it is often possible to remove at the initial cutting that material which is then salable and to retain as cover those stems which may develop to become salable during the time they are retained.
The felling and removal of the poor hardwood under either system is the major part of the cost and the only possibility of reducing this is by ring barking. In the extreme southwest of England where there are extensive areas of poor oak coppice it has been possible to do the entire work by a complete ring barking of all stems either at the time of planting or shortly afterward, at a total cost of about £5 per acre (U.S.$34 per hectare). The crowns and stems of the ring-barked trees disintegrate slowly and the total cost of weeding has been 96 per acre (U.S. $40 per hectare).
Since no felling has been done there has been no trouble from coppice regrowth and no expense in stacking unwanted waste wood.
However, in many parts of Britain the need to preserve public amenities totally precludes the methods of ring barking or girdling.
Conclusion
1. The silvicultural advantages of the shelterwood system as opposed to the system of clear felling are. as follows:
(a) Many species which are capable of the highest volume production can be more readily established.(b) The forest soil conditions are preserved.
(c) The water table does not fluctuate to the same extent. The system is particularly valuable on intractable heavy clays where considerable variations in the water table arise and where bad herbage types tend to dominate.
(d) There is a greater survival of those trees which have been recently planted.
(e) More protection is provided from frost and wind exposure.
(f) Less weed growth is present under cover.
(g) The brushwood is more evenly returned, after decomposition, to the soil rather than being burned unevenly over the area. This is because the waste being out over a period of several years can generally be left to rot whereas by clear felling there is too large a bulk which has in consequence to be burned.
2. The management advantages are as follows:
(a) There is less weeding which has, of necessity, to be done within a short period during the summer. Such weeding as may be needed can be conducted over a longer season.(b) The removal of the overhead cover can be done at any convenient time and when other work is not available. It is very suitable work for the winter months for there is more shelter for the men and since the leaf is off the cover at this time of year less damage is done to the young crop.
(c) Hunting values are enhanced.
(d) The transformation of a hardwood scrub crop to a coniferous crop is effected under a system which has higher amenity value than if the whole woodland were clear felled.
Finally, it must be emphasized that in such a system as that described it is quite impractical to explain all the various conditions with which the forester will be faced. Flexibility is therefore essential in the application of the system and, once the broad principles have been explained and understood, the local staff must be given full scope to obtain the best silvicultural, management and financial results possible.
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· Chief of the United States Forest Service, Richard E. McArdle, in January received from former President Eisenhower the nation's top civilian career service award - the President's Gold Medal Award for distinguished federal civilian service. This is probably the highest accolade ever bestowed by the Government on the young profession of forestry. Accomplishments cited as the basis for the award were: "His dynamic leadership and vision in the development of the nation's forest resources; his wise and effective action in meeting the rapidly-rising public use of the national forests; building and strengthening working relations of the federal government with state governments and private forest industry; for an increasingly effective forest research program nation-wide; for leadership in world forestry and the conservation of natural resources which has promoted international co-operation and friendship. · There has recently been established in Canada a Federal Department of Forestry, which, however, no way infringes on the constitutional rights of the individual provinces to control and administer the publicly owned forests within their borders. The objectives of the new Department of Forestry are to support, through means appropriate to the Dominion Government, improvement in the management and protection of forest resources, a fuller utilization of forest products, and improvement in the competitive position of Canadian forest industries. J.D.B. Harrison has been appointed as Deputy Minister. After successive positions of increasing responsibility in Canadian forestry, he was appointed to the Forestry and Forest Products Division of FAO in 1946, serving for 4 ½ years in Washington, D.C. In 1951 he returned to Canada to become Chief of the Forest Research Division. In 1956 he was appointed Director of the Forestry Branch. |
