Arboriculture

ARBORICULTURE, the scientific management of trees (Lat. arbor, a tree). In its broadest sense arboriculture includes that part of horticulture which deals with selection and cultiva tion of ornamental and fruit trees and shrubs and that part of forestry known as silviculture. Because of its broad meaning the word arboriculture is seldom used in practice. More specific meanings can be conveyed by such words as pomology which means the scientific cultivation of fruit trees or silviculture which means the scientific management of trees for commercial timber purposes.

The growing of woody plants for educational and scientific purposes has been practised since the middle of the 16th century when a collection of trees was made at Touvoys in France by Rene du Bellay. Many of the different phases of arboriculture have received particular attention from scientists and there are specialists in different fields; e.g., systematic relationships, dis eases of trees and shrubs, insect pests, pruning of trees and shrubs, soil management, ecology and propagation.

The systematists classify plants according to likes and unlikes, placing them in their proper genera and families. The pathologist studies diseases of plants and works out rational control meas ures. The entomologist is interested in insects that feed on eco nomic plants and, like the pathologist, tries to find ways to ex terminate or control the pests. The ecologist studies the habitats and environmental effects on plants, ever trying to find ideal conditions which would insure their maximum development. The plant propagator specializes in the multiplication of plants by means of seeds, cuttings or grafting. It is his problem to find the best and most economical methods to multiply valuable plants. If plants do not come true from seed, as is the case with varieties and hybrids, he must work out methods for vegetative propaga tion. (P. W. Z.) In arboriculture the cultivator's aim is to produce specimen trees with fine trunks carrying evenly balanced heads of well de veloped branches, whereas in silviculture well-grown trunks free from branches, produced in the shortest possible space of time, are the first requirement, and the minimum branch area only is tolerated that will keep a tree in good health and help in provid ing the necessary food required to build up the trunk. Isolated, the one is seen as a handsome specimen, the other as a gaunt trunk bearing meagre branches about the upper part. Further, in arbori culture trees are widely spaced in order to facilitate branch devel opment and display, but in silviculture they are grown close to gether so as to suppress side branches, encourage height growth, and to obtain the largest possible quantity of timber from the area on which they stand. However, there are common grounds on which the two sections meet and function together. The best marked instances are park and hedgerow trees which are grown partly for decorative purposes and partly for profit. Such trees are often allowed to form large heads of branches but they are usually felled at the time of their greatest commercial value.

Propagation and Pruning.—Arboriculture may be regarded from many different standpoints. The scientist considers it from the point of view of collections of different kinds of trees drawn together from many parts of the world for the purpose of study. The owner of a large estate practises it in connection with the amenities of his property; the ordinary householder in connection with the adornment of his garden ; municipal and county council authorities as a means of beautifying the parks, gardens, streets and roads under their control, and so on. However, in whichever way arboriculture may be practised there are certain fundamental operations. Of these propagation is one. There are many methods of propagation. Some trees are raised from seeds, others from cuttings, grafts or layers. In some instances a tree may suc ceed quite well if propagated by one of two or three methods, but there are others that must be raised by one and one only of the several means if the best results are required. Propagation must be followed in most cases by one or more years in nursery quar ters, where, by careful attention and training the foundation for the future tree is laid.

Pruning constitutes a very important item in this early train ing. A tree must be encouraged to develop with a single strong leading shoot. Branches are very liable to develop at the expense of the leading shoot. If side branches are suppressed or removed too rapidly there is a tendency for the tree to become spindly and unable to support its own weight, therefore pruning is so di rected that the correct balance is maintained between the main axis and leaf-bearing branches, by first shortening and later on very gradually removing side branches. Root pruning is also prac tised during the nursery period. This is done by biennial trans planting. At that time long roots are shortened to encourage fibrous roots, for the more fibrous roots a tree possesses, the bet ter is it likely to re-establish itself when removed from the nur sery to a permanent position. Trees may sometimes be transferred to permanent positions when no more than two or three years old and a foot or two in height, or they may be allowed to remain many years. Street and avenue trees are often allowed to remain in nursery quarters until they are eight or 10 years old and 15 to 18f t. high. Some kinds of trees cannot be transplanted success fully after they have attained a height of 3f t. or 4f t. Pines suc ceed very much better if they are planted in permanent positions when less than 'ft. high. As a rule deciduous trees transplant better than evergreens after they have attained a height of several feet.

Securing Vertical Growth.

Trees that are growing in open positions have a very decided tendency to develop in a lateral rather than vertical direction and to counteract this defect it is necessary to shorten and sometimes remove side branches. Trees should be pruned every second year until such a time as a single, strong leading shoot is keeping well in advance of side branches. After that less frequent pruning may be required. For a consid erable time, however, it will be necessary to remove the lower branches now and then. Once the trees have attained the desired height branch development may be allowed to go on unchecked. In the case of old trees it is often necessary to remove one or more lower branches to balance the tree or lift the lower branches from the ground in order to disclose distant objects, open out vistas, or expose the trunk, for in some instances a view of the trunk adds very materially to the effectiveness of the tree. The removal of dead and broken branches from old trees is a very necessary practice in arboriculture. When dead branches are allowed to remain on the trees they are not only a source of danger to living parts but to other trees and dangerous living branches should also be shortened or cut off.

When the higher branches of trees such as oaks or elms die, the tree may often be rejuvenated by lowering the whole head to a point well below the dead part. With care the contour of the head can be left rather similar to the original, and in the course of a few years new branches will be formed, which will effectively hide the wounds and form a new head. Whether branches are re moved from young or old trees they should, whenever possible, be removed in a line parallel with the bark of the trunk. A wound formed in this manner is almost sure to heal well but should only a short snag be left it will not heal properly. All wounds must be covered with a coat of coal-tar or some other antiseptic and pro tective substance as soon as made. Where cavities of dead wood occur in trees they should be cleaned out, as much as possible of the dead wood removed, the surface painted with a strong solution of carbolic acid, and afterwards with coal-tar if possible. The cavity should then be built up with cement, concrete or asphalt, the surface being so finished that water will be thrown off. Ivy should not be allowed to encircle young trees but there is little use in removing it from old and fully matured trees. In some in stances it may even help to support decaying trees and is certainly effective during winter. Pruning may be carried out during sum mer, autumn or early winter.

Botanical Grouping.--When

arboriculture is practised for scientific purposes it is a good plan to arrange the trees in botanical sequence, placing the species of each genus together and allowing the related genera to follow each other. Soil conditions may not always allow this ; then digressions must be made. More than one tree of each kind should be planted and they should be allowed enough space to develop to their full size. This will probably necessitate nurse trees being planted between the permanent ex amples, which can be removed as occasion warrants. Proper records must be kept of ail collection trees, whence obtained, when planted, height and condition when planted and any other pertinent matters. They must also be properly labelled.

Garden Plantings.—When trees are planted for decorative purposes care must be taken to select kinds suitable for the avail able positions and they must be given ample space for their full development. Conifers should not upon any account be chosen for gardens in or about manufacturing towns where the atmosphere is heavily smoke-laden, or impure through chemical fumes. De ciduous trees give better results, but they must be very carefully chosen for such places. When the atmospheric conditions are fairly clear of impurities and the soil is good, almost any hardy tree will thrive. Ericaceous plants, arbutuses, rhododendrons and the like, however, do not thrive where lime is present in the soil to any appreciable extent. For public parks arboriculture should take an educational form; trees suitable for the neighbourhood should be planted and given every chance of developing into fine specimens, in order that residents in the neighbourhood may make a choice of those suitable for their own gardens.

Street Plantings.

Too little thought is given to trees planted in streets and by the sides of roads, yet they have a very great bearing upon the amenities of towns and the countryside. Trees should not be planted in very narrow streets and roads; they are in the way, are often injured by traffic, and their branches must be constantly cut back to keep them within bounds. Where gardens adjoin narrow roads it is better to encourage the owners to plant small trees in their gardens in order that they may over hang the footpath, rather than plant trees on the sidewalk. In wide streets trees can be used with very great advantage. Even in such positions, however, they are often planted too close to gether and it is doubtful whether upon any occasion they need be spaced closer than 6oft. Close planting results in severe pruning and it is wise to avoid heavy pruning as much as possible. In the past, street and road trees have had a great deal to contend with by the presence of water and gas-pipes, etc., laid near the roots. Each time one of the services requires attention the roots of the trees are injured and very often premature death of the trees can be traced to such injury. New roads should be provided with proper accommodation for such services in order that injury to the trees may be avoided.

Care is necessary in selecting the right kinds of trees for definite soils and positions, and vigorous trees should be specially prepared for the purpose. Park, hedgerow and cop pice trees should either be native trees or such exotics as will harmonize with native trees. Kinds with variegated foliage for instance are more in place within the confines of the garden than planted in association with common species. (See also ARBORE