30
In studying soil or subsoil in relation to tree growth, the degree of porosity and its water-retaining capacity is of far greater importance than chemical composition.
The chemical composition of soil and subsoil is of far less importance in forestry than in agriculture, for trees as a general rule take only about one-twelfth of the mineral substance required for the successful growth of field crops. Trees have their likes and dislikes, but few soils are so poor that they cannot support a crop of some species or other of tree. For example, Pinus pinaster (Cluster pine) will grow practically anywhere except on limestone, whereas Pinus halepensis (Aleppo pine) nourishes on the poorest of limestone ridges.
The only safe method of assessing the quality of a locality is by a close study of the trees which have previously grown on the same area and the conditions under which they have developed.
The forester may decide that in order to fulfil his objects of management more than one species of tree are necessary, in which case he has the alternative of growing the different species in "pure forest" on separate areas, or together in a "mixed forest" on the same area. In the case of a mixed forest the management is more difficult and great care must be taken to see that one species does not outgrow and suppress another. Certain species known as "light demanders," require full enjoyment of direct sunlight. Other species, known as "shade bearers," are content with much less light, and will flourish under the shade cast by a light-demanding species.
Method of Regeneration.—If the forester has decided to continue growing a species already established on the area, he will seek to establish the new crop by some system of "natural regeneration." It is more economical and satisfactory to start a fresh crop of trees with seed from "mother trees" already established on the area than to employ "artificial regeneration." If it is necessary to resort to artificial regeneration, then the seed must be collected, and it may be sown directly on to the area where it is desired to establish the new crop, or it may be sown in a nursery and the young plants carefully tended until they become large enough to plant out.
In this country the forester is often blamed for sowing or planting his trees too close together, but in this he is simply following the example set by Nature. In a virgin forest from which fires are excluded Nature fills up open spaces with many thousands of seedlings to the acre; as they develop, the stronger suppress the weaker, and this struggle goes on throughout the whole life of the forest. The forester steps in and, working on the principles laid down by Nature, causes a struggle between the individual trees in order that they may grow into fine straight timber trees without side branches, but hastens their development by thinning out the crop at intervals.
A "sylvicultural system" sets out the method by which the formation, regeneration, tending and exploitation of the woods which compose a forest are effected. Every forest requires individual treatment, so that the sylvicultural system only sets out the general lines of working. The main consideration in the management of any forest is to ensure that the sylvicultural system adopted will at least maintain the fertility of the soil on which the forest is growing. Herein lies the great distinction between forestry and the general practice of agriculture. Field crops rob the soil of essential mineral salts and make the use of fertilisers and manures necessary. Forest crops, if properly managed, increase the fertility of the soil by returning to it rich organic matter in the form of humus.
