Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/39

33 If the forester is to maintain a continuous supply of timber he must know how many years it takes a jarrah tree to grow from seed to a mature tree large enough for commercial purposes. He learns from measurements which have been taken to ascertain the rate of growth of existing jarrah trees of various sizes, that it will probably take one hundred years for a tree to grow to the required size.

Consequently, he divides his total area of 50,000 acres by 100 and finds that he can afford to cut the timber on 500 acres each year. If he cuts 500 acres this year and makes provision for a new crop to take the place of the trees removed and continues this procedure each year, by the time that the last 500 acres are to be cut the first 500 acres which were cut will be carrying a crop of trees 99 years old. The trees on this area will be 100 years old the following year and the forest will then be "normal," for it will consist of 100 compartments each of 500 acres in extent and each carrying trees, of one age. A regular series of compartments is thus established in which there will always be one in process of regeneration, and one in process of being cut out, and between the first and last 98 compartments in order of age, from the one one year old to the one 100 years old. The "normal forest" is the aim of every forester, but although he seldom reaches his objective the success of his efforts may be judged by the degree of normality of cultivated forests under his control. In practice the forester is never allowed a full rotation of 100 years to convert an abnormal into a normal forest, but for the sake of illustration we will follow the probable procedure of the forester in charge of the 50,000 acres already referred to.

His first care would be to choose the 500 acres to be cut over during the coming year. On this area all the sound trees will be sawn into timber; many faulty trees will be suitable for firewood only. Before felling operations commence the forester must mark certain trees which, although the trunks may be damaged, have strong healthy tops, and these must be left standing as "mother trees." One or two mother trees to the acre will shed sufficient seed to establish a fresh crop on the area.

When all the timber and other forest produce are cleared off the area the remaining tops and debris must be roughly stacked and a strong fire run through the compartment towards evening. Before the fire is started a firebreak must be cleared around the outer boundary of the compartment to prevent the fire spreading through the remainder of the forest.

The burning over of the area and the dropping of seed by the mother tree will result in a crop of seedlings springing up in the compartment. As soon as this new crop gives indication of being established by surviving one summer, the mother trees which have served their purpose may then be ringbarked.

For the next ten years the forester will have no worry other than keeping the fires out. A thinning may then be necessary, but this and subsequent thinnings must leave sufficient trees to maintain a continuous canopy of leaves over the area during the whole life of the forest.

When the trees have finished their main height growth and are not likely to develop side branches, a large number of trees may be taken out in the periodic thinning to enable those remaining to grow to a larger size. Height must, however, be secured before diameter growth is encouraged. The value of the final crop will depend on the length of clean timber free from knots in the trunk of the trees.

Another very important matter which must be attended to as rapidly as funds permit is the complete fire protection of the whole forest. Fires have effects other than the more apparent scorching of the leaves and the burning of the butts of the older trees. The running of a hot surface fire over the root system of trees, which are often necessarily shallow rooted owing to impermeable layers of ironstone, must have an injurious effect on the finer absorptive roots. The burning of