Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/31

25 they have been introduced, it has been ascertained that they grow at a greater rate than in their native land, and it is supposed that is because the rainfall in California is fairly well spread over the year. Jarrah, karri, marri, blaekbutt, yate, tuart, and certain other varieties of eucalypts are found only in the South- West part of Western Australia, that is, in the region where the rainfall is greatest. Some other varieties—wandoo for instance—are abundant in the South-West, but they are also found in the districts to the eastward, where the rainfall is much less. On the Eastern Goldfields, where the rainfall is very scanty, salmon gum and gimlet grow to a considerable size, producing timber that is of great use in mining and for other local purposes. Certain of the eucalypts have developed a marked resistance to extremes of heat and cold. Baron von Mueller mentions that one variety of eucalypt—the coolibah of tropical Western Australia, the Northern Territory, and Queensland—withstands unscorched a frequent heat of 156° Fahr. in Central Australia, nor is this tree affected by exceedingly severe frosts (18° F.). Soil and climate are among the determining factors in tree growth, but there are some varieties of trees which flourish well under conditions that are widely divergent from the normal.

CHAPTER VI.

UNDERGROWTH AND HUMUS.

Trees are not the only members of the vegetable world that grow in forests. One cannot enter a forest without at once being attracted by the undergrowth, consisting of shrubs and herbs of many kinds that are growing side by side with the trees. Another thing that one notices is that the ground is covered with a soft carpet of leaves and small branches that have dropped from the trees, and this carpet, is thick or thin according to the class of trees in the forest. When this decays, it forms the rich leaf-mould known as humus. It will be readily seen that deciduous trees (those which shed their leaves annually) must form a thicker humus than those trees that do not so lose their leaves every year. Both the undergrowth and the humus are essential to the healthy life of a forest, and anything that destroys or reduces it below its natural quantity is an enemy of the forest. It is the humus which provides much of the food of the trees. In the course of time, through the action of the sun and rain and wind, it is gradually decomposed and mixed with the upper layers of the soil and becomes available as plant food. The continual decomposition of humus forms several important products. In the first place, the soil receives all the mineral matter contained in the humus; secondly, large quantities of carbon dioxide are produced, which accelerates the decomposition of the mineral part of the soil rendering it soluble; finally the decomposing humus renders many important substances such as potash, magnesia, and lime soluble in water, so that that become available for the use of the tree. So it will be seen that humus plays an important role in the healthy growth of a forest. Undergrowth has also a special value. It sheds leaves, which add to the stock of debris that falls from the trees, and when the undergrowth dies off, it decomposes, and adds to the amount of food available for the production of essential nourishment for the tree. When a fire passes through a forest, even when it may do apparently little damage to the trees, it may destroy part or all of the humus, and thus indirectly cause serious damage to forest growth.