The value of the forest as a producer gains additional significance in the economy of a nation from, the fact that it yields a return on land which for other purposes may be useless. Timber is the crop which we can raise on our wastes and barrens, and which enriches and improves the soil instead of exhausting it. For the forest-tree generates most of its substance from the carbonic acid of the atmosphere through assimilation in the leaves, while from the soil it requires mainly water, most of which and of the exceedingly small amounts of mineral food that enter its composition, are derived from the deeper strata. The greater part of this mineral food is returned by the falling leaves to the surface of the soil, and thus that circulation of matter is set up which makes forest-growing a means of improving poor soils.
The prospects of private gain might seem to be sufficient to insure the raising of timber as well as of grain or vegetables to the full extent desirable. But against the advantages of the wood-crop just mentioned must be set certain drawbacks. The agricultural crop is produced in one year, during which it is easy, by constant cultivation, to keep up favorable conditions, and the expenditures yield their profit within a year's time. The forest-crop requires ten, twenty, forty, nay one hundred and more years to grow to useful size, does not admit of much aid on the part of the cultivator, and must have the favorable conditions for its development provided in the methods by which it is originated. It not only requires a greater amount of capital, but a greater amount of foresight, to carry on a systematic forestry with similar objects in view to those of systematic agriculture.
Here, however, enters the national interest in the business of forestry, based upon the indirect significance of the forest, namely, its influence on climate, water-flow, and soil. Even in ancient times this significance was vaguely realized, when Critias spoke of the "sickness of the country in consequence of deforestation." The earliest written expression which ascribes to the forest a definite influence upon climatic conditions we owe to the Spaniard, Fernando Colon (about the year 1540), when he states that, "on Madeira, and the Azores and Canary Islands, the rains have become rarer since the trees, which spread their shade, were cut down." In later times we find similar observations and allusions to this connection between forests and climatic or agricultural conditions in the literature of almost all civilized nations.
But these occasional notes assumed a practical significance only when, after the extensive clearings which were perpetrated by an unbridled populace during the French Revolution, the injurious consequences upon some of the most fertile districts of France made themselves felt, when fields and pastures which had sustained a thrifty and prosperous population were turned into sand-wastes or made sterile by torrential action of mountain-streams, carrying away the fertile