In a pine forest, therefore, intended for revenue, eighty years is generally considered the fiscal age of the trees. To derive good revenue the trees should be cut at about that age, at which time they will have reached on an average a diameter of thirteen inches breast high. After removal of the timber where natural regeneration does not prevail, the blank spaces must be planted.
If the forest is extensive one would need in due time to have a skilled forester make a map of it and a working plan, showing the character of surface and soil, location of roads and trails; kinds, localities and age of trees, and all other facts indicating the work necessary to be done to maintain a sustained yield. The forest should be fenced as animals do injury to trees. After a forest crop has been cut enough of the brush to prevent future danger should be burned at a safe time by piling it upon a fire. Fire in a forest should never be allowed to run nor left to smoulder.
A good rule for estimating the number of board feet in a tree is this one, called Doyle's: from the average diameter in inches deduct 4 (as representing slabs); square one quarter of the remainder and multiply by length in feet. For example, suppose a tree is 65 feet high and its average diameter 16 inches; deduct 4 leaves 12 inches, one quarter of which is 3, which squared makes 9; this multiplied by the height of the tree, 65, gives 585 board feet as the contents of the tree.
We have thus seen that by forestry, refuse land can be converted into interest yielding capital as good as government bonds.
In a normal forest the average annual growth on third and fourth rate land amounts to about 280 feet board measure per acre. At that rate we shall expect that our planted forest of red (Norway) and white pine would in eighty years contain on an average 22,000 board feet per acre of merchantable timber, a fairly conservative estimate. I have seen on my native farm in New Hampshire white pine grow to a diameter of two feet breast high in fifty years, but it was on fresh loamy soil, such as is most favorable for the white pine.
We can assume that the expense of planting per acre will be as follows: cost of land, $5, trees, $6, planting, $10, total, $21, being the capital invested in one acre of forest. This sum at 3 per cent, compound interest will amount in eighty years to $223.44. The price of pine stumpage in this country has trebled in the past twenty years, and we can very safely assume that in eighty years its value will at least be $20 per thousand board feet. The acre crop of 22,000 board feet will then be worth $440, so that the average acre of forest will have yielded a net revenue of 3 per cent, on the capital invested, and left a surplus of $216.56 which will have paid the expense of management and taxes. There will also be a little revenue from thinnings and from game.