Читать книгу A Brief History of Forestry - B. E. Fernow - Страница 25

In scientific direction, the mathematical disciplines were the first to be developed; the natural sciences received attention much later.

A considerable amount of mathematical knowledge was required for this work of forest organization. The mathematical apparatus of the foresters even at the end of this period was rather slender, but its development went hand in hand with the development of these methods of regulation; and even elaborate mathematical formulæ for determining felling budgets were not absent.

Until nearly the middle of the 18th century, surveys of exact nature were almost unknown; only when the division into equal or proportionate felling areas became the basis for determining the felling budgets, did the necessity for such surveys present itself.

Plane table and compass were the instruments which came into use in the beginning of the 18th century. But not until the latter half of that century were extensive forest surveys and maps of various character made, especially in Prussia under Wedell, Kropff and Hennert.

The methods of measurement of wood developed still later. Until Oettelt’s time no method of precise determination of volumes was known, everything being estimated by cords or by diameter breast-high and height, or by the number of boards which a tree would make (board feet?).

The diameter was sometimes used as a price maker, the price increasing in direct proportion to the diameter increase. Oettelt calculated the volume of coniferous trees as cones, and Vierenklee, who wrote a book on mathematics for the use of foresters, calculated timbers with the top removed by using the average diameter, to which Hennert added the volume of a cone with the difference of the two diameters as a base, to make the total tree volume.

Most measurements of standing trees were, of course, made on the circumference, for, in the absence of calipers, the diameter could be directly measured only on the felled tree. Doebel had already measured the height by means of a rectangular triangle, and the first real hypsometer with movable sights was described by Jung in 1781; and a complete instrument, which could be used for measuring both height and diameter at any height, similar to some more modern ones, was constructed by Reinhold.

Determination of the real wood contents in a cord of wood and of the volume of bark by measurement was taught by Oettelt, and the method of immersion in water and measuring the displaced volume, by Hennert (1782).

In 1785, Krohne first called attention to the variation of the increment in different age classes and the need of determining the accretion for each separately.

In 1789, Trunk taught how to determine average felling age increment, and also the method of determining the change of diameter classes, which is now used by the United States Forest Service: “On good soil a tree grows one inch in three years, on medium soil in four years, on poor soil in five years.” With this knowledge, the attainment of a given diameter, or the change from one diameter or age class to the next could be calculated.

Volume tables were at Trunk’s command, and Paulsen in 1787, Kregting in 1788, mention periodic yield tables; but generally speaking “ocular taxation” or estimating was the rule, checked by experience in actual fellings, the method of the American timber looker. Generally, of course, only the log timber was estimated as with us, and only the very roughest estimating or rather guessing was in vogue until near the end of the period.

The first attempt at closer measurement was made by Beckman (1756), who surrounded the area to be measured with twine, drove a colored wooden peg into each tree, one color for each diameter class, when, knowing the original number of pegs that had been taken out, the difference gave the number of trees in each diameter class, and by multiplying the average cubic contents of a measured sample tree in each class by the number in the class its volume was found.

The method, often employed at present, of ascertaining by tally the diameter classes on strips forty to fifty paces wide, the so-called strip survey, was described by Zanthier in 1763.

These measurements were usually confined to sample areas, the use of such being already known in 1739. The contents of the sample area, if a special degree of accuracy was desired, were ascertained by felling the whole and measuring.

Oettelt, of mathematical fame, was the first to publish something about the determination of the age of trees by counting rings, although the practice probably antedates this account. He knew of the dependence of the ring width on the site and on the density of the stand.

It seems that long before this time the French had made the determination of yield in a more scientific manner, Réaumur reporting in 1721 to the French Academy comparative studies of the yield of coppice and of volumes of wood.

Oettelt, too, laid the foundation of forest financial calculations when he ascertained the value of a forest by determining the value of an acre of mature wood—the oldest age class—and multiplying it by half the acreage of the whole forest, suggesting the well known expression for the normal stock (I r2) soon after to be developed by an obscure Austrian tax collector.

Even the first forest finance calculations with the use of compound interest, and a comparison of the profitableness of the different methods of management, are to be recorded as made by Zanthier in 1764, bringing the beginning of forestal statics into this period.