302 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OP AGRICULTURE. 



In a stand of trees, an acre of forest growth, tbe progress of wood production is, to be sure, 

 different from that in the individual tree, for here the amount of wood to the acre at any time 

 depends on the number of trees as well as their volume. And this number, as we have seen, 

 rapidly decreases as the trees grow older and crowd each other, when some are killed and 

 eliminated from taking part in the total w<>d production, while the remaining, with the increase 

 iu light and food supplies, increase their production. This increase in the rate of volume growth 

 per acre is very rapid in young woods and on good soil; it reaches a maximum and then declines 

 more or less rapidly according to species and site, very much according to the diameter growth of 

 the individual. 



The question as to the number of trees which should be allowed to grow per acre, so as to 

 produce not only the largest amount of wood, but of useful sizes and best quality, which means 

 freedom from knots and technically most serviceable in form and grain, is one of the foremost 

 problems of both the technicist and the manager. 



The capacity of our unmanaged virgin forests in this respect is no criterion of the possibili- 

 ties, and on the other hand the experience of other countries is only partially applicable to our 

 conditions. But as an example of what our white pine forests, for instance, may eventually 

 produce, we may cite the experience with spruce in Germany, which on good soil is capable of 

 producing at the rate of 40 cubic feet per acre each year during the first decade, as much as 120 

 cubic feet in the second decade, and at the rate of 200 cubic feet at the age of 40, while at one 

 hundred and fifty it shows only an average of 80 cubic feet per acre annually; having declined 

 from about the seventieth year on. 



On poorer soils about one-half of this production may be expected, and if we inquire into the 

 total quantity per acre we may find at thirty years 4,200 cubic feet of wood, more than twice that 

 amount at sixty years, and 14,000 cubic feet at one hundred years, which appears an enormous 

 yield compared to those of our virgin forests, whose yield is depressed by the presence of much 

 valueless material and lack of density, and which would in double the time hardly have produced 

 such amount. With other species, to be sure, entirely different aggregate amounts would result, 

 but in general the march of progress would be in a similar proportion. 



If, however, we have to deal not with seedling trees, but with coppice growth like the sprout 

 lands of our Xew England States, the progress is entirely different. There are several million 

 acres of hard-wood coppice in these States, which, when fairly stocked, produce annually for the 

 first twenty-five to thirty years at the rate of a cord or a little less (i. e., about 100 cubic feet solid) 

 per acre, but after that time very rapidly decline in production without an equivalent value 

 increase, and hence must be cut when the maximum amount of wood production has been attained; 

 this is also necessary from silvicultural reasons, as the stocks, if left too long, are impaired in 

 reproductive power. 



To be sure, such woods yield hardly any other material than firewood and fence rails. There 

 are many trees to the acre, 1,500 to 2,000 at least, but each one is small, not more than 10 to 12 

 inches in diameter at best, hence the supply of firewood is in excess of the home demand and the 

 price obtained hardly covers the expense of getting the material to market. 



To produce materials of size and quality such as we now require in the lumber market, nature 

 has taken from one hundred and fifty to five hundred years, and for the giants of the Pacific, two 

 thousand years and more. Even with the best skill in managing the crop, not less than seventy- 

 live to one hundred years from the seed will be required to produce logs tit for the mill, such as 

 are now considered hardly worth sawing. 



From such measurements and considerations of the quantitative and qualitative development 

 of the crop, the economist will learn that the time at which a forest growth is utilized has an 

 important bearing on the more or less intensive and prolitable use of the resource. 



When the crop, accumulated during a longer or shorter period, is ripe for the ax depends not 

 only upon silvicultural and forest-technical considerations influenced by soil and climatic condi- 

 tions and the species composing the forest, but, from a business point of view, upon market con- 

 ditions and financial considerations. The material would hardly be useful for anything but firewood 

 or small posts and fencing material at best before twenty years, and again lor lumber or purposes 

 of construction it may bu considered fit for use not before one hundred and more years. 



