244 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



winds makes it desirable to have the fellings progress from the south and west toward north and 

 east. Altogether it will have become apparent that the distribution of successive fellings is an 

 important matter, not only from the standpoint of regulated administration, but also of successful 

 culture. 



In the accompanying map (PI. XXXII) we have attempted to give aii idea of the matter on 

 which a "manager's map" is constructed, and how ideally in a forest of the plain the arrangement 

 of age classes would appear when the forest regulation is perfected. 



YIELD CALCULATIONS. 



When the distribution of areas has been effected in accordance with the considerations set 

 forth, the yield calculations are made. These are computed after careful measurements and by 

 various methods of calculation, which have been developed after much experience during more 

 than one hundred years. 



Since the different compartments are cut at different times, not only the present " stock on 



10 20 30 40 50 60 70 80 90 100 110 120YE.ARS 

 Fid. 24. Diagram showing comparative progress of yields of spruce, fir. pine, and lieech on best and poorest site classes. 



hand" needs to be measured, but also the accretion for each age class from the present to the 

 middle of the period in which it is to be utilized as to total quantity (decreasing in arithmetical 

 proportion as the stock on hand is diminished by fellings), when by adding the two quantities and 

 dividing the total by the number of years in the rotation or time of regulation the equalized 

 yearly quota to be utilized, or "felling budget" (Haubarkeitsertrag or etat), can be calculated. 



The determination of existing stock is made by measuring diameter breast high by means of 

 calipers, estimating the average height, and calculating contents with the aid of tables which give 

 the corresponding volumes of timber wood (above 3 inches diameter). These tables are constructed 

 after numberless detail measurements, from which the " factor of shape" for each species, soil, or 

 climate is derived, for, since the tree is neither a cylinder nor a cone, which could be calculated 

 from the base and height, the modification from either of these two forms, the "factor of shape" 

 must be determined experimentally in order to arrive at the approximately true contents. In 

 very irregular growths and with skillful valuators a simple estimating of contents or the use of 

 so-called normal yield or " experience tables," which give for the various species, soils, and climates 

 the amount of wood that would normally be produced per acre at a given period, is not excluded. 



