METHOD OF 1883 235 



plus the growth on the old wood class while it is being cut. But in many 

 former working plans the growth of the old wood class was not com- 

 puted ; the result was therefore a slightly more conservative yield. 



The method was designated for selection high forests of tolerant 

 species, where the regeneration could be secured in at least one-third 

 the rotation, and where a sustained yield was important. It is based 

 on the conception that a selection forest, normally constituted, is just 

 like an even-aged forest (where, on equal areas, stands of all ages, up 

 to the rotation age, are found), except that the various aged trees are 

 intermingled. In the latter case an equal cut is secured by cutting each 

 year areas of the same size and productivity. But in the selection 

 forest the cutting must remove only ripe trees here and there over the 

 entire area without any comparison of surface. Therefore in this case 

 volume must be substituted for surface. 



The method is based on the assumption that the volume of the old 

 wood is five-eighths and the volume of the average wood three-eighths 

 the total merchantable volume, presuming that the young wood is un- 

 merchantable. According to the French Secretary of Agriculture the 

 data furnished by research on the mean annual rate of growth of high 

 forests shows that this relationship is approximately as 5 is to 3. There- 

 fore, whenever in a selection forest the volume of the old wood and the 

 average wood is as 5 is to 3 it can be taken for granted that these two 

 groups are similar to the first two periodic blocks of a high forest. To 

 demonstrate that the volume covering the first two periodic blocks of a 

 regular high forest (divided into three periodic blocks) is about as 

 5 is to 3, which represents their average age respectively, it suffices to 

 note that the trees of the second periodic block are the average wood, 

 which has arrived at a state where the annual growth is very uniform 

 and just about equal to the average of the stand and at a period when 

 it is safe to figure the future growth as equal to the past average. Sup- 

 pose a high forest with a 150-year rotation were divided into three 

 periods of 50 years each. The average age of the first (old wood) and 

 second (average wood) periodic blocks will be 125 and 75 years and 

 will be separated by a length of time equal to a period of 50 years. 

 In admitting that the future growth will be equal to the average growth, 

 the volume of the 125-year wood will be equal to that of the 75-year 

 old wood increased by an amount equal to 50 times the annual growth. 



"average age of the average tree in each diameter class")- For each class the cut 



,,. , . , ^, Number of trees per acre „, ,. ^ , , • i i * n 



IS = Volume of class X ^ : 7 — ^ , The summation of the yield for all 



Years to grow to next class 



classes is the cut per acre which can then be increased or decreased according to the 



surplus or deficit in the growing stock. According to Recknagle's example the surplus 



is reduced in one cutting cycle (which is made equal to the number of years to grow to 



the highest diameter class for the preceding class). 



