FOREST MANAGEMENT 131 



according to the recognised methods of land-surveying which 

 are based chiefly on trigonometry. 



The Normal Forest 



The laws of nature make it impossible for us to take away 

 annually from each individual tree its yearly production of 

 timber. This must be allowed to accumulate till the tree 

 has reached a useful size, and then, on felling it, the accretion 

 of years ^s at once harvested. In a forest consisting of sections 

 equal in number to the years of the rotation, the oldest 

 section should represent the year's increment for all the forest ; 

 upon its removal the quantity this represents is realised. 

 Yields from thinnings are here not taken into consideration. 



A forest permanently placed under sylvicultural control is 

 under rules regulating sustained management. It is the foun- 

 dation of systematic forestry to secure to the proprietor 

 continuously (annually or periodically) a regular yield or 

 income. When the timber output is approximately equal 

 every year, it is the most positive evidence of sustained 

 management. This is possible only in the so-called " normal 

 forest" (see Fig. 30, p. 138), of which in its simplest form 

 the following is true. With a rotation or felling age of 

 r years (for instance, 100 years) a forest has r acres (say 

 100 acres) divided into a number of sections, the ages of 

 which are graded evenly from the youngest to the most 



mature (gradation I r years). Immediately upon felling 



the oldest wood, matters may be represented as o (r - i), 



because the newly felled area is not restocked, and the ninety- 

 nine-years-old wood requires one year before it reaches its 

 full rotation period. Under such conditions the volume of 

 the r-years-old section is exactly equal to the annual incre- 

 ment amassed during the last growing season over the whole 

 forest area, and does not vary from year to year. The stock 

 of timber in the normal forest is called the normal growing 

 stock. 



The amount of the normal growing stock, apart from the 



