64 



duals of the lowest class, the average stem of the crop is constantly 

 moving upwards into one of the (up to the present) larger stem- 

 classes" and on the other hand the average stem at any given age 

 is constantly receding into a smaller class and ultimately takes its 

 place in the subordinate crop. So that the exploitable crop eventu- 

 ally consists for the most part of individuals which in their youth 

 belonged to the highest or over- topping class. Hence investiga- 

 tions into the course of growth of a crop, by means of measurements 

 and ring-countings made on existing already exploitable indivi- 

 duals, give the heights, diameters, basal areas, volumes, etc., at 

 various periods, not of the representative or average individuals at 

 those periods, but of the largest class of stems of those periods. 



B. Basal area. 



According to the universal convention adopted of measuring the 

 diameter at breast-height (4j feet), the basal area of crop is 

 obviously nil until that height is attained ; it then increases 

 rapidly up to middle age, and thenceforward more slowly but 

 steadily up to a great age. The diminution of the rate of increase 

 is very conspicuous in the case of quick-growing or shade-avoiding 

 species, but is comparatively slight in the case of shade-enduring 

 or slow-growing species. Thus the basal area in good crops of the 

 European spruce or silver fir, at the age of 140 150 years aggre- 

 gates as much as 348 square feet per acre, whereas in the best 

 crops of Scotch pine or beech it seldom exceeds 217 square feet per 

 acre. In inferior soils and localities the b^sal area, for one and the 

 same age, is considerably less than in good soils and localities in 

 spite of the number of stems being larger. In Germany, in un- 

 favourable localities, the bas,al area in mature crops is only 130 

 square feet per acre for pine and larch, and 196- 1 21 7 square feet per 

 acre for spruce and silver fir. In Germany the basal area of a crop is 

 on an average about 0*5 per cent, of the area covered by the crop ; 

 in the best soils and localities the average percentage is 0*8 for 

 spruce and silver fir. 



C. Volume. 



The volume of a crop, as well as the rate at wtrich it increases, is 

 very sm:ill in early youth. The volume then increases rapidly up 

 to the end of the middle age of the crop, after which the rate i 

 increase is much less rapid, but is maintained up to a very advanced 



