65 



period. A complete cessation of increase can occur only when tlie 

 increment of the growing stems is just counterbalanced by the loss 

 due to decay and death. 



The current increment of volume becomes rapidly larger until 

 it attains its maximum in 30 — 40 years in the case of quick-growing 

 species under favourable conditions, and in 70 — 80 years in the case 

 of slow-growing species and in unfavourable soils -and localities. 

 After reaching its culminating point, it sinks rapidly in places 

 where favourable conditions exist, more slowly where circumstances 

 are not so suitable. 



The mean annual incrennent obviously begins by being identical 

 with the increment of the first year. As the current increment- 

 now goes on increasing steadily year by year, as long as this in- 

 crease continues, and also for some time afterwards, the mean in- 

 crement obviously keeps below the figure of the current increment. 

 Ultimately it catches up and gets ahead of the latter, and as the 

 current increment goes on steadily declining, the mean increment 

 maintains its superiority to the end. The mean annual increment 

 obviously attains its maximum when it becomes equal to the current 

 annual increment, for the continued diminution of the latter must 

 cause it to decline also from that point. As the current increment 

 diminishes only gradually, especially in unsuitable soils and locali- 

 ties, the mean annual increment, after attaining its culminating 

 point, continues nearly at the same level for a considerable period, 

 particularly so in unfavourable places, where this period may extend 

 over several decades. 



Ciapter VII. 



On the compilation of tables of yield. 



In yield tables are collected figures representing the course of 

 growth of different classes of crops that have developed under nor- 

 mal conditions of growth and density. These figures give the 

 volume and increment per unit of area of the crops in question at 

 different ages and under different conditions of growth, and some- 

 times also the corresponding factors which contribute to the pro- 

 duction of volume, viz., number of stems, basal area, and height 

 of crop. 



As shown higher up, the course of development of a crop can- 

 not, like that of the individual tree, be deduced by a single series 



