Ill] MENSURATION AND INCREMENT 21 



For a whole wood, the present annual increment may be 



1-1 i_x ■ JT. i, jr , ,. mean annual increment 



roughly obtamed by the formula * = 100 x — =-. -r — -— 



° ■^ ■' ^ present cubic contents 



when the mean annual increment is found by dividing the present 

 volume of the growing stock by its age, and this method would 

 give fair results in the case of a middle-aged crop that has just 

 passed its maximum mean annual increment. If the volume of 

 the crop has been calculated by forming groups of diameter- 

 classes, the mean percentage of increment should be estimated 

 for each group from sample trees of that group, and then the 

 current annual, increment in cubic feet will be estimated for 

 each size-class, and the sum of them added together will give 

 the increment of the whole crop. 



A comparison of the course of the current annual increment 



per acre with the m ean annual increment ( = j during 



the life-time of a crop will always yield information useful for 

 purposes of forest management. Curves to indicate the current 

 and mean increments may be plotted, with a horizontal co- 

 ordinate for age, and a vertical co-ordinate to represent cubic 

 feet of increment year by year. These increment curves must not 

 be confused with volume curves. The current increment rises 

 rapidly at first, and reaches its maximum towards the end of 

 the pole stage, when the height-growth culminates; earlier on 

 good soils, and with light-demanding species. It then falls 

 gradually. The mean annual increment rises more slowly, and 

 reaches its maximum often about thirty or forty years later, 

 and it is at its maximum when it is equal to the current annual 

 increment. It is at this period that the production of volume 

 per acre per annum is at its maximum. Later on the mean 

 increment gradually decreases, but much less rapidly than the 

 current increment. 



A single tree growing in a free open position would have a 

 higher increment than an average tree grown in a close crop, 

 but the fully-stocked wood would have a larger increment per 

 acre than the open wood, because the number of stems is so 

 much greater, although the crowding diminishes the growth in 

 diameter. 



