Forest Mensuration 65 



c. With the help of the average annual increment of the woodlot (Par- 

 agraph XCIX.). 



The increment of a past period is never exactly equal to that of a 

 future period, unless the age of the woods is close to that year at which 

 the increment culminates. The increment percentage during a past period 

 is always larger than the increment percentage during a coming period 

 (aside of temporary increase due to light-increment). 



The general laws (Paragraph LXXV.) relative to the culmination, 

 increase and decrease of increment hold good for the volume increment 

 of woodlots as well as for that of trees. 



PARAGRAPH XCVIII. 



ASCERTAINING THE INCREMENT OF WOODLOTS BY SAMPLE TREES. 



The current annual volume increment and the volume increment per- 

 centage of a wood, from which its maturity largely depends, can be cor- 

 rectly found only by a valuation survey, combined with an investigation 

 into the present rate of growth exhibited by a number of sample trees. 



Borggreve recommends to gauge the increment of the sample trees by 

 the Schneider increment percentage. This is usually insufficient. 



The correct volume increment percentage p of a woodlot is obtained 

 from the volume increment percentage pi, p2, ps, P4 and p fi of the class sam- 

 ple trees which represent class-volumes vi, vz, vs, v< and v$ as 



= Vi Pi + v* Pz + v s Pa 4- v 4 P 4 + v 5 P 5 



v i + v 2 + v 8 + v 4 + v s 



Where the form heights of the classes differ slightly only, the sectional 

 areas of the classes may be substituted for the volumes of the classes. 



Again, where classes of equal sectional area are formed (after Robert 

 Hartig), there the volume increment per cent, of the woodlot equals the 

 arithmetic mean of the volume increment percentages of the sample 

 trees, so that 



_ Pi + P2 + Pa + P* + Ps 



PARAGRAPH XCIX. 



CURRENT INCREMENT ASCERTAINED FROM AVERAGE INCREMENT. 



Within certain limits, a short time previous and a short time after the 

 culmination of the average annual increment, the annual average incre- 

 ment equals the current increment and can be used in its place as a basis 

 for yield calculation. European Governments frequently prescribe this 

 modus operandi for yield forecasts in working plans. 



