METHOD OF SAMPLE PLOTS. 67 



then the following proportion is assumed to exist : 



v : V=a: A 

 and 



Again, if 



v _vxA 



N= number of trees in the wood, 

 n = number of trees on the sample plot, 

 then 



v: V=n:N, 

 and 



n 



In the first case it is necessary to ascertain the areas, and in 



the latter the number of trees in both sample plot and wood. 

 As, however, the counting of all trees gives hardly less trouble 

 than measuring them, it yields only a small saving of labour, 

 and it can only come into consideration when the area of the 

 wood is not known, or cannot readily be ascertained. 



2. Selection of Sample Plots. 



The proportion given above will hold good only if the sample 

 plot represents a fair average of the whole wood, so that it can 

 be considered as a model of it ; in other words, if a measure- 

 ment of the trees on it yields an average basal area of stems 

 per unit of area, an average height and the same form factors. 

 Hence, the sample plots must be selected accordingly. 



Here several cases must be distinguished : 



(a) The quality of the wood is the same throughout the area. 



In this case the sample plot may be selected anywhere, 

 as long as the density of stocking represents an aver- 

 age. In very large woods it may become desirable to 

 take several sample plots and calculate the mean. 



(b) Several qualities occur, which are clearly separated 



according to area. Here each quality is treated sepa- 

 rately, and one or more sample plot taken in each 

 part (Fig. 27). 



F 2 



