68 VOLUME OF WHOLE WOODS. 



(c) Several qualities exist, which change gradually from one 

 to the other. In this case the sample plot may take 

 the shape of a strip, which runs through the whole 

 wood, so as to include a due proportion of each 

 quality (Fig. 28). As this is difficult to accomplish, 

 it is generally better to follow the method given 

 under (b), to divide the wood into several parts, and 

 to take a sample plot in each. 



Fig. 27. Fig. 28. 



(d) Several qualities prevail irregularly over the whole 

 wood. Here a sample plot of average stocking must 

 be selected, a matter frequently beset by great 

 difficulties. 



3. Extent and Shape of Sample Plots. 



The sample plot must be of sufficient extent to contain the 

 different classes of trees in the same proportion as the wood. 

 Hence, its size depends on the degree of regularity of the stock- 

 ing ; the more uniform this is, the smaller may be the sample 

 plot. It follows that they may be made smaller in young fully- 

 stocked woods, than in old irregularly stocked areas. 



Very small sample plots have the disadvantage that propor- 

 tionately too many trees fall into the boundary lines. 



The absolute extent of the sample plot depends on the 

 desired degree of accuracy. In mature woods it should not be 

 less than 5 % of the whole area, but in young woods it may be 

 much less, down to a quarter of an acre in very young woods. 



