COMPOSITION OF STANDS AS TO SPECIES 393 



yields is to select plots which show a fairly complete crown density, 

 not seriously reduced by avoidable factors of damage, and to con- 

 struct the table of yields entirely from such plots. This is supposed 

 to give the normal relation between yields at different ages for well- 

 stocked stands. There remain many variable factors, the chief of 

 which is the number of trees per acre in the plots measured. It has 

 been suggested that the age or ages at which the final yield is to be 

 harvested shall be taken to indicate the normal number of trees per 

 acre and that stands of lesser age having this number or more trees, 

 while not showing the full yield for these ages may be regarded as fully 

 stocked, if not to be cut until the final age. The only difference between 

 such stands and stands which remain fully stocked would be found in 

 the thinnings in the interval and in the quality and limbiness of the 

 timber. 1 



Yield tables based on a given standard such as described may be 

 discounted to predict the average degree of stocking for average areas, 

 which are known as empirical yields. In some instances efforts have 

 been made, by collecting data on large areas, to obtain these empirical 

 yields or averages directly in the field instead of by discount from 

 yield tables. In either one or the other of these forms, the empirical 

 or actual average is the final result desired, and the normal or standard 

 yield table is but the means to this end. The arguments in favor of 

 obtaining a normal or standard yield table by the selection of plots 

 are that the variables represented in the average or empirical stocking 

 by differences in form or mixed ages, differences in density and dif- 

 ferences in composition of the forest, are eliminated from the table, 

 which is confined to showing differences in yield based on site qualities 

 and age. The relations of more than two variables can not be accu- 

 rately set forth in a single table. 



302. Composition of Stands as to Species. Stands composed of 

 a mixture of species may vary in yield from pure stands. Species may 

 differ considerably in their capacity for growth and yields even on the 

 same site. They vary in height growth and consequently are affected 

 differently by the factor of suppression when in mixed stands. The 

 rate of survival and the dimensions vary so that the composition of 

 the stand changes with its growth. Finally, the original composition, 

 independent of these later changes, varies greatly. For these reasons the 

 prediction of yields in stands of mixed species has always been regarded 

 as extremely difficult. Approximate rather than accurate results must 

 be accepted. Recent investigations indicate that for certain character- 

 istic types and mixtures of species naturally growing together, yields 



1 The Use of Yield Tables in Predicting Growth, E. E. Carter, Proc. Soc. Am. 

 Foresters, Vol. IX, No. 2, p. 177. 



