408 NORMAL YIELD TABLES FOR EVEN-AGED STANDS 



The latter tables thus become the basis or minimum from which such 

 increased yields may be computed for fully stocked areas. 



314. Yield Tables for Stands of Mixed Species. Practically all 

 stands are composed of more than one species, though some conifers 

 as Western yellow pine and lodgepole pine grow in practically pure 

 stands. So prevalent is the mixture that a stand which is composed 

 of 80 per cent and over in volume for the given age class of a single 

 species is termed a pure stand of that species. There may exist a large 

 number of trees in an under-story of different species, and yet the volume 

 of the trees of other species in the main stand may not exceed 20 per 

 cent. 



In even-aged stands composed of two or more species in mixture, 

 two methods have been proposed for the determination of yields. One 

 is to prepare yield tables for pure stands of each species, and then to 

 determine the per cent of these species in the mixed stand. The further 

 yield of such a stand is predicted by applying the per cent thus indicated, 

 to each yield table, and taking the sum of the two partial yields as the 

 yield of the mixed stand. 



In applying these tables on this basis to get yields for the future 

 from young stands, the question of survival may affect the result, in 

 case one species tends to crowd out another. But when stands are 

 even-aged, the association is apt to be of species which customarily 

 grow in mixture and maintain their places in the stand. The yields, 

 however, will be for the per cent of future, not of present mixture. 



"V^Hiere species differ radically in their characters, and grow in a 

 mixed stand, such as a hardwood species with conifers, there is apt to 

 be greater variation in yields, but with trees of similar habits, such 

 as mixed sprout hardwoods or mixtures of two or more conifers, the 

 stand behaves much as it would for pure stands. 



For all such even-aged mixed stands, it is possible to prepare yield 

 tables by disregarding the per cent of mixture, or recording it merely 

 as a descriptive item, and proceeding as if the stand were pure. 



An example ^ of a yield table for mixed stands of second-growth hardwoods in 

 New England is given below. The conchisions based on this study were, first, that 

 in spite of wide variation in percentages of species in mixture, for a given age, site, 

 and density, the volumes in board feet, cubic feet and cords were constant, and, 

 second, that the volumes of trees of given height and diameter in cords and cubic 

 feet were the same, regardless of species. 



1 Bulletin of the Harvard Forest No. 1. Growth Study and Normal Yield Tables 

 for Second-Growth Hardwood Stands in Central New England. By J. Nelson 

 Spaeth, Cambridge, Mass., 1921, 



