CHAPTER XXVII 

 FACTORS AFFECTING THE GROWTH OF STANDS 



293. Enumeration of Factors Affecting Growth of Stands. The 



rate of growth per acre or total vokiine production of stands is the result 

 of five classes of factors, namely, site, form, treatment, density, and 

 composition. 



Under site are included all factors of local environment such as soil, 

 exposure and altitude, which influence growth (§ 294). 



The term form alludes to age, and the forms of stands distinguished 

 in yield studies are even-aged and many-aged (§ 259). 



Treatment refers to the sHvicultural management of the stand, 

 in the form of thinnings, and protection; untreated stands are those 

 grown under natural conditions (§ 300). 



Density means primarily the completeness of crown cover, but this 

 factor is also influenced by the number of trees per acre (§ 301). 



Under composition, pure and mixed stands are distinguished. Pure 

 stands are those in which a single species comprises 80 per cent or more 

 of the volume. Mixed stands are those made up of two or more species, 

 none of which amounts to 80 per cent of the volume. Stands may be 

 alluded to as pure if 80 per cent or more is composed of trees of the 

 same genus, such as pure pine or pure oak stands. 



Natural enemies such as insects and fungi, and climatic factors 

 such as tornadoes and ice storms reduce the density of stocking and 

 lower the rate of growth, thereby widening the gap between average 

 and fully stocked stands. 



294. Site Factors, or Quality of Site, In estimating the volume 

 of stands, the forest type is made a distinct unit of area for the purpose 

 of increasing the probability of accuracy in obtaining an average stand 

 per acre, or in securing a curve of average height on diameter (§ 225 

 and § 227). In the measurement of growth and yields, not only is 

 the forest type also a fundamental factor, since it determines the 

 species and composition of the stand, whose capacity for growth under- 

 lies the results obtained, but these types must be further subdivided 

 into site classes. 



The rate of growth per year or total yield for a given period for 

 different species depends directly upon the combination of factors 



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