THE EFFECT OF TREATMENT ON GROWTH 391 



which this same area produced during the period when the final crop 

 was only occupying a portion of it. The latter problem may be illus- 

 trated best by the yield or rate of growth per year of stands which 

 have come up to spruce following poplar or white birch on a burn. 

 In the period required to produce a mature crop of spruce, a crop 

 of poplar and birch has also been produced. The mean annual growth 

 for the whole period must include the total yield of both species. 



Owing to the difficulty of adjusting these yields on one of these 

 three bases, it is customary to employ a substitute method of determin- 

 ing the rate of growth, not for the total period by any of these adjust- 

 ments, but for a partial period, measuring the current periodic growth 

 based upon trees or stands which have already reached a given diameter 

 or average age. This will be discussed in Chapter XXXI. Its effect 

 is to eliminate most of the uncertainty attending the adjustment of 

 the factor of competition in many-aged stands, but it introduces the 

 question as to whether the current growth measured represents the 

 true mean or average for the site over a complete period of crop pro- 

 duction. 



300. The Effect of Treatment on Growth. The fact that the growth 

 of individual trees demands expansion of their crowns influences 

 not merely the yield per acre which may be attained, but more especi- 

 ally the dimensions of the individual trees in the stand. Since the 

 production of lumber and of certain piece products and the value of 

 products grown on a given acre depend much more largely upon dimen- 

 sions and sizes and upon quality than upon total cubic volume, yields 

 attained in board feet are profoundly influenced by the number of trees 

 brought to maturity in stands of equal degrees of crown density or 

 stocking. It has been commonly assumed that a normal or fully 

 stocked stand simply meant one which showed a complete crown density 

 throughout its life regardless or independent of the number of trees 

 which composed it. This conception neglects the fundamental idea 

 of the tree as an individual. Stands which are fully stocked when 

 young, so that crown density is early established, usually become over- 

 stocked almost immediately. The normal number of trees, to attain 

 best results or highest yields, is least on good sites with strong growing 

 species, rapid height growth and correspondingly rapid diameter growth, 

 and increases as the sites become poorer. The danger of over-stocking 

 and stagnation of both height and diameter growth increases with 

 poor sites, even-aged stands, and tendency to abundant reproduction. 

 These natural tendencies are affected tremendously by artificial control. 

 All operations such as planting, in which the initial spacing is fixed, 

 and subsequent thinning by which the resultant number of trees per 

 acre at each decade is determined, have a direct effect upon the diam- 



