CLASSES OF GROWTH DATA, CHART GROWTH STUDIES 327 



ration of volume tables, but the few trees analyzed for growth may- 

 be either cylindrical or neiloidal in form. We therefore may have a 

 perfect record of the past growth of certain selected trees which vary 

 in form and volume at least 10 per cent from the average desired. 



Even if this difficulty can be overcome by careful selection of trees 

 of average form quotient, and a few of these average trees analyzed 

 for past growth, how are these past results to be applied in predicting 

 future growth? It is evident that the growth of individual trees is 

 only a part of the problem, for the average tree in a well-stocked stand 

 at a given age does not remain the average tree for future periods and 

 was not the average tree at any period in the past. The trees which 

 die in a stand are naturally the smaller, more suppressed specimens 

 with the smallest diameters. In the lapse of a ten-year period, the 

 loss of a number of trees from the lower diameter classes will raise the 

 average diameter and volume of the remaining trees so that the tree 

 which is now the average is in ten years dropped into a class below the 

 average. 



There is but one way of even approximating the growth of a stand 

 in the future by means of the analysis of volume growth of individual 

 trees. If the number of trees which will probably survive to a given 

 age can be predicted (which can best be ascertained by the method 

 of comparison and yield tables), the selection of this number from a 

 younger stand, taking trees wholly in the dominant class, will indicate 

 the character of tree which must be cut and measured to determine 

 the growth for the future. Yet even here it is better to take a tree 

 which is fully mature and shows the growth for the entire period, in 

 which case the stand, rather than the tree, is the better unit. 



255. Relative Utility of Different Classes of Growth Data, and 

 Chart of Growth Studies. To sum up these principles: past growth 

 is measured in order to predict future growth. Growth on an area 

 and not the growth of single trees is wanted. The three essentials 

 of growth are volume, time and area. For even-aged stands the time 

 element is the total age and may be determined by counting rings on 

 one or two sample trees. This requires a minimum of investigation 

 in addition to volume measurements. 



Diameter growth of trees comes next in importance and is used 

 when size must be depended upon to determine age either for the total 

 period or for shorter current periods of growth when diameter is sub- 

 stituted for age. 



Height growth of trees comes third in importance since it is used 

 to indicate site quality (§ 296). It may also be used together with 

 diameter growth, to predict the volume growth of trees by a method 

 much shorter than volume analysis. (§ 288). 



