RELATION BETWEEN VOLUME AND AGE OF STANDS 449 



in separating stands or units of volume estimating, such as small legal 

 subdivisions, e.g., 10 acres, except where, by the aid of topography, 

 the site qualities can be mapped to conform more closely with natural 

 boundaries. Types are commonh' separated in the forest survey by 

 mapping the areas, and the estimate is usually separated to coincide 

 with the divisions thus made (§221) though on forties this is not always 

 done. 



346. Relation between Volume and Age of Stands. Density of 

 stocking, as shown, is not determined l)y the total merchantable volume 

 of a stand, but by a comparison of the existing volume with the index 

 volume which stands should have at given ages. Density when deter- 

 mined by comparison of volumes, is therefore a function not solely of 

 area but also of age. To determine density for large areas, therefore, 

 a basis of separation of the volume into age classes is required. This 

 means either the direct mapping of areas of separate age classes, or a 

 tally of diameters and a stand table for diameter classes in the stand. 

 IMethods of forest survey which utilize diameter tallies to obtain volumes 

 (§ 207 and § 209) naturally lend themselves to the securing of such 

 a stand table. The use of such talhes for determining age groups and 

 average ages are shown in § 320 and § 323. In general, density of stock- 

 ing for mature age classes will be found not in the field, but after the 

 volumes have been computed or stand tables prepared, and by means 

 of a comparison of volumes with the yield table, on the basis of similar 

 ages. 



Age classes and their actual ages may be determined directly during 

 timber survey only when the areas which they occupy are separate, large 

 and easily distinguished, and when time permits of the testing of trees 

 for age. In intensive management, this method will be followed on small 

 areas; but for large areas of mixed ages, the general method of depending 

 upon diameters to indicate age should be relied on; hence the stand 

 table is the basis of this age class division, both for age and area (§318 

 to § 323.) 



347. Averaging the Site Quality for the Entire Area. Site qualities, 

 when not con-elated with type, present difhculties in classification, 

 so much so that on large extensive projects site qualities may for the 

 time have to be waived and an average yield table obtained for all 

 sites. (This method was adopted in the preliminary working plan 

 for the Coconino Nationa' Forest, Arizona.) A composite stand table, 

 including stands on all sites, is best for this purpose. Its application 

 to the average site will depend on the average density or reduction 

 per cent found for the area. Only when the divisions of the total area 

 into site qualities can be coordinated with similar divisions of the esti- 

 mate and stand can these divisions be made the basis of separate growth 



