378 JOURNAL OF FORESTRY 



senting the selected age thus become, in effect, a key to the curves 

 themselves. The height growth index must therefore be considered 

 simply as a ready means of correlating height growth data previously 

 secured. Its function is to identify and give a name to the site. 



The site interval of 10, 15, or 20 feet (see Roth's table on a pre- 

 ceding page) provides for a considerable range in height at 100 years. 

 In many cases the height growth curves of a number of different 

 species will undoubtedly fall within a possible set of limiting curves 

 which may be drawn through the site interval points set up at 100 

 years. An illustration is the set of curves for southern upland hard- 

 woods illustrated in the January issue of the Journal.* The similar- 

 ities in the rate of growth were such as to appear to warrant a single 

 set of site curves for a number of Southern Appalachian hardwoods, 

 based at the outset on chestnut. Each species has, of course, its own 

 characteristic growth habit ; but the differences are insignificant, usu- 

 ally well within the limits of consistent accuracy imposed by other 

 factors. The height growth of these intolerant hardwoods is similar 

 enough to be regarded as identical for many practical purposes. 



It should be emphasized, however, that in the proposed plan sites 

 are classified for one species only. If other species fall in closely 

 enough to be grouped, well and good. Generally this is very secondary, 

 but natural groupings of species of similar height growth on the same 

 sites, and therefore belonging to the same "standard," will be of mani- 

 fest value. At 100 years the height growth of most of our important 

 species on most of their sites closely reflects the site conditions.^ For 

 these species the height at 100 years may be (somewhat figuratively) 

 said to ejMtomize the height-growth capacity of the site. The century is 

 a rather appropriate time unit in forestry, corresponding with a fre- 

 tiuently expressed rotation period and also, roughly, with the stage in 

 the life of a majority of our species at which they are "flattening off" 

 in height growth. 



Before a species can be assigned to its proper "standard" in the 

 height classification scheme its height at 100 (or 50) years on its best 



' "Site Determination and Yickl Forecasts in the Southern Appalachians," by 

 E. H. Frothingham, Jocrxal of Forestry, 19:1-14. 1921. 



- At this age the differences in growth, as a result of dififerences in site, are 

 well marked. For short-lived species and those which (like poplars, loblolly 

 pine, etc.) flatten their growth curves at an early age, 50 years is better, as 

 suggested by Watson (Journal of Forestry, 15 :.552-fi3. 1917). There are few, 

 if any, species of such sustained growth as to require the placing of the reference 

 point at greater than 100 years. 



