1 70 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



varying conditions, the number of sample plots is so large that 

 the thinnings cannot all be carried out under the immediate 

 supervision of one officer. When revising old thinnings, uni- 

 formity in the carrying out of the work and a true apprehension 

 of previous thinnings are easily obtained if the trees are definitely 

 classified. 



The changes which the trees have undergone between the 

 revisions can be recorded in a more complete manner by 

 transferring the trees to a new tree-class than by the registration 

 of the increases of dimension only. 



The system of classification is based on the position of the 

 height of the trees in the stand and on the condition and 

 character of the crowns and, in lesser degree, the stems. It is 

 fairly easy to distinguish in the stand certain levels to which the 

 heights of the trees reach. These levels are called "Crown 

 Strata," and are grouped as follows : — 



I. Crown Stratum of the Dominant Trees : — Consisting of 

 dominant trees, the largest in the stand. 

 II. Crown Stratum of the Co-Dominate Trees : — Including trees 

 of somewhat less height, less developed crown, and often 

 smaller diameter than in Group I. 



III. Crown Stratum of the Dominated Trees : — Composed of 



trees reaching about two-thirds of the height of the trees 

 in Group I., the top shoots being generally short. 



IV. Crown Stratum of the Under-Stand : — The trees reaching a 



height of 50 to 60 % of the height of the trees in Group I.; 

 this group includes the suppressed trees and isolated trees 

 of the same height in gaps. 



In assigning the trees to the different crown strata the 

 undergrowth (u) and the standards (o) are not included, as 

 these two groups, strictly speaking, do not belong to the 

 actual stand. 



In the undergrowth may be included trees of other species 

 and younger age (usually more than forty years younger) than 

 those composing the actual stand. When the undergrowth 

 reaches half the height of the trees in Group I., it is assigned to 

 the stand and put in the proper group. 



Standards are such trees as are at least forty years older 

 and usually taller and larger than the stand proper. 



Every crown stratum is then divided into tree-classes based on 



