50 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY, 



whether the method is capable of being used with more than 

 fair success by anyone not already able to estimate the 

 possibilities of a locality equally well by intuition or by careful 

 observation, while the use of units of value must give a quite 

 unwarranted sense of accuracy and security to one of less 

 experience, to whom it is really of greater importance that some 

 ready means of evaluation should exist. 



The occurrence and vigour of a plant are determined by the 

 sum of the environmental factors to which it is subject. Hence 

 a plant must give some indication of the soil (and other) 

 conditions of the locality in which it grows, that is to say, in the 

 first instance, that the trees growing on an area indicate to a 

 fair extent the forestal possibilities of that area (provided they 

 have not been subjected to abnormal conditions). Thus the 

 best method of arriving at an estimation of the quality of any 

 locality for the growth of trees is the quality of the trees already 

 produced by it, and this may be brought to a very exact point 

 if a mixture of species form the crop, the best species of the crop 

 being most suited to the locality. There is no difficulty here. 



Where there are no trees on the area which is to be planted 

 there is greater difficulty. But trees are subject to the same 

 distributional laws as other plants, and respond to the same 

 factors as affect these, although not in quite the same way. 

 Not only are individual plants affected by variation of edaphic 

 conditions, but also the actual character of the vegetation, 

 which may be noticeably affected by unobservable environ- 

 mental variations. Thus the ground vegetation as a whole is 

 really a very definite indicator of the environmental conditions of 

 its habitat, and is therefore to some extent a means of evaluation 

 of the silvicultural quality of the locality. The work of Moss, 

 Rankin, and Tansley on The Woodlatids of Efig/and show that 

 distinct vegetational types may be correlated with each of the 

 types of natural and semi-natural woodlands in England. 

 Hence the vegetation in a woodland is an indication of the 

 silvicultural value of the locality, since it may be correlated with 

 the natural crop growing upon it. The objection that ground 

 vegetation is not an indication of possible effects of exposure or 

 of abnormal subsoil conditions, to which trees may be subject 

 in any locality, has greater apparent than real grounds, since 

 exposure and subsoil conditions are reflected to a considerable 

 degree in the surface soil. It may be noted also that the factors 



