17] LANDSCAPES AND VEGETATION 553 



These ' indicator ' plants, communities, and other signs are prone 

 to vary from one region to another, sometimes even of comparable 

 climate, so that considerable local knowledge is necessary for effective 

 practice of the principles involved. Imparting such local informa- 

 tion, even for a single limited area, is beyond the scope of this 

 introductory book : recourse must be had to detailed local floristic 

 and ecological works such as are now available for many parts of the 

 world. But even with much book knowledge, field experience is 

 usually necessary for reliable interpretation and use, as observations 

 have to be analyzed into usable data and synthesized into applicable 

 forms. Thus most of us living on either side of the North Atlantic 

 know that Willows and Poplars like an abundance of water in the 

 soil, yet there are wide variations in this respect between different 

 species of Willows and Poplars, some especially of the latter favouring 

 rather dry habitats. Indeed one species, popularly known as the 

 Trembling Aspen {Popiiliis trenniloides), in certain circumstances and 

 places can be a fair indicator of rather arid conditions — which 

 emphasizes the need for specific observations and local knowledge. 

 Moreover, members of an individual species often vary in their 

 needs for water, etc., in different regions, according to other habitat 

 factors and to their particular state, while it must also be remembered 

 that species within a genus are sometimes difficult to tell apart — 

 even to taxonomists, which too many ecologists are not. Still more 

 liable to be confused in our minds are members of lower taxa which 

 may yet exhibit marked ecological preferences or needs, and indeed 

 different habitat preferences may be shown by physiological strains 

 lacking any evident differences in form. 



Especially do plants tend to become more limited to conditions 

 expressed by certain habitats as they approach the periphery of 

 their range. This principle should always be borne in mind to 

 avoid mistakes in interpretation. Thus Beech forests in western- 

 most Europe are largely limited to calcareous soils, in England being 

 commonly associated with chalk, whereas to the east this is no longer 

 the case. The European Beech does, however, seem everywhere to 

 be indicative of fairly dry, well-drained conditions, and it has been 

 suggested that its western association with chalk, especially, is due 

 to the drier conditions usually found thereon as compared with 

 adjacent clayey and other soils. 



In view of the local variability of soil and some other conditions, 

 it is only to be expected that individual species of plants are in 

 general unreliable as indicators, being of relatively low indicator 



