each usually occupying an extensive area, have been proposed 

 by Knapp'-^ — for example, the oak-hornbeam principal-associa- 

 tion extending from western to eastern Europe. The characteriza- 

 tion and delimitation of these large units are based on sociolog- 

 ical analyses and descriptions of almost innumerable stands, 

 such as those of the characteristic species Carpinus betulus, Prunus 

 avium, and Potentilla sterilis in the oak-hornbeam forest, in which 

 the chief attribute for delimitation is high constancy. Subdivisions, 

 called regional-associations (Gebiets-assoziationen), are distin- 

 guished by differential species, floral history, and some features 

 of the habitat. Examples in the United States of Knapp's prin- 

 cipal-association appear to be the oak-hickory forests of the 

 central and eastern parts of the country and the Bouteloua-Buchloe 

 grasslands of the Great Plains. 



Ecological Relations or Habitat 



Classification on the basis of ecological relations (synecology) 

 has been widely used. This is a causal approach that requires an 

 analysis of the environment and the segregation of habitat types, 

 which involves difhcult problems, for the habitat factors govern- 

 ing the occurrence of species and communities are not readily 

 measured and interpreted. Usually observations and interpreta- 

 tions of relationships have been limited to the major conditions 

 such as geographic location, physiography (altitude, slope, soil), 

 and general climatic influences, without including detailed 

 quantitative data. However, large environmental complexes can 

 be used advantageously in distinguishing the main kinds of veg- 

 etation (Figure 1-4), later dividing them into their compo- 

 nents, as was done in a study in the mountains of Scotland ^^^ in 

 which most of the communities fit into a framework of the 

 following five factor complexes: 



(1) Altitudinal zonation: forest, up to about 2100 feet; above 

 it, subalpine, potential birch woodland; low alpine; very 

 little middle alpine 



(2) Oceanicity: change from oceanic climate near the coasts 

 to continental climate in the interior, including higher 



Bases axmd Uimits • 181 



