CAUSES OF ZONATION 153 



is light. Its intensity is greatest at the edge of the for- 

 mation, and just below the primary layer: the light becomes 

 increasingly diffuse toward the center of the forest, and 

 toward the ground. In response to this, both lateral and 

 vertical zones appear. The former are more or less in- 

 complete, and are only in part due to differences in illumin- 

 ation. The vertical zones or layers are characteristic of 

 forest and thickets, and are caused directly by differences 

 in light intensity. 



The physical causes of zonation are by far the most im- 

 portant. They arise from differences in temperature, 

 water, and light. In the large, temperature differences are 

 the most important, producing the great zones of vege- 

 tation. In a particular region or habitat, variations of 

 water content and humidity are controlling, while light, as 

 shown above, is imnorant in the reactions of forest and 

 thicket. Physical factors produce zonation in a habitat or 

 a series of habitats, when there is either a gradual and 

 cumulative, or an abrupt change in their intensity. 

 Gradual, slight changes are typical of single habitats, 

 abrupt, marked changes of a series of habitats. This modi- 

 fication of a decisive factor tends to operate in all directions 

 from the place of greatest intensity, producing a character- 

 istic symmetry of the habitat with reference to the factor 

 concerned. If the area of greatest amount is linear, the 

 shading-out will take place in two directions, and the sym- 

 metry will be bilateral, a condition well illustrated by 

 rivers. On the other hand, a central intense aroa will shade 

 out in all directions, giving rise to radial symmetry, as in 

 ponds, lakes, etc. The essential connection between these 

 is evident where a stream broadens into a lake, or the latter 

 is the source of a stream, where a mountain ridge breaks up 

 into isolated peaks, or where a peninsula or landspit is cut 

 into islands. The line that connects the noints of accumu- 

 lated or abrupt change in the symmetry is a stress line or 

 ecotoiie (oiKos, 6, habitat, tovos, 6, stress). Ecotones are well- 

 marked between formations, particularly where the medium 

 changes: they are less distinct within formations. It is 



