ASSOCIATION 207 



is equally true of dense thickets and wastes, except that the vertical distance 

 is less, and the diffuseness of the light is correspondingly modified. In these 

 formations, the dominant trees, shrubs, or herbs, the facies, constitute a pri- 

 mary or superior layer. The degree of subordinate association, as a result 

 of which inferior layers will arise, is entirely determined by the density of 

 the facies. In open woodlands, which are really mixed formations of wood- 

 land and grassland, the intervals, and usually the spaces beneath the trees 

 also, are covered with poophytes, showing an absence of subordination due 

 to light. This is the prevailing condition in the pine formation (Pinus pon- 

 dercsa-xcrohylium) of the ridges and foot-hills of western Nebraska. 

 When, however, the trees stand sufficiently close that their shadows meet or 

 overlap throughout the day, the increasing diffuseness begins to cause modi- 

 fication and rearrangement of the individuals. By photometric methods, the 

 light in a forest is found to be least diffuse just below the facies, while the 

 difTuseness increases markedly in passing to the ground. The taller, 

 stronger individuals are consequently in a position to assimilate more vig- 

 orously, and to become still taller and stronger as a result. Just as these 

 have taken up a position inferior to that of the facies, so the shorter or 

 weaker species must come to occupy a still more subordinate position. This 

 results, not only because the light is primarily weaker nearer the ground, 

 but also because the taller plants interpose as a second screen. The complete 

 working out of this arrangement with reference to light produces typical 

 subordinate association, which finds its characteristic expression in the lay- 

 ering of forests and thickets. Layers tend to appear as soon as open wood- 

 land or thicket begins to pass into denser conditions, and up to a certain 

 point, at which they disappear, they become the more numerous and the 

 more marked, the denser the forest. 



In the Otowanie woods near Lincoln (Qucrctis-Hicoria-hyllum), layering 

 usually begins at a light value of .1 (i^rnormal sunshine in the open). 

 Thornber^ has found the same value to obtain in the thickets of the Missouri 

 bluffs. In these, again, layers disappear at a value of .005, the extreme 

 dififuseness making assimilation impossible except for occasional mosses 

 and algae. A number of herbaceous plants are present in the spring, but 

 these are all preyernal or vernal bloomers, which are safely past flowering 

 before shade conditions become extreme. In the Fraxinus-Catalpa-alsimn, 

 all inferior holophytic vegetation disappears between the light value of .004 

 and that of .003. The spruce-pine formation {Picea-Pinus-hylium) of the 

 Rocky mountains, with a light value of .01, usually contains but a few scat- 

 tered herbs, mostly evergreen ; in some cases there are no subordinate plants 



'Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 

 5 :36, 46. 1901, 



