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- 
derosa-xerohylium) 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 catise 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 
diffuseness 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 (Quercus-Hicoria-hylium), layering 
usually begins at a light value of .1 (1=normal 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 .o05, the extreme 
diffuseness making assimilation impossible except for occasional mosses 
and algae. A number of herbaceous plants are present in the spring, but 
these are all prevernal or vernal bloomers, which are safely past flowering 
before shade conditions become extreme. In the Fraxinus-Catalpa-alsium, 
all inferior holophytic vegetation disappears between the light value of .co4 
and that of-.003. The spruce-pine formation (Picea-Pinus-hylium) of the 
Rocky mountains, with a light value af .o1, usually contains but a few scat- 
tered herbs, mostly evergreen; in some cases there are no subordinate plants 
1Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 
5:36, 46. 1901. 
