340 George E. Nichols, 



the trees in such a forest there may be no vegetation whatever, 

 but only a dry layer of dead spruce needles, comprising what 

 the forester familiarly refers to as "duff." The absence here of 

 plants does not seem to be attributable directly to insufficient 

 light. In remarking recently on this same phenomenon, Moore 

 ('17, pp. 156, 157) has concluded that the lack of vegetation is 

 due to the dryness of the soil which results from the interception 

 of the precipitation by the crowns of the trees. The writer had 

 already arrived at a conclusion somewhat as follows. During 

 the development of a group of young spruces in the open, at 

 first there is ample light for all. But later on, in the competition 

 for light which ensues as they become larger, many of the trees 

 are killed. The accumulation on the ground beneath, both of 

 the needles which fall from these dead trees and of needles 

 derived from the shaded branches of the living trees, may 

 take place so rapidly that the ground vegetation is buried. The 

 formation of this thick, loose layer of dry needles not only 

 wipes out the original ground cover, but, because of its dryness, 

 prevents any new vegetation from getting a start. This process, 

 initiated while the tree growth is still open, continues during 

 the transition from the grove to the forest stage in the succes- 

 sion. A layer of needles several inches thick may collect on the 

 forest floor, and all the mosses and herbaceous plants, as well as 

 the seedling trees described in the preceding paragraph, may be 

 exterminated. The extreme paucity of vegetation on the forest 

 floor which results in this manner is a very characteristic feature 

 of young coniferous forests. Later on, as the forest matures, 

 the trees becoming greatly decreased in number by the constant 

 competition for light, and in consequence becoming more widely 

 spaced, the rate of leaf-fall gradually slackens so that a certain 

 degree of equilibrium is brought about on the forest floor. It 

 then becomes possible for a new ground cover to establish 

 itself : Polytrichum commune and Hypmtm Schreberi reappear, 

 followed shortly by Hylocomium splendens, and a moss carpet 

 is gradually reestablished, on which woodland herbs and shrubs, 

 together with seedlings of balsam fir and other trees of the 

 coniferous forest association-type become increasingly abundant. 

 The history of the forest beyond the grove stage of the succession 

 is practically identical with what has been described in connec- 

 tion with primary successions. 



