350 BOTANICAL GAZETTE [November 



yet but little disintegration, the principal forms present are the lichens, 

 but even here occasional shrubs and grasses may be found growing 

 in crevices where a little soil has collected. On loose gravel slopes 

 denuded by erosion, herbs, shrubs, and even trees from neighboring 

 formations, may pass in indiscriminately. Along railroad cuts and 

 embankments and in new soils generally, which are close to highways 

 or other paths of ruderal migration, those forms are among the first 

 invaders, but they are not exclusively the first to enter. Associated 

 with them are forms from the surrounding formations, and these may 

 be either herbs or shrubs, while in some instances trees are the first 

 invaders of a new soil. As an instance of the latter we have the inva- 

 sion of sand flats on Boulder Creek by Populus occidentalis. 



All of the forest formations of the higher mountains have been 

 extensively denuded by fire {figs, j, id). The Pinus scopulorum 



burned 



form 



them. While 



m 



mobility of their seeds and the readiness with which they reproduce 

 from roots left uninjured in the burned area, their share in reforesta- 

 tion is not exclusive. Thus any other of the forest trees may 

 reforest a denuded area without the intervention of any aspen stage 

 in the succession, while pines and aspens may reforest a denuded 



w 



area simultaneously. ■- 



In the numerous meadows of dacial origin in the higher mountains, 



^x^v.^«,j. wxxq^x^ XX* ^xxv, x*XQ 



meado 



m 



mosses 



^ sedges, and grasses; these in turn 

 to be replaced by beeches and willows, and these latter finally to 

 make way for the forest. These stages, however, are not sharply 

 defined. The forest may succeed the sedges and grasses directly, 

 without the intervention of willows and beeches, or the latter may 

 succeed the grasses and sedges simultaneously with the forest. 



The life-zones of the mountains in Boulder County may be outlined 

 as follows, based upon the forests of the region: Alpine zone, char- 



3300 to 3450^; Hudson 



) 



Aphius flexilis and Abies lasiocarp 





