ioo A Study of the Vegetation of 



In case of a limited but severe burn in the midst of a mixed 

 forest, provided a few lodgepole seed-trees are present, the fol- 

 lowing generation is usually predominated by the lodgepole pine. 

 Provided the various tree seedlings have an equal start, the lodge- 

 pole, because of its more rapid height-growth, gains an ascend- 

 ency over its neighbors. Likewise, by its habit of early repro- 

 duction (I have frequently found trees only seven years old bear- 

 ing cones) it furnishes seeds for any unoccupied areas or for the 

 entire area if seeds of other trees for any reason fail to be present. 

 I have seen a few such areas dominated by lodgepole pine but 

 comparatively they are not abundant. 



But because of its early maturity and its inability to reproduce 

 in its own shade, in spite of the advantage gained in the middle- 

 aged forest, the lodgepole is the first tree to drop out of the 

 mature forest. Unless aided by another fire before its competitors 

 reproduce, it assumes a less and less important role. Indeed, in 

 most of the forests it occurs only as scattered individuals, al- 

 though, as previously stated, in areas cut and burned-over thirty 

 to fifty years ago the lodgepole pine is not infrequently an im- 

 portant tree. 



THE HYDROSERE 



We shall now take up the development of the hydrosere. An 

 outline of the vegetational units of this sere together with their 

 relation to the xerosere may be found on page 19. Because of 

 the peculiar topography of the region, combined with the arid 

 climate, hydrophytic and meso-hydrophytic communities are not 

 widely represented. However, along the streams and in small 

 ponds and lakes, the latter occurring especially in the north- 

 western part of the region, the following communities are found. 

 The three well marked associes of the water sere showing strik- 

 ing changes of habitat and life form as well as definite floristic 

 differences, are those dominated by submerged, floating, and 

 swamp plants respectively, in their successional sequence. The 

 last stage is succeeded by a wet meadow community which is 

 usually followed by a consociation of the Agropyron-Festuca 

 association but may be replaced by the flood-plain forest associes. 



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