NORTHERN COLORADO PLANT COMMUNITIES 227 



a south exposure. At Pine Cliff, still farther down the valley, the rock 

 pine is the general association. 



Scrub. — A willow-birch-honeysuckle scrub is present everywhere 

 along streams, lagoons and ponds, usually taking a position slightly 

 back from the water, the intervening space being occupied by a strip of 

 sedge moor. Ponds, as they become filled in through silting and the 

 growth of vegetation, are first occupied by sedges and then by these 

 shrubs which in turn give way to coniferous trees (Engelmann spruces) 

 as the soil moisture becomes diminished. Occasionally aspens become 

 established before the conifers make their appearance but this can only 

 occur where drainage is good and the soil not "sour." 



Potentilla scrub is generally quite open, that is the bushes are far 

 apart. Grasses, sedges and flowering herbs occupy the interspaces. 

 As a rule this association occurs in situations slightly drier than where 

 willows are found. In the flatter parts of the park it is followed in time 

 by grassland; on steep hillsides it may give way to aspens or to conifers. 

 This community is always to be thought of as proximate and never 

 ultimate. It is continually being replaced 1 in any one locality by other 

 associations adapted to a more stable habitat. On the other hand, 

 such a community as the lodgepole pine association is an ultimate com- 

 munity. It does not become replaced by anything else, for it depends 

 on climatic conditions, which do not change. 2 



There are some scattered Potentilla bushes in certain dry grassland 

 areas of the park. In such situations their presence is to be accounted 

 for on historical grounds. They were present at an earlier period when 

 these particular spots of ground had more moisture and they have not 

 yet been driven out by competition with grasses and other herbs, nor 

 have they died out for lack of water. 



Across the park from Tolland on the south-facing hill there is a ridge 

 covered with sage brush {Artemisia tridentata or possibly Artemisia 

 arbuscula). Although Boulder Park seems very high for such a plant 



1 The replacement of one association by another is known technically as succession and furnishes an 

 interesting subject for investigation. 



* Naturally when it is stated that any particular type of environment does not change it is meant that 

 change will not occur without profound alterations in climate, and these, as is well known, do not take place 

 except at great intervals, and very slowly 



