PLANT COMMUNITIES OF SANDY SOIL 323 



III. GENERAL DISCUSSION 



All of the plants collected or recorded in the present study- 

 are well-known members of the Colorado flora; most of them occur 

 elsewhere in the neighborhood of Georgetown and are not en- 

 tirely confined to sandy soil. Some, however, reach in this 

 sandy area a higher altitude than is usual for the species in this 

 part of Colorado. Examples are: Eriocoma, Bouteloua, Dis- 

 tichlis, Peritoma, Rhus trilobata, Opuntia, Pediocactus, Anogra 

 albicaulis, Artemisia brittonii. No doubt, the character of the 

 soil rather than any local climatic condition is the determining 

 factor. Three of the plants just named, viz.: Eriocoma, Peri- 

 toma, and Anogra albicaulis may be characterized as "sand 

 dwellers" since they seldom occur abundantly in other soils at 

 any altitude. 



In our area of study, mixtures of various plant communities 

 occur in every possible combination because of diversity of soils. 

 For example, a former rock- wash fan has recently, i.e., within a 

 few years, been covered by a layer of sand, now about 4 dm. 

 thick. The sand has been deposited by succeeding storms proba- 

 bly a few centimeters at a time. Some of the former plants re- 

 main, chiefly Artemisia frigida, but there is an almost equal 

 amount of Chrysothamnus with some Bouteloua. These are 

 plants that belong, in our area, especially to sand. The Arte- 

 misia is. then, a remnant of a totally different ecological com- 

 munity. Chrysothamnus and Bouteloua belong properly to the 

 developing arenicolous flora. Reverse conditions frequently oc- 

 cur. A short grass growth may be covered completely with rock 

 wash. Here a mixed inceptive grassland will develop, dominated 

 at first by Artemisia frigida. The shrub growth all along the 

 lower part of the canyon walls is in unusual community for the 

 mountains. The same association, it is true, may be recognized 

 on isolated sunny slopes here and there throughout the region. 

 Scrub also, as is well known (9, 10), is an important feature of 

 the vegetation along parts of the eastern mountain front; but well- 

 developed and clearly-marked scrub belongs rather to the Great 

 Basin and the Pacific States. One naturally compares the scrub 

 at Georgetown with the chaparral of California. Like the chap- 



