[From ihe Brt.i.ETiN of the Torrky Botanical Club, 4a: 629-642. 15 D 1915.] 



Phytogeographical notes on the Rocky Mountain region 

 V. Grasslands of the Subalpine and IVIontane Zones 



p. A. Rydberg 



In a previous paper, I have discussed the forest trees of the 

 mountains proper. Next to the trees the most important vege- 

 tation consists of the grasses. In fact, it may be said that in 

 most cases where the land is not covered by the forest, the grasses 

 are the predominant plants. The principal exceptions are the 

 following: (i) Lakes and ponds, as the aquatic grasses are few 

 and there are no aquatic trees. (2) Rock-slides and bare cliflfs, 

 where the soil is too scant. As soon as humus and sediments 

 have had time to increase the soil, regions of this type will grad- 

 ually change into either a forest-formation or a grassland. (3) 

 Shrub formations. These are not very common in the moun- 

 tains, but more so in the foothills. They are mostly found on 

 the borders between the forest and the grassland, and in time will 

 change into either, usually into the former. (4) Certain brook 

 banks, especially in the subalpine region, where the vegetation 

 consists of species of Mertensia, Senecio, Delphinium and other 

 dicotyledonous plants. 



Usually the forest occupies the slopes, while the grasslands 

 occupy either the elevated ridges or so-called "hog-backs, " or else 

 the valleys or so-called "parks." The cause of this division is 

 pointed out, or at least suggested, in one of my preceding papers on 

 the alpine region.* The " hog-backs " are exposed to the desiccating 

 winds and are therefore too dry for forest growths. In the valleys 

 too much snow lodges during the winter and melts too slowly 

 during the summer and the trees would be suffocated. There may 

 also be other causes. In the valleys and parks, the drainage is 

 often too poor and the soil has become too alkaline or saline for 

 tree growth. 



The grasslands are, however, not wholly confined to the ridges 

 and valleys. In many places, the slopes where the soil and 



* Bull. Torrey Club 40: 681, 682. 1913. 



629 



