Botany in Colorado 29 



broad forest belt are usually much more clearly marked than the 

 boundary lines between the three forest zones. These five 

 zones are characterized about as follows as they are found in 

 Colorado. 



1. The Plains Grass Zone. 



There are no true lowlands near the mountains, but on the 

 contrary one passes from a grass or chaparral formation at an 

 altitude of from 5000 feet to 6000 feet directly to the foothill 

 forest zone. The plains grass zone is essentially a grass forma- 

 tion surrounding the bases of the mountains and extending for 

 many miles out on the plains. Many grasses occur in the zone 

 the principal ones being Bouteloua, Bulbilis, Aristida, Stipa, 

 Sporobolus, etc., etc. In some localities the grasses are inter- 

 spersed with woody composites and in some places such woody 

 plants as Artemisia and Bigelovia constitute the bulk of the 

 vegetation. Many shrubs and small trees are found along the 

 stream courses. On the higher stretches of the zone, i. e., nearer 

 the mountains, on the low rock ridges just before we enter the 

 foothills proper pinon pine, Pinus edulis, is very commonly 

 found. The most common associate of the pine is the one-seeded 

 Sabina, Sabina monosperma. These two trees appear to enjoy 

 the driest of dry situations, where they form characteristic open 

 stands of low trees. Some of the small trees are of great age. 

 The rocky soil of the ridges is usually very coarse with scattered 

 surface bowlders. The water content is extremely low especially 

 on south exposures. Besides the conifers the soil supports a 

 widely scattered stand of grasses and coarse herbs. Most of 

 the ridges are covered with either granitic or sandstone soils. 



It is in this zone where most interesting problems in forest 

 encroachment may be investigated. The pinon and the sabina 

 seldom leave their home on the ridges to wander out over the 

 more level areas with finer soil. More often does the bull pine, 

 Pinus ponderosa, migrate to lower levels and out among the 

 grasses; even the occurrence of this species on the plains is not 

 very common. The pinon and the sabina possess large, bony, 

 wingless seeds so perhaps we may account in part for their ab- 

 sence farther out by the poorly developed devices for dissemina- 

 tion. But the seeds of bull pine are provided with a large 

 membranous wing and hence they may be carried for considerable 

 distances by the wind. It is certainly true that many more 

 seeds of this species find their way out on the plains than is in- 

 dicated by the trees found there. The reasons for this are not 

 clear at all. I have said that the zone on the whole is grassy. 

 In many places it is well sodded, and the water content, es- 



