106 FORUST DISTRIBUTION 



ures attending the above experiments indicate something of the 

 hazards involved. - Of all native species of forest trees the yellow 

 pine undoubtedlj' is the most capable of surviving in such situ- 

 ations, from the standpoint of its water requirements, but in 

 addition to this we may consider also the fact of considerable 

 amounts of seed produced, their high percentage of germina- 

 tion and the facility with which they are sown by the wind. 

 Millions of seeds mast be produced for every tree that shows its 

 head above the herbaceous vegetation, and such are very few. 

 Thesie soils absorb practically all the rain that falls, except what 

 is lost by evaporation from the surface of plants. There is 

 rarely any run-off. The soil is porous and stony and absorbs 

 quickly any rain that may fall upon it. 



In the invasion of the grassland the constant companion 

 of the yellow pine is the Douglas spruce (Pseudotsuga taxifolia). 

 No more adaptable species is to be found among the forest trees 

 in the northwest. Partial as it is to abundant moisture, never- 

 theless it resists almost as much drouth as the pine, and grows 

 to large size, albeit more slowly than elsewhere. It is this spe- 

 cies which more than any other marks the transition from the 

 open yellow pine to the m.esophytic or near-mesophytic forest. 

 On account of its greater tolerance it is capable of supplanting 

 the pine. In the.se situation, however, it is slow to develope 

 density and grows with the pine in an orchard-like stand and 

 not crowded, as its habit is on more favorable ground. 



The western yellow pine and Douglas spruce are, however, 

 characteristic of the semi-arid zones and belts of irregular form 

 and determined by elevation, exposure, precipitation and wind 

 in the tension zone between woodland and prairie. On the Mis- 

 souri drainage they often occur associated with Juniper us scopu- 

 lorum, either on the foothills or alontj' the rim-rock bordering 

 the benches on the Yellowstone and other streams far out into 

 the plains. Of the two species the yellow pine has a little the 

 best of the struggle as the pioneer in the invasion of the grass- 

 land, but owing to its intolerance, is at a decided disadvantage 

 in competing with the Douglas spruce in the better watered soils. 



One of the most striking features of the forests at middle 

 elevations, especially on the western face of the divide is the 

 strong influence of topography in the matter of forest distri- 



