FOKKST DISTRIBrTlON 



itti'iidinir the above experiments indicate something of the 



ha/ards involved. Of all native species of forest trees the yellow 

 j)ine undoubtedly is the most capable of surviving in siu-li situ- 

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

 addition to this \ve 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 must be produced for every tree that shows its 

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

 These soils absorb practically al! the rain that falls, except what 

 is lost by evaporation from the surface of plants. Then- is 

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

 <|iiickly any rain that may fall upon it. 



In the invasion of the grassland the constant companion 

 of the yellow pine is the Douglas spruce (Pseudotsiif/n t<i.n folia). 

 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 mesophytic or near-mesophytic forest. 

 On account of its greater tolerance it is capable of supplanting 

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

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

 not crowded, as its ha'>i1 is on more favorable ground. 



The western yellow pine and Douglas spruce are. however, 

 'hara<-trri<ti<- of the semi-arid /ones and belts of irregular form 

 and determined by elevation exposure, precipitation and wind 

 in tin- tension /one between woodland and prairie. On the Mis- 

 souri drainanv tln-y < Hen occur associated with J n iii />/ rus srafni- 

 lorum, either on the foothills or ?ilonr the rim-rock bordering 

 the benches on the Yellowstone and other streams far out into 

 tin- plains. Of the two species the yellow pine has a little t he 

 iiLTL'le as the pioneer in the invasion of the grass- 

 land. but ouin-j to its intolerance, is at a decided disadvantage 

 in i oniprtiiiL' with the Douglas spruce in the better watered soils. 



One of the most sinking features of the forests at middle 

 elevat dly on the western face of the divide is the 



-tronir inflm n<-.- of topography in the matter of forest distil- 



