272 BULLETIN UNIVERSITY OF MONTANA 



the soils in which the meadows occur. It must not be supposed from 

 what has been said that the trees mentioned above never grow in other 

 situations, for they do, as will be seen later. In the spruce associations 

 they are the predominant trees. In some of the other situations, they 

 are subordinate. 



Around the drier borders of the spruce association, and sometimes 

 on peninsulas or islands in it, are stands of trees other than those men- 

 tioned. These trees are clearly associated with soil in which the water 

 level is still further beneath the surface. Because the western larch and 

 the Douglas spruce are the most common trees in this stand, it will be 

 given the name of western larch — Douglas spruce association. Far the 

 greater part of Swan valley is occupied by this association. Other trees 

 in it are the lodgepole pine, lowland fir, silver pine, and occasionally 

 an arbor-vitae. The Engelmann spruce and the birch are sometimes 

 present also, though they seldom form a conspicuous element. The lodge- 

 pole pine (Plate LII) occupies vast tracts of this area. Its presence here 

 is clearly due to fires. Often the older trees of western larch and Doug- 

 las spruce, their charred trunks telling the story of former fires, are seen 

 standing above young forests of lodgepole pine. Sometimes almost con- 

 sumed trunks show that the fires have been more destructive. The 

 western larch and the Douglas spruce are the last to be killed by fires, 

 because they can resist them best. Where fires have not been so de- 

 structive, the lodgepole pine is less conspicuous. Indeed, in places it is 

 almost entirely wanting. The lowland fir, silver pine, and Engelmann 

 spruce are more abundant in the moister parts of the western larch- 

 Douglas spruce association. In isolated patches the bull pine is also 

 found. 



Just as the meadows form treeless places in the forest formation so 

 occasionally the soil may be too dry to form forests. It is a well known 

 fact that clay soil holds water better than sandy soil. The rainfall may 

 be sufficient to support trees in the former, where it would not do so in 

 the latter. This fact may account for the prairie "islands" in the forest 

 formation, to be seen in Fig. 5. 



Surrounding these prairie islands and bordering on the prairie forma- 

 tion is another type of forests which is due primarily to the fact that there 

 is more water in the soil than in the prairie, and less than in the western 

 larch-Douglas spruce association. This type of forest may be called the 

 bull pine-Douglas spruce association, because these two trees are the 

 predominating ones. Sometimes the western larch is found with these, 

 but it never occupies the drier soils. In other places the Rocky mount- 

 ain juniper is present. The bull pine-Douglas spruce forest usually is an 

 open one, with grass patches between the trees. It grades imperceptibly 

 into the prairie formation 



In the foregoing it is shown that the type of plant associations in the 

 forest formation depends on the amount of water in the soil. Warming, 

 a Danish botanist, who was the first to fully perceive this relation be- 

 tween the grouping of plants and the amount of water in the soil, classi- 

 fied plants into hydrophytes, mesophytes, and xerophytes. Those plants 

 that grow in soils with a great amount of water in it are known as hydro- 



