THE PLANT ASSOCIATION. 271 



The Plant Associations. 



Harry Nichols Whitford. 



From what has been said in the preceding pages it can readily be 

 seen that the climate affects the distribution of plants. The so-called 

 plant formations are the direct result of certain climates. However, the 

 landscape of a forest formation (Fig. 5) does not show an equal distribu- 

 tion of the kinds of trees in it; nor does it show an unbroken mass of 

 trees. On the other hand there is a tendency for trees of certain kinds 

 to be growing together, and there are gaps in the forest, island-like open- 

 ings, as it were, in the sea of trees. The gaps may contain a 

 meadow, a heath, or even a prairie. A prairie formation may also con- 

 tain trees in certain favorable situations (Fig. 5) where there is a suffi- 

 cient water supply. In distinction from the formation these groups have 

 been called plant associations or plant societies. It is now in order to 

 inquire what causes have brought about the division of formation into 

 associations. This can be done best by selecting a limited region in the 

 forest formation, for example, and by noting what associations are found 

 and in what conditions they are growing. The region around the Mon- 

 tana Biological Station at Big Fork is a good one to Illustrate the point. 

 Any region in the state can be studied in the same way, whether it be 

 in the prairie or forest. 



In the forest formation near Big Fork there are five more or less dis- 

 tinct plant associations. In the low places in the Swan river valley along 

 streams and around ponds are moist areas that are usually submerged 

 during the spring and early summer months. These are known as 

 meadow associations (Plate LII.) The grass type prevails in these places, 

 grasses and sedges of various sorts being here associated. Oftentimes 

 peat moss is abundant, and with it may be found the interesting carnivor- 

 ous plant called sundew. Around the borders of the meadows, and 

 sometimes in them, are willows, alders and birches. Shallow ponds con- 

 taining water lilies and pond weeds are often found in the meadows. It 

 is very evident that the reason why trees do not grow in these meadows 

 is because there is too much water, which, like too little water, is in- 

 jurious to them. 



Bordering on the meadows are localities, not quite so damp, where 

 the Engelmann spruce and other plants growing with it are found, form- 

 ing what may be called the Engelmann spruce association. On the edge 

 of the meadow the trees of this association are small and scattered. De- 

 pending on the amount of water in the soil, there may be peninsulas and 

 islands of spruce in the meadow. Sometimes there are almost pure 

 stands of spruce forests; again the spruce element is scattered along 

 streams. Very often lodgepole pine, narrow-leaved cottonwood, aspen, 

 and birch are associated with the spruce. These particular combinations 

 of trees are always found in soil that is quite damp, but not so damp as 



