PLANT ASSOCIATIONS 277 



dependent upon the habitat. The start may be on bare 

 rock, on sand, on clay, in a drained swamp, or in an un- 

 drained swamp, and then each kind of succession will follow. 

 It is also evident that the succession cannot go on indefinitely, 

 but that some final association will be reached which is called 

 the climax association, for that region. In general, some 

 type of forest is the climax association, but there are 

 obvious reasons why that type cannot be reached in certain 

 regions. 



153. Forest succession. Since forests represent the 

 most important natural vegetation, an illustration of forest 

 succession will be given. It will serve to illustrate not only 

 an important succession, but also the facts that must be con- 

 sidered in any effective study of forestry. One of the best 

 known forest regions is the white pine region of Northern 

 Michigan, from which the trees have been swept, with no 

 thought of their continuance, and the evolution of this forest 

 will indicate the forest problems in general. 



The succession of plant associations which led up to the 

 white pine forests started on sand, rock, clay, or in swamps, 

 but the series beginning on a sandy beach will be used in 

 the illustration. The first stage was the lower beach, 

 washed by the summer waves, and therefore with no vegeta- 

 tion, but with an accumulation of sandy soil. The second 

 stage was the middle beach, rising higher above the water, 

 and therefore washed only by the larger winter waves. This 

 freedom from waves during the summer permitted the growth 

 of certain annual plants, whose bodies added some humus 

 to the sand. The third stage was the fossil beach, that is, 

 a beach that was once washed by the waves, but is now beyond 

 their reach. This ^continual freedom from wave action per- 

 mitted the growth of more plants, and therefore resulted in 

 the accumulation of more humus, but the soil would still 

 have looked rather bare, as the plants would not cover the 

 surface. The fourth stage was made possible by the accu- 

 19 



