188 PLANT COMMUNITIES 



length in later chapters but it will aid us in grasping the significance 

 of the individuality of plant commimities if we trace briefly the de- 

 velopment of one at this time. 



Let us take for example a beech and maple forest such as may be 

 found in Indiana or Ohio as well as in a number of the other states 

 east of the Mississippi River. Let us suppose that after the glacial 

 period an area of bare rock of fairly high elevation was left exposed 

 somewhere in Indiana or eastern Illinois. The bare rock was, of 

 course, not a suitable place for trees to grow, but, since it was a 

 bare area, that is, an area unoccupied by plants, it was a good place 

 for the birth of a plant community. The first plants that appeared 

 on the rock were some xeric lichens. This was the birth or beginning 

 of the new plant community. 



The lichens grew very slowly, but gradually they eroded the sur- 

 face of the rock, causing it to crumble somewhat and they also col- 

 lected dust and other materials blown upon the rock by the wind, 

 so that after a number of years they had formed a small amount of 

 soil upon the rock. Because of this soil, which would hold a certain 

 amount of moisture, it was now possible for a few kinds of mosses to 

 grow here. With the coming of the mosses the formation of soil 

 went on more rapidly and some years later, therefore, it was possible 

 for some of the more xeric herbaceous plants to come in. Probably 

 the first ones were some of the common weeds and these were later 

 followed by grasses and the perennial plants that usually grow along 

 with the grasses. Notice, now, that as more plants grew here and 

 more soil was formed the place w^as becoming less and less xeric. 

 Finally, after many years, there was soil enough and water enough 

 that woody plants began to appear. Perhaps the first was the 

 juniper, or some xeric shrubs, but later other and larger trees came, 

 especially the black and white oaks and the hickories. Our plant 

 community had now developed into a real forest and, because of the 

 shade and the increasing layer of leaf mold, it was becoming quite 

 mesic but it was still too dry for the beech and maple for these trees 

 are confirmed mesophytes. Of course the oaks and hickories had 

 many other trees, shrubs, and herbaceous plants associated with 

 them, each of which had its part to play in the community, and they 

 flourished for many years. But gradually the place became more and 

 more mesic, the oaks and hickories gave place to the beeches and 

 maples, and the community had at last reached its maturity 

 (Fig. 82). 



