The Community of Trees 



105 



the icecap moved slowly down from the 

 polar regions, Temperate Zone trees, 

 which at one time grew almost to the 

 Pole itself, migrated southward ahead 

 of the freezing weather. In Europe the 

 trees finally came to the Alps, which 

 made an east-to-west barrier directly 

 across their path. The climate of the 

 mountains was already cold, and, be- 

 cause none of the scattering seeds 

 lodged where they could grow, many 

 species disappeared; only a few were 

 left to repopulate the land when the 

 glacier receded. Today Europe has 

 only a few native kinds of trees. In our 

 country, the mountains run north and 

 south, and none lay in the way of the 

 trees that were retreating before the 

 ice. Consequently a rich variety sur- 

 vived the ice age and gave us our pres- 

 ent wealth of species. 



The second source of change is the 

 forest itself. It is a living community of 

 trees; through its own internal work- 

 ings it is constantly adjusting itself. 

 Within the community, plants and 

 animals live, grow old, and die; some- 

 times they help their own kind to 

 inherit their places; more often they 

 hinder them from doing so; always, 

 however, they alter the environment, 

 and, through that alteration, change 

 the forest itself. 



On any tract of land, these continu- 

 ing adjustments bring about a natural 

 development of the vegetation that re- 

 sembles the evolutionary development 

 of an animal or plant. Early plant com- 

 munities give way to more advanced 

 forms in a succession of infancy, youth, 

 and maturity that, while the climate 

 stays unchanged, is regular and pre- 

 dictable. These regular and predictable 

 changes are of utmost importance : We 

 can modify them by the way we treat 

 the forest ; we can speed up natural suc- 

 cession or delay it, depending on the 

 kind of forest most useful to us. 



Let us, then, take a closer look at 

 natural forest succession, and consider 

 ways in which we can modify it. 



Every forest area began once as a 

 stretch of bare rock or of water. If it 

 was a lake, algae and other floating 



plants first appeared. As they died and 

 sank they made the lake more shallow, 

 and plants could grow that must have 

 their roots in the bottom and their 

 leaves above the water. The remains 

 of these in time built the land still 

 higher, making the area less suitable 

 for floating and rooting acquatic plants 

 and more favorable for land plants. 

 These, in turn, took over and helped 

 to build up the ground with decaying 

 leaves and stems and to dry it out by 

 transpiring quantities of water. Herbs 

 gave way to bushes and they to for- 

 ests, because climate was favorable. 



If the forest area began as bare 

 rock, lichens first got a toehold in the 

 crevices that could catch a few drops 

 of moisture. Gradually, as one genera- 

 tion after another added its substance 

 to the fragments of rock broken away 

 by weathering or the corrosive action 

 of the lichen juices, a thin layer of soil 

 was built up in which mosses could 

 take root and the process continued. 

 As each type of plant spread its shade 

 and added more humus, the air near 

 the ground was made cooler so that 

 evaporation was lessened, and the soil 

 became more spongy and could hold 

 more rain water. The soil deepened 

 and got more moist, the shade in- 

 creased, and new plants were favored 

 over those already there. Finally again, 

 because the climate permitted, a forest 

 resulted. 



None of us in his lifetime can see all 

 these stages over a single area. The 

 whole process may take hundreds or 

 thousands of years, and some of the 

 steps may change so slowly that they 

 seem interminable. But in one spot we 

 can find lichens helping the slow dis- 

 integration of rock, in another poly- 

 pody ferns growing on soil so thinly 

 spread over a ledge that they dry to 

 tinder during drought, and in another 

 blueberry bushes where the soil is a 

 few inches deep. Elsewhere we can 

 find pitch pines and scrub oaks in dry 

 situations and a forest of maple and 

 beech in a deep, moist cove. By going 

 from one place to another we can pic- 

 ture the slow process of succession. 



