72 MAN ON THE LANDSCAPE 



The geographical range of plants is almost entirely determined 

 by climate. Climate is a complex which pays no attention to politi- 

 cal boundaries, nor even to latitude and longitude in many cases. 

 The tropical Mexican coastal plain is only a hop, skip, and high 

 jump from the temperate inland high plateau. Within the bound- 

 aries of a farm, climate and topography usually have directed the 

 construction of numerous environments. One section of the farm 

 may be fit only for forest, another for meadow, another for tilled 

 crops. Unless consideration is given the fitness of the domestic plant 

 for the environment, neither plant nor environment may be able to 

 sustain itself. Corn will grow on a fertile hillside, but it is only a 

 matter of ten or twenty corn crops until there is no topsoil left, and 

 no corn. (Fig. 25.) 



The Genesis of Succession. 4 Succession has been treated in a 

 symbolic or general way in Chapter 6. Succession is a development 

 of life and habitat toward a climax formation. Succession occurs 

 on every part of the earth's surface where life is possible and where 

 a climax does not exist at the moment. It occurs on land and in the 

 waters. The end result of succession is not only a climax, but it ap- 

 proaches as near as possible a median (mesophytic) condition; that 

 is, a middle status between extremes, of moisture particularly, and 

 temperature. Succession starting in a lake eventually results in 

 filling up the lake with silt and acquatic vegetable matter; followed 

 by an invasion by land herbs, and finally by shrubs and trees. Start- 

 ing on relatively dry, rocky areas, the succession may end in forest, 

 for instance, where much more iroisture is retained than originally. 

 Water is held in the leaf litter, and the now granular and humus 

 laden soil. 



Of course, the climax formation is determined by the overall 

 climate: forest in the humid areas, savannah (trees and high grass) 

 in less humid or transitional areas, grasslands in the subhumid cli- 

 mate, short grass or scrub in the semi-arid, and desert vegetation in 

 the arid regions. 



What causes succession to get underway? In the beginning of 

 the earth, all places were barren, land and waters alike, and the 

 whole evolutionary process got underway. Today, succession occurs 

 wherever barrenness is produced, or any degree of barrenness short 

 of the climax of the area. The causes of barrenness may be due to 

 man's culture or to natural forces. 



Topographic Causes. The fragmented surface of the earth is 

 movable. The removal of earth material from one place must result 

 in its deposition somewhere else. Both actions, except when very 

 slow, result in damage to vegetation. Removal of soil by wind, 

 water, ice, or gravity is called erosion. 



The results of water erosion are bare or partially bare gullies, 

 ravines, drains, arroyos, flood plains, stream islands, banks, shores, 



4 Weaver and Clements, Plant Ecology, McGraw-Hill Book Company, 1938, 2nd 

 edition, passim, 



