EDAPHIC CAUSES — CLIMATIC CAUSES. 55 



EDAPHIC CAUSES. 



Nature. — In the preceding account of topographic processes which produce 

 bare areas it has frequently been shown that the critical results are the soil 

 structure and the amount and kind of water-content. This is equally true 

 of new areas due to climatic and biotic agents. In the case of all initial 

 causes, therefore, the basic control is exerted by water-content, which is con- 

 trolled in its turn by the physical character of the soil. A change in the kind 

 of soil-water may seem an instance of an edaphic initial cause. The real 

 cause, however, is topographic in the case of a change from salt to fresh water 

 or the reverse, climatic in the increasing alkalinity of the lakes and pools of 

 arid regions, and biotic when acid or other injurious substances accumulate 

 in the soil. All seres are consequently more or less edaphic in nature, and hence 

 the term edaphic can not well be used to distinguish one kind from another or 

 to contrast with climatic causes. If seres are grouped in accordance with 

 initial causes they can be distinguished only on the basis of the forces which 

 lie behind the changed conditions of soil and water. These are topographic, 

 climatic, and biotic agents. In a developmental classification of seres such a 

 basis is believed to be of secondary value. While such a grouping is simple 

 and convenient, it is artificial because it ignores development, and because 

 of the fact that very dissimilar initial causes produce identical bare areas and 

 seres. 



CLIMATIC CAUSES. 



Role. — Climate may produce new areas for succession, or it may modify 

 existing seres by changing the rate or direction of development, by displacing 

 the climax, etc. As a cause of modification, the discussion of climate belongs 

 elsewhere, and will be found in the chapters on direction, climax, and eosere. 

 We are concerned with it here only as an initiator of new seres. In this r61e 

 it acts usually through the agency of the ordinary changes and phenomena 

 which constitute weather. A distinction between climate and weather is 

 manifestly impossible. It is clear that climate would produce less effect in 

 the course of its ordinary oscillations than when it swings beyond the usual 

 extremes. A change of climate can produce bare areas by direct action only 

 when the change is sudden. A slow departure, even if permanent, would act 

 upon existing vegetation only by modifying it through ecesis or adaptation. 

 Indirectly, of course, climatic factors and processes may cause new areas 

 through the cooperation of topographic or biotic agents. 



Bare areas due to climatic factors directly. — The direct action of climatic 

 factors takes place regularly through the destruction of existing vegetation. 

 When the destruction is complete or nearly so, a bare area with more extreme 

 water conditions is the result. The factors which act in this manner are: 

 (1) drouth, (2) wind, (3) snow, (4) hail, (5) frost, and (6) Ughtning. In addi- 

 tion, evaporation, which is the essential process in drouth, produces new areas 

 from water bodies in semiarid and arid regions. It may have the same effect 

 on periodic ponds in humid regions. While the process is the same, the 

 degree to which it acts varies widely. Evaporation may merely reduce the 

 water-level to a point where the ecesis of hydrophytes is possible, or it may 

 continue to a point where islands, peninsulas, or wide strips of shore are laid 

 bare to invasion. Finally, the lake or pond may disappear entirely, leaving a 

 marsh, a moist or dry plain, or a salt crust (plates 9 a, 58 b). 



