52 INITIAL CAUSES. 



ELEVATION AND SUBSIDENCE. 



Elevation and subsidence. — ^These are likewise examples of processes which 

 are opposite and complementary in many instances. This is seen in the 

 relation of syncline and anticline, in earthquakes, in the movements of certain 

 sea-coasts, etc. Naturally they affect the development of the vegetation 

 directly only where land and water are in contact. The relative rise or fall 

 of an inland area could produce no effect upon succession, except as it changed 

 the climate or produced flooding or draining. A direct effect upon vegetation 

 is possible only along sea-coasts and lake-coasts, and of course upon islands. 

 Here, however, the rate of emergence or submergence is the decisive factor. 

 Physiographically, it is much the same thing whether a coast rises or falls a 

 foot a year or in a thousand years. From the standpoint of succession, the 

 rates of elevation and subsidence assumed for the Atlantic coast, the coast of 

 Sweden, and for the Great Lakes are so slow as to be wholly insignificant. A 

 rise of 2.5 feet in a century is equivalent to a rise of less than a centimeter per 

 year. Over such a minute area as would result from such a rise each year 

 there would be no chance for extreme conditions and no place for even the 

 most incomplete and fragmentary succession. Each annual increment of 

 space would be controlled by the association at hand and quickly made an 

 intrinsic part of it. 



With subsidence the case is different if it results in flooding and consequent 

 destruction of the vegetation. In the case of a low-lying coastal forest even 

 the slow rate of 3 feet per century would eventually flood the forest floor and 

 kill the trees. It is conceivable that the flooded forest floor might serve as a 

 new area for the invasion of a coastal swamp, thus causing an apparent 

 "retrogressive" succession. It seems much more probable, however, that the 

 action of waves and tides would erode the soil, and thus destroy the forest or 

 other vegetation piecemeal year after year. 



While elevation and subsidence are largely negligible to-day as initial causes 

 of succession, this is obviously not true of the past. Crustal movements 

 were of the first importance in changing the outlines of continents, in building 

 mountains, and in producing cycles of erosion. As a consequence of such 

 changes they exerted a profound effect upon climate. The consequent effect 

 upon vegetation resulted in the change of climax populations and in the initi- 

 ation of a new eosere. Accordingly, the further discussion of this matter will 

 be found in Chapters XII to XIV. 



New areas due to elevation. — As assumed above, elevation produces new 

 areas for succession only where it is relatively rapid, or where the new area is 

 not in contact with an existing vegetation. Cases of this kind are practically 

 confined at the present day to volcanic cones formed in lakes or in the ocean, to 

 islands due to volcanic disturbances, and to coral reefs and islands. The latter, 

 however, belong properly under biotic initial causes, though their formation 

 and behavior are often intimately associated with volcanic islands. Appar- 

 ently no studies have been made as yet of the development of vegetation on 

 new islands due to volcanic action. It seems evident, however, that they 

 would exhibit rock and cinder seres generally, with the water sere of coral and 

 other oceanic islands at the lowest level. 



Subsidence. — ^The evidences for the recent subsidence of the Atlantic coast 

 of the United States are summed up by Johnson (1913:451), as: 



