April, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 171 



replacement of one plant association by another, I can see no adequate reason 

 for not recognizing it as one kind of succession. 



Considering the various types of succession which have been distinguished 

 in relation to cause, biotic succession is typically progressive. 50 Physiographic 

 succession may be either progressive (as in the building up of flood plains 

 and coastal swamps), retrogressive (in the sense illustrated in the preceding 

 paragraph), or cataclysmic (as in the destruction of forests by erosion or by 

 moving dunes and their consequent replacement by more primitive communi- 

 ties). Climatic succession may be either progressive or retrogressive; it may 

 even be cataclysmic, as in the changes brought about by protracted drought, 

 to say nothing of effects due to hurricanes and tornadoes, severe ice storms, 

 and the like. Pyric succession is invariably retrogressive ; most commonly it 

 is cataclysmic. Anthropeic succession likewise is primarily retrogressive and 

 commonly cataclysmic ; but man himself may purposely inaugurate secondary 

 successions 57 which are progressive in character. Certain of the changes 

 which are induced by the forester, for example, through his application of 

 ecological principles in silvicultural practice, may be looked upon as being in 

 the nature of progressive anthropeic succession; so also the replacement of 

 prairie grassland by woodland as a result of planting, the changes in vegetation 

 which are effected by irrigation, and, in a very general way at any rate, various 

 of the progressive changes which are instituted in agricultural practice. 



D. Types of Succession in Relation to Origin 



The character of every successional series of plant associations is deter- 

 mined in very large degree by the nature of the area in which the succession 

 takes place. With reference to their origin. Cooper ('13) has distinguished 

 series of two types, viz., xerarch and hydrarch. A third type is here sug- 

 gested, viz., mesarch. 



Xerarch series are those which, " having their origin in xerophytic habitats, 

 such as rock shores, beaches, and cliffs, become more and more mesophytic 

 [or developmentally advanced] in their successive stages/' Hydrarch series 

 are "those which, originating in hydrophytic habitats such as lakes and 

 ponds, also progress toward mesophytism," or toward a developmentally more 

 advanced condition. Mesarch series are those which originate in mesophytic 

 habitats, such as are afforded by moist, rich soils, and in which the vegetation 

 likewise becomes progressively more and more advanced as a result of develop- 

 ment. The advance here may take the form, more especially, of increasing 

 complexity. 



63 Exceptions to the rule are illustrated by the effect of chestnut blight and similar 

 destructive organic agencies, which may be distinctly retrogressive and even cataclysmic. 



" The term secondary is here applied to successions which are subsequent on the 

 destruction of original vegetation by either man or fire. 



