PREFACE. 



The present book is intended to be a companion volume to "Plant Succes- 

 sion." The latter was planned to contain several chapters on the applications 

 of ecology, but these were omitted on account of the lack of space. Chief 

 among these was the consideration of succession as the primary basis for a 

 system of indicator plants, and this has been made the theme of the present 

 treatise. For the sake of clearness, it has been necessary to give a concise 

 account of the climax communities of the region concerned. The original 

 plan included a brief summary of the priseres and subseres of the various 

 climaxes, but the limitations of space have precluded this. The same reason 

 has made it desirable to deal with principles and examples in the three fields 

 of practice, rather than to attempt a complete account of the host of climax 

 and serai communities which serve as indicators. The general principles and 

 specific indicators have been tested repeatedly during the field work of the 

 past five years, and the treatment has profited from the fact that a special 

 inquiry into indicator relations throughout the West was made during the 

 season of 1918. 



It is believed that succession and indicators constitute the most essential 

 and useful form into which the results of research can be put for practical use. 

 They are the fundamental responses of plant and community to the conditions 

 in control, and hence contain their judgment as to the fitness of the environ- 

 ment in which they grow or are to be grown. Such responses require transla- 

 tion into familiar terms, and for this purpose the quantitative analysis of 

 habitats and responses by means of instruments and phytometers is indis- 

 pensable. Moreover, habitat and response vary not only with the develop- 

 ment of the community, but also in accordance with the phases of the climatic 

 cycle. The importance of the latter can hardly be overestimated, and it seems 

 certain that the climatic cycle must be accorded a unique position in all future 

 research and practice. 



The indicator method will naturally have its greatest usefulness in new or 

 partly settled regions. While the results given apply only to western North 

 America, and to the western United States particularly, the principles and 

 methods are of universal application. They should be of especial value on 

 other continents where there is still a distinct frontier. Australia, South 

 Africa, and South America should furnish fertile soil for indicator investigations 

 and applications, while large portions of Asia and northern Africa should 

 possess almost equal promise for this work. Even in Europe and in other 

 thickly settled regions, indicator studies will have much value, and this will be 

 true everywhere that natural or semi-natural vegetation is found. Indeed, 

 it is probable that indicator methods in some form will come to be applied to 

 all cultural vegetation with the advance of quantitative ecology and the dis- 

 appearance of the artificial barrier between science and practice. 



The author is under especial obligation to Dr. H. L. Shantz for many 

 helpful suggestions arising from the reading of the manuscript. Grateful 



