PLANT INDICATORS. 53 



and the struggle for existence is more and more intense. This 

 competition is the factor which reduces the number of species 

 and forms of life to a mediocrity in the climax formation. The 

 climax of the development is reached when a dominant 

 vegetation occurs which can exclude another dominant. It 

 may be taken as indicating the complete action and reaction of 

 the locality, and of the plants themselves. It is a form of 

 vegetation that normally would remain stable for many years, 

 and its existence is determined by edaphic and climatic 

 conditions, such as temperature, relative humidity, wind, and 

 rainfall. Forest is a well-marked climax formation. It covers 

 immense areas as such, and holds its own by reason of its 

 powers of dominance over other kinds of vegetation. 



Stable as the climax formation may be, yet there are 

 conditions which cause it to break down. Forest fires may 

 destroy large tracts of woodland, creating blank areas. In old 

 stands fire does not always end the existence of forest in the 

 region where it occurs. There are in most cases groups and 

 single trees left, and these produce seeds which in course of 

 time would regenerate the forest. In the interval a sub-climax 

 stage would occupy the ground. The continued occurrence of 

 destructive influences may destroy the forest so that ultimately, 

 under certain conditions, it is replaced by prairie or steppe. 

 Mayr has stated that the eastern prairie of North America 

 probably occupies the position of forest which had been destroyed 

 by fire. Others, e.g. Shimek and Gleason, have considered, 

 however, that fire did not originate the prairie but rather that 

 it acted as a deterrent to the advancing forest. The treelessness 

 of the prairie is also said to be due to the exposure to evapora- 

 tion as determined by temperature, wind, and topography. 

 One might ask if this is not a complication of cause and effect. 

 There are facts suggestive of an answer in later studies, which 

 showed that invasion of prairie by certain species of tree may 

 take place where the rate of evaporation is high. Where this 

 occurs evaporation is reduced. There is in this an example of 

 the reaction of the vegetation on the locality. 



In many parts of Britain there can be found abundant remains 

 of old forest life. The peat bogs are full of them. Other 

 natural forest tracts in Britain have been destroyed within 

 the period of written history, and the place of these is now 

 occupied by a sub-climax stage. In the continuation of this 



