factors affecting plant distribution 17 



The Factors in Plant Succession 



The most concise grouping of the factors of plant succession 

 that can be made is : climatic, edaphic, and biotic. 



1. Climatic Factors 



In the climatic factors the air environment of the plant is 

 studied. There is no intention of raising the old metaphysical ques- 

 tion of "Where does the sky begin?", and it is not likely that the 

 statement will cause confusion. The air is that which begins at the 

 soil surface in the climatic factors and envelopes the above-ground 

 portions of the plant. In water plants it v/ould be whatever por- 

 tions are not submerged. The atmosphere contains carbon dioxide, 

 water, and oxygen, materials needed by the plant. In addition, the 

 duration and quality of light, the amount of heat, and the velocity 

 of wind are sources of energy for the plant. Animals or plants af- 

 fecting the above-ground portions of plants are grouped among the 

 biotic factors. The processes of absorption and distribution of mate- 

 rials and energy taken in and released by plant activity have no 

 place in this discussion, rightly belonging to a text on physiology or 

 experimental ecology. 



From our viewpoint of the development of the vegetation, 

 extremes of temperatures are the most important factors in lim- 

 iting plant grov/th ; for the extremes, and not the means, determine 

 which species may become a part of the vegetation and which may 

 be killed from too great cold or too intense heat. Once species be- 

 come established, the mean temperatures are important because 

 they determine the average rate at which plant activities proceed. 

 The rate may double or treble within certain narrow limits for each 

 rise of ten degrees Centigrade, but Blackman ('05j and Blackman 

 and Tansley ('05) have shown that beyond a set limit further rises 

 do not accelerate growth. To regard temperature means as any- 

 thing more than a condition for photosynthesis is to put too much 

 weight on the wrong factor. 



What has been said for temperature applies largely to light 

 also. The minimum and maximum amounts of light in which a 

 species may develop are the important factors in the distribution of 

 plants. As with the temperature light intensity, varies with the 

 season, the altitude, and the latitude. While some plants are active 

 photo-synthetically at 2° C, their common range is from 20 to 

 perhaps 40 or 45° C. Light less intense than full sunlight is sufR- 



