CLIMATIC FACTORS 93 



position of certain isotherms. These attempts have usually over- 

 looked the fact that boundaries of vegetation can be uniform and com- 

 parable only when based upon the occurrence of a definite plant 

 species or upon a well-defined plant community. For example, the 

 much discussed tree line, or limit of forest, is not, biologically speaking, 

 based on uniform and equivalent data, since each of the tree species 

 which helps to form the boundary has its own ecologic reactions. 



Proposals for characterizing climates according to temperatures 

 and correlating them with conspicuous species or boundaries of vege- 

 tation have been offered by Merriam (1894), Vahl (1911), Samuelsson 

 (1915), Brockmann-Jerosch (1919), Enquist (1924), and Livingston 

 and Shreve (1921). 



Merriam (1894) states that the northern range of organisms is 

 determined by the sum of the positive temperatures for the entire 

 season of growth but that the southern limit is due to the mean tem- 

 perature of a short period at the hottest time of year. This concept 

 has been modified by Livingston and Shreve (1921) by recognizing that 

 the efficiency of a degree of temperature for plant growth varies much 

 in different parts of the temperature scale. 



Enquist (1924) considered of first importance the extremes of 

 temperature and their duration. These determine the boundaries of 

 vegetational areas. He distinguished between the heat require- 

 ment which bounds an area on the colder side and the cold requirement 

 which bounds it on the warmer side. In the first case a certain 

 maximal temperature and a certain minimal temperature must be 

 exceeded during a definite number of days. In the second case a 

 definite number of days with a certain maximal temperature and a 

 definite number with a certain minimal temperature must not be 

 exceeded. The cold limit of the beech is attained where less than 217 

 days reach maxima above 7°C. ; that of the holly, Ilex aquifolium, where 

 less than 345 days have maxima above 0°. 



Temperature Zones. — Next to mean annual temperature the most 

 important temperature factor for determining the general character 

 of vegetation is the annual range of temperature. Mean annual 

 temperature, dependent upon the distance of an area from the equator, 

 determines the beltlike arrangement of the great vegetational zones of 

 both hemispheres and on the slopes of mountains. This fundamental 

 arrangement, however, suffers many variations and displacements, 

 because of variations in temperature due to the position of the great 

 continental land masses. The mean temperatures gradually decrease 

 from the equator to the poles, but the range of temperature increases. 

 The range also increases very widely and rapidly from the seacoast to 



