510 CORRELATION OF DISTRIBUTIONAL FEATURES. 



moisture ratios have also been seen to be closely similar for these two 

 forests, and their minimum values are almost identical (see fig. 25). 

 There is also little difference between the amplitudes and extremes of 

 the humidity conditions for these two forests and for the Northwestern 

 Hygrophytic Forest as well. The narrowest amplitudes for the South- 

 eastern Evergreen Forest are found in the evaporation, relative 

 humidity, and moisture ratios, and these must be regarded, therefore, 

 as the most important of the various conditions in controlling the dis- 

 tribution of this vegetation, in so far as its control is a matter of 

 climate. A comparison of the position of the isoclimatic lines for the 

 values 0.100 and 0.110 of the moisture ratio based on the conditions of 

 the frostless season and the preceding 30 days (plate 59), with the 

 position of the boundary of the Southeastern Mesophytic Evergreen 

 Forest, shows a close correspondence. This is least satisfactory in the 

 vicinity of Arkansas, where the Southeastern Forest extends into a 

 region with lower values for the ratio. This region, however, is the 

 one in which there are the largest areas of mixed forest forming a tran- 

 sition to the Deciduous Forest region (see plate 1). 



The moisture-temperature index reaches its highest values in this 

 vegetation, and has an amplitude almost exactly equal to that for the 

 Deciduous Forest. The high values of the physiological summation 

 of temperature are responsible for the high values of this index, when 

 the relatively low values of the moisture ratios are taken into account. 

 The very high values which are rapidly attained by this form of the 

 moisture-temperature index on approaching the southeastern corner 

 of the United States may be taken to signify that this region presents 

 the optimum conditions for plant activity in the entire area studied, as 

 far as climate is concerned. 



Northern Mesophytic Evergreen Forest, western section (fig. 34) . — The 

 Northern Mesophytic Evergreen Forest, when considered as a whole, 

 is so widely distributed and so varied in its character and specific com- 

 position that it assumes a unity only when contrasted with the other 

 evergreen-forest areas of the country. The only natural subdivision 

 of this region is that which is made possible in the United States by 

 the geographical separation of the eastern and western portions. In 

 the study of the correlation of this vegetation with the climatic condi- 

 tions it has seemed desirable to determine the climatic extremes 

 separately for the eastern and western sections, which are sharply 

 separated by the northern arms of the Grassland and the Grassland 

 Deciduous-Forest Transition. 



The most striking feature of the diagram which shows the leading 

 climatic features of the western section of the Northern Mesophytic 

 Evergreen Forest (fig. 34) is the relatively narrow amplitude of the 

 majority of the conditions which accompany this widely and irregularly 

 distributed forest area. There is a particularly strong contrast in this 



