CORRELATION OF DISTRIBUTIONAL FEATURES. 513 



No vegetational region exhibits a narrower amplitude of the mois- 

 ture-temperature index than does the western section of the Mesophytic 

 Forest. This derived and complex expression of climatic conditions 

 has been shown to differentiate the evergreen-forest regions of the 

 United States, in spite of its failure to bring out consistent differences 

 between the other regions. Its extremely narrow amplitude for the 

 western section of the Mesophytic Evergreen Forest points to its impor- 

 tance as an expression of the conditions controlling this vegetation. 



Northern Mesophytic Evergreen Forest, eastern section (fig. 35). — A 

 comparison of the climatic extremes of the eastern and western sec- 

 tions of this forest shows a general similarity of temperature condi- 

 tions, and a dissimilarity of moisture conditions and the expressions 

 derived from them. The length of frostless season is much narrower 

 in its amplitude in the eastern section, ranging from a length of 85 

 days at Thomaston, Michigan, to 167 days at Fitchburg, J\lassa- 

 chusetts, values which lie well within those of the western section. 

 The normal daily mean temperature for the coldest 14 days of the 

 year ranges through lower values in the eastern section, as does also 

 the normal daily mean temperature for the year. 



The only precipitation condition in regard to which the eastern and 

 western sections of the Northern Mesophytic Evergreen Forest are 

 at all alike is the normal annual total. The normal daily mean for the 

 frostless season ranges in the eastern section through values which are 

 well above those of the western section, the maximum of the latter 

 being 0.070 inch and the minimum of the former being 0.091 inch. In 

 accordance with this fact the number of days in the longest normal 

 rainy period of the frostless season is greater in the eastern section than 

 in the western, and the number of days in the longest normal dry 

 period is pronouncedly less in the eastern section. 



TlMrCMTum 

 0*r« IN NORMIL TKOlTkltS SusoN (F. S.) 



Hot D*y«, F. S. 

 Cold Dats, F. S. 

 Phvsiologimi Summation, F. S. 



NoRMAi. Daily Mean, colocst 14 OAr^ or Year (Z 

 NonMAL Daily Mcan. Yian ■ 



Precipitation 

 Normal Daily Mean, F. S. d 



Days in longest Normal Rainy Pchiod. F. S. [ 

 Days in longest Normal Dry Period. F, S. 

 Mean Total. Year 



CVARORATtON 



Daily Mean, 1687-8. F. S. 



Moisture Ratios 

 Normal P/E. F. S. 

 Normal t/E, F, S. 

 Normal P/E, Year 



Humidity 

 Normal Mean, F, S. 



Sunshine 

 Normal Daily Duration. F. S. 



Moisturc-Temrerature Indices 

 Normal P/E i T, F, S,, Physiological Method 



Fig. 35. Climatic extremes for eastern section of Northern Mesophj-tic Evergreen Forest. 



