542 CORRELATION OF DISTRIBUTIONAL FEATURES. 



occurs, and is remarkable in having a number of relatively narrow 

 amplitudes for a tree of such wide distribution. Its widest amplitudes 

 are in the number of days in the longest normal rainy period and in 

 the moisture-temperature index. The number of hot days and the 

 number of cold days appear to be of about equal importance in limiting 

 the range of Pinus echinata. The number of days in the longest normal 

 dry period and the mean total precipitation of the year appear to be 

 still more important as limiting conditions, while the amplitude of 

 relative humidity is also comparatively narrow. 



TCM^CHATURt 



D«r9 IN NoRMiL Frostlcs* Scoton IF. S.I 



Hot D«vs. F. S. 



Cold Dhys. F. & 



PHTSiOLOcicAt. Summation. F. S. 



Normal Dailv Mean, coldest 14 davs of Yean 



Normal Daily Mean. Year 



Preciritation 

 Normal Daily Mean. F. S. 

 Days in lonocst Normal Rainy Period, F. S. 

 Days in lonocst Normal Dry Period. F, S. 

 Mean Total, Year 



Evaporation 

 Daily Mean, 1887-6, F. S. 



Moisture Ratios 

 Normal P/E, F. S. 

 Normal rr/E, F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean, F. S. 



Sunshine 

 Normal Daily Duration. F. S. 



Moisture-Tcmrerature Indices 

 Normal P/E > T, F. S., Phtsiolocical Method 



Fig. 50. Climatic extremes for Pinus echinata. 



Pinus strobus (fig. 51). — The distribution of Pinus strobus is coex- 

 tensive with that of the eastern section of the Northern Mesophytic 

 Evergreen Forest, and exceeds it to some extent toward the south and 

 west, carrying the tree into the Deciduous Forest region and into the 

 Grassland-Deciduous Forest Transition region. The conditions in the 

 area of Pinus strobus are therefore similar to those of the vegetational 

 area in which it reaches its greatest abundance. The southernmost 

 extension of the tree carries it into a region with longer frostless season 

 and with no cold days, in our sense of this term. A slightly greater 

 value for the physiological temperature summation and a greater 

 normal daily mean temperature are also encountered by Pinus strobus 

 in its extension toward the Atlantic Coastal Plain and southward to 

 Georgia. The number of days in the longest normal rainy period and 

 the number in the longest normal dry period both reach maximum 

 values which are greater for this tree than for the Evergreen Forest, 

 although the minimum values are the same for the two. 



There are no very narrow amplitudes for this pine. The narrowest, 

 however, are those for the moisture ratios and for relative humidity, 



