CORRELATION OF DISTRIBUTIONAL FEATURES. 543 



and those for the number of hot days, the normal daily mean pre- 

 cipitation, the physiological temperature summation, and the number 

 of days in the longest normal dry period. 



From a comparison of the distributional limit with the positions of 

 various isoclimatic lines, the southward range of Pinus s^robws appears 

 to be determined by temperature conditions, of which the physiological 

 summation and the number of hot days in the frostless season are the 

 most important, while its westward range appears to be determined 

 by moisture conditions, of which the normal daily mean precipitation 

 and the number of days in the longest normal dry period are the most 

 important. The position of the isoclimatic lines for the moisture 

 ratio, ir/E, would indicate that this compound factor is one of strong 

 importance in determining both the southern and the western limits 

 of this tree. 



TCMKnarunt 

 D*T* m Normal FaosTicn fluaen (P- 8J 

 Hot Oty, F. S. 

 Cold D*vt, F. S. 

 Phvsiolooical Summation, F. S. 

 Normal Oailt Mcan, colocst 14 o«T* Of Year 

 Normal Oailv Mcan. Ycar 



Prcciritation 

 Normal Daily Mean, F. S. 



0aT8 IK LONSCRT NORMAL RaINT PcRIOO, F. S. 



Oat* in lohocst Normal Ort Pcriod, F. S. 

 Mian Total, Viar 



cvaroration 

 Dailt Mcan. 18ST.a, F. S. 



Moirturc (Utio* 

 Normal P/E, F. S. 

 Normal n/C, F. S. 

 Normal P/t, Vu» 



HuMiorrr 

 Normal Mcam, P. •. 



SUNRHinl 



Normal Oailt Duration, F. S. 



MoiSTVRK-TCMRCRATURt INDICC* 



Normal P/Z x T, F. S., PxTSiOLOeiCAL Mmtoo C 



Fig. 51. Climatic extremes for Pinus strobus. 



Quercus alba (fig. 52).— This oak is found throughout the eastern 

 United States, with the exception of northern Minnesota and Michigan 

 and peninsular Florida. In keeping with its wide distribution it 

 encounters a very wide range of practically all of the climatic condi- 

 tions, exceeding in a number of cases the extremes for the Deciduous 

 Forest region, in which it finds its greatest development. The nar- 

 rowest amplitudes for Quercus alba are those of the number of days in 

 the longest normal dry period, the normal mean relative humidity of 

 the frostless season, and the moisture ratios. The first and last of these 

 conditions appear to be responsible for the western limit of distribu- 

 tion. This edge is roughly paralleled by the line for 25 days in the 

 longest dry period and by the line for a value of 0.60 for the moisture 



