CORRELATION OF DISTRIBUTIONAL FEATURES. 



533 



trees, not on the ideal basis of the ecological distribution of all species 

 of that type, but on the only basis which is now practicable — the 

 geographical distribution of the most abundant species. 



It will be noted that there are a number of cases, particularly among 

 the temperature conditions, in which the upper climatic extreme 

 grows higher as we pass from the center (with 13 species) through the 

 subcenter (with 8 or more species) into the fringe (with 1 to 7 species). 

 Thus, the number of species of deciduous trees grows less as the frost- 

 less season grows longer, as the number of hot days increases, as the 

 number of cold days increases, etc. In several cases the extremes are 

 nearly the same for the center and the subcenter, or for the subcenter 

 and the fringe. In the case of humidity we have no deciduous trees 

 growing in the lower half of the gamut of this condition; with increasing 

 humidities above 53.2 per cent we have an increasing number of 

 deciduous species; on approaching the region with highest humidities 

 we first leave the fringe, then the subcenter, and finally the center. 



Rather wide amplitudes characterize all oif the temperature condi- 

 tions, and in most cases the conditions of the subcenter and fringe 

 shade off very gradually from the conditions of the center. The 

 minimum number of cold days is the same for all three areas, inasmuch 

 as all of them range into the region with no cold days. A much more 

 irregular set of relations is exhibited between the extremes for the 

 moisture conditions of the three areas. The shortening of the longest 

 rainy period brings us rapidly from the center to the limit of the entire 

 group. Increasing evaporation also brings us, within very narrow 

 limits, from the center to the edge. 



TCMPCRATUnC 



lAYS IN Normal Frostlcsb Season (F. S.) 

 loT Days, F. S. 

 OLD Days, F. S. 



'HYSIOLOGICAL SUMMATION, F. S. 



lORMAL DailV Mean, coldest 14 days of Year 

 loRMAL Daily Mean, Year 



Precipitation 

 loRMAL Daily Mean, F. S. 



>AYS IN longest NORMAL RaINY PERIOD, F. S. 

 lAYS IN LONGEST NORMAL DRY PERIOD, F. S. 



lEAN Total, Year 



Evaporation 

 )ailv Mean, 1887-8, F. S., 



Moisture Ratio* 

 Iormal p/E, F. S. 

 iORMAL rr/E, F. S. 

 Iormal P/E, Year 



Humidity 

 Formal Mean, F. S. 



Sunshine 

 Iormal Daily Duration, F. S. 



Moisture-Temperature Indices 

 Iormal p/E x T, F. S., Physiological Method 



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Fig. 43. Climatic extremes for eastern Deciduous trees; center of distribution (black), subcenter (shaded), 



fringe (dotted). 



