540 CORRELATION OF DISTRIBUTIONAL FEATURES. 



ture are the two conditions which appear to be most critical in the 

 hmitation of this species with respect to its entire range. There are 

 few localities in its distributional edge, however, in which the moisture 

 conditions would fail to be of importance in connection with the tem- 

 perature conditions just mentioned. 



Pinus ponderosa (fig. 48). — The range of Pinus ponderosa is similar 

 to that of Pseudotsuga, but is somewhat more extended from Colorado 

 northward to the Canadian boundary, where it exceeds the eastern 

 hmit of the western section of the Northern Mesophytic Evergreen 

 Forest, and is less extended in the extreme northwest, where it fails to 

 enter the Northwestern Evergreen Hygrophytic Forest. 



Except for the shghtly narrower amplitude in length of frostless 

 season, the temperature conditions are similar for Pinus ponderosa 

 and for the western section of the Northern Mesophytic Forest. The 

 number of hot days and the physiological summation of temperature 

 again appear to be factors of importance in hmiting this tree. 



On account of its extension into the western edge of the Grassland, 

 the moisture values of Pinus ponderosa are of somewhat wider ampli- 

 tude than those of the forested area in which it is so abundant. This 

 species does not, however, encounter such low evaporation values nor 

 such a high range of humidities as does the forest itself. It is doubtful 

 if the former of these facts would be confirmed by data from a larger 

 number of critically located stations ; the latter is due to the absence of 

 Pinus from the extreme coast of northern Cahfornia. 



The conditions expressed by the moisture-temperature index appear 

 to be of considerable importance in limiting this tree, together with the 

 moisture ratios, the number of hot days, and the physiological summa- 

 tion of temperatures. 



TCMPCRATURC 



Oavs in NoRMAt Frostccss Season (F. S.> 



Hot Days. F. S. ( 



Cold Days. F. S. 



Physiological Summation, F. S. 



Normal Daily Mean, coldest 14 days or Year C 



Normal Daily Mean, Year C 



Precipitation 

 Normal Daily Mean. F. S. 

 Days in longest Normal Rainy Period, F. 5, 

 Days in longest Normal Dry Period, F. S. 

 Mean Total. Year C 



Evaporation 

 Daily Mean. 1867-6. F. S. C 



Moisture Ratios 

 Normal p/e, F. S. 

 Normal tt/E, F. S. 

 Normal P/E, Yeah 



Humidity 

 Normal Mean, F. S. C 



Sunshine 

 Normal Daily Duration. F. S. C 



Moisture-Temperature Indices ^ 

 NoRMAi. P/K. X T. F. S., Physiological Method C 



Fig. 48. Climatic extremes for Pinus ponderosa. 



