OROGRAPHIC FACTORS 



267 



Alps in the Tirol, and the south slope of the Great Atlas. Similar 

 extreme conditions exist in the mountains of Idaho, according to Gail 

 (1921). There in summer the mean relative humidity on northeast 

 exposures is fully 22 per cent above that of the more windy southwest 

 side. The curves of transpiration, evaporation, and soil moisture run 

 similarly (Fig. 135). 



The maximum of evaporation coincides with the minimum of soil 

 moisture. For this reason at the beginning of August on southwest 

 exposures all annuals are dried out. The dominant forest tree here is 



ccm June 

 300\ — 



August Sept. 



0. 5. 22. 30. 7. 16. 22. 30. 6. '3. 20. 27. 3. JO. 17. 

 Fio. 135. — Evaporation in a southwest open exposure (il) : and in a northeast 

 exposure (B) , under Pinus ponderosa and Pseudotsuga taxifoUa (weekly mean) . {After 

 Gail.) 



the drought-resisting P. 'ponderosa, while the northeast slope is occupied 

 by the more mesophytic Pseudotsuga forests. 



The map of vegetation by A. Luguet (1926) furnishes an excellent 

 survey of the dependence of plant communities upon exposure in the 

 Mont-Dore region. The rainy northwest side of the mountains is 

 covered by gorse and ericaceous heaths, Agrostis meadows, and beech 

 woods and extensive fir forests. On the southeast slope Abies is 

 entirely absent, and in place of the Ericaceae and Agrostis there are 

 fields of grain. 



Differences due to exposure can everywhere be found, even in the 

 smallest areas. Plant colonies which enjoy the warmth and dryness of 

 the Mediterranean and Sarmatic climates are almost excluravely 



