SETTING 



Site Conditions 



The different and distinct climates that occur within the Intermountain area are 

 due to several factors. 



The area extends for 900 miles of latitude and markedly different temperatures are 

 exhibited within this great range. 



The north-south orientation of mountain ranges exerts important influences on 

 climate. Areas on the windward side receive greater amounts of precipitation than areas 

 on the leeward side. Mean annual temperature decreases about 3° F for each 1,000-foot 

 rise in elevation. Other factors, such as wind, humidity, cloudiness, and radiation, 

 are similarly modified by the topography. 



In northern Idaho and northwestern Montana, a strong maritime influence penetrates 

 to the Continental Divide. The long, wet winters and comparatively short summer drought 

 period of this area are more favorable for tree establishment than are climatic condi- 

 tions throughout the remainder of the study area. 



The country east of the Continental Divide gets frequent cold and dry air masses 

 from Canada during the winter and persistent warm southerly winds from Mexico and the 

 Gulf of Mexico in the summer. 



The pattern of climates throughout the Intermountain area (U.S. Department Commerce 

 1968) suggests that, in general, conditions are probably most favorable for tree estab- 

 lishment in northern Idaho and northwestern Montana, becoming progressively less favor- 

 able farther south and abruptly less favorable east of the Continental Divide. 



The diversity of climates is accompanied by an equally impressive diversity of 

 geology and soils. Major portions of the area are represented by uplifted sedimentary 

 formations and by volcanic, granitic, and basaltic formations. Soils range from highly 

 erodible to quite stable, from nutrient rich to somewhat nutrient deficient and from 

 droughty to moist. When combined with environmental differences fostered by geographic 

 position and varied topography, soil diversity creates a complex pattern of site 

 conditions . 



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