VERTICAL RANGE OF CONIFEROUS SPECIES. 57 



This arises from the fact that the lower boundaries of most of the spe 

 cies are ill defined. We shall find that the cause of this is the in finite 

 variety of climatic conditions, which is such a marked feature of the 

 western extensions of the Bitter hoot Range. 



The tortuous courses of the mountain ridges and the canyons are 

 the main causes of the innumerable local variations of climate. The 

 mean annual temperature of any given portion of a ridge or canyon 

 varies witli its direction of exposure, and this does not simply refer to 

 the cardinal points of the compass. Almost every degree of inclination 

 to any of these directions involves a corresponding change in the pre- 

 vailing temperature conditions. This occasions differences in the pre- 

 cipitation, and, though we have no data by which we can demonstrate 

 the difference between a south and a north slope, yet it is a fact well 

 established by observation that more rain and snow fall on the latter 

 than on the former. 



It must here be taken into consideration that the evaporation from 

 the southern slopes is greater than from the northern, and that the 

 visible effects upon the vegetation, from the same amount of rain and 

 snow, would therefore be less on the meridional sections of the ridges 

 than on any of the others. It is not temperature conditions alone which 

 influence the precipitation. There is another very potent factor to be 

 found — the mechanical impact of the wind. Let anyone who doubts 

 this go to the summit of some of the high ridges, say from 1,500 to 

 2,100 meters (5,000 to 7,000 feet) altitude, and remain there during a 

 rain or snow storm. It will be seen then that a large quantity of the 

 air which is driven against the southern face is deflected upward with 

 great force and velocity, carrying with it enormous quantities of clouds, 

 which sink and are thickly massed as soon as the calmer and cooler 

 north or lee side is reached. The rending asunder of the cloud masses 

 by this upward current diminishes the amount of moisture precipitated 

 from them. Part of the deflected air and clouds will, if the ridge is 

 high enough, pass through the low gaps or saddles in the mountain. 

 The heavier growth of timber in these saddles than elsewhere in the 

 neighborhood is due to this rather than to the drainage from the higher 

 parts of the ridges, which would not flow toward the depression unless 

 the dip of the strata favored it. The phenomenon which I have desig- 

 nated the dominant precipitation point plays a part here; but however 

 much this may vary from year to year the condition of the forest as a 

 whole proves that there is a general mean in it, and that the variation 

 is not sufficient to exert a permanent influence in the distribution of 

 the species. 



From a consideration of these climatic conditions we shall not be sur- 

 prised to observe the poorly defined vertical limits of the Comr d'Alene 

 conifers, and as great humidity is the predominant climatic feature of 

 the region, we may expect the species of the elevated portions, which 

 can endure these very conditions, to descend to low levels. On the 



