7() BOTANICAL SURVEY OF THE CCEUE D'aLENE MOUNTAINS. 



absorbed during the day, and therefore more frosty nights. Not 

 only are the valleys within the mountains affected, but the phenomena 

 of frosty nights during the summer will make itself felt more severely 

 in the valleys <>f the plains which have their head in the adjacent 

 mountains. The living forest warms the cold night air that niters 

 through it during the night. As the sun nears the western horizon 

 on a clear afternoon an interchange of air between the crests of the 

 ridges and the bottoms of the valleys takes place. The cold air 

 being the heavier descends, and sinks into the lowest depressions in 

 the ravines and canyons. The heated air of the valleys ascends by 

 the slopes. In a dense forest where free radiation is prevented the 

 mean temperature of the several seasons varies but little either during 

 the day or night. Let us suppose the mean summer temperature in a 

 section of normal forest in the valley bottoms to be C° 0. (43° F.) 

 at water level in the soil a meter or two beneath the surface. I think 

 this temperature approximately correct for elevations of TOO to 900 

 meters (2,300 to 3,000 feet). During the night the air temperature to 

 a heighth of 50 to 90 meters (160 to 300 feet) above the surface will 

 not be more than 2° C. lower. The cold air from the crests littering 

 through such a forest is warmed near the earth and emerges upon the 

 open meadow lands at a considerably higher temperature than it other 

 wise would. Frosty nights are doubtless averted in many instances 

 through this means and the severity of others much mitigated. 



The timber should be conserved for the part it plays in regulating 

 the drainage of the annual precipitation. Vast quantities of snow fall 

 every winter in the western ranges. Bain storms occur frequently 

 during the late fall and early spring and load the mountain snows with 

 water. The amount thus held by the snow is sometimes immense. 1 

 have melted the snow on the mountain summits in the month of March 

 and have found it in some seasons so heavy with absorbed water that 

 from a layer 18 cm. (7 inches) in depth 10 em. (3.8 inches) of water 

 would be obtained. In the spring the snow disappears much faster and 

 more suddenly on the open and exposed places than it does where 

 afforded protection by the shady forest. This precipitates a great volume 

 of water into the streams and destructive freshets occur. During the 

 summer the streams become abnormally low and the supply is insuffi- 

 cient to serve the purposes for which it is needed. Alternate freshets 

 and scarcity of water in the streams are the universal results of the 

 cutting away of the forests in all portions of the world, and arc too 

 well known to be dilated upon here. In the Cienr d'Aleues and indeed 

 everywhere throughout the Pacific Slope an element not so univer- 

 sally experienced comes in, namely, tire. 



The streams are supplied in two ways, by springs and by slow per- 

 colation of the water held back in the moss and humus and in the top 

 soil among the multitude of roots that ramify throughout it. The 

 springs issue forth from the rocks, their water representing the drain- 

 age of the precipitation that has fallen upon the crests and rockier 



