ii2 Studies in Forestry [CHAP. v. 



in the track of the Gulf-stream, have a high relative humidity ; 

 whilst those from the east, which come from about the central 

 area of Asia and Europe, are comparatively dry, although (in 

 consequence of the moisture acquired in passing over the 

 North Sea) not so dry as on the continent, where the relative 

 humidity of the air decreases with longitudinal progress east- 

 wards. 



As aqueous precipitations must occur whenever the air is 

 reduced so far in temperature that the corresponding saturation- 

 point renders it impossible for all the moisture to be retained, 

 rainfall is more frequent at higher elevations than at lower 

 levels, whilst at the same time aerial disturbances of various 

 kinds are also more frequent and violent. 



For the maintenance of normal transpiration through the 

 foliage of woodland crops enormous supplies of moisture are 

 requisite in the soil. When a period of drought occasionally sets 

 in during summer for about a fortnight to three weeks, this often 

 forces vegetation to close its activity for the year in the case 

 of broad-leaved trees, or can even be fatal to species like the 

 Spruce, which transpire freely *. Though the actual amounts 

 of soil-moisture requisite for the various species of trees of 

 woodland growth are not accurately known, yet observation 

 and experience show that Spruce, Black Pines, Alder, Maple, 

 Sycamore, and Ash thrive best in a humid atmosphere, whilst 

 Beech, Birch, and Silver Fir grow better in damp than in dry 

 localities. If, therefore, the former species be planted out in 

 situations where the air is usually dry, they make considerably 

 greater demands as to soil-moisture than when growing in 

 localities where, owing to a higher relative humidity of the 

 atmosphere, the transpiration of water through the foliage is 

 not so much stimulated 2 . 



The Aspect, or Exposure towards one or other of the 

 cardinal points of the compass, exerts great influence on the 



1 Compare Chapter VIII, Section i. 



2 See British Forest Trees, 1893, pp. 27, 28. 



