22 Forests — their Influence upon fall. [January, 



vated ridges or mountains of moderate elevation, they may have 

 the effect of extending the influence of the latter a step further in 

 producing an upward current to the cooler regions, or condensing 

 area, and in this manner greatly assist local precipitation. It is 

 now a settled fact that high mountains augment the rainfall in them- 

 selves or even to some little distance from their bases. The Alps 

 of Switzerland are known to modify and greatly influence the 

 course of storms. We repeat it then, that forests resemble high 

 altitudes as regards their mechanical action in affecting the rainfall, 

 but owing to their meagre height, can scarce be said to have any 

 influence (mechanically) over this phenomenon except they are 

 situated upon the latter, in which case their action may tell con- 

 siderably. Forests do, however, affect local precipitation through 

 certain vital functions, as will be seen by and by. 



No other influence which forests exert upon atmospheric con- 

 ditions can claim so large a share of importance as that exercised 

 on its humidity. The explanation of their effect on this meteor- 

 ological element is to be found mainly in a study of some of the 

 organic processes carried on by trees, but to a slight extent also 

 to a mechanical action. The evaporation from the soil is inter- 

 fered with by the vegetable canopy above, which prevents, in a 

 great measure, the sun's rays from reaching the earth and heat- 

 ing it so as to facilitate evaporation. Again, by forming a more 

 or less perfect screen interposed between sky and earth, forests in 

 a measure intercept the dew and lighter rains, allowing but a por- 

 tion of this moisture to reach the earth. 7/ has been estimated 

 that the evaporation from the soil of the forest is rather more than 

 one-third as great as that from open soil, but this lessened surface 

 evaporation is much more than compensated for by transpiration of 

 the forest, as zvill be indicated by the remits of our investigations. 



The question of the influence of the organic functions of plants 

 on the humidity of the air, is one of paramount importance and 

 great philosophic interest. Whatever effect plants have through 

 these processes must be due either to the exhalations of moisture 

 from the leaves (transpiration) or to the absorption of moisture by 

 the leaves. The latter idea, as formerly taught and until recently 

 held by most authorities, is now most probably shown to be erro- 

 neous. According to the researches of Unger 1 the theory of the 

 absorption of the watery vapor by the leaves is untenable. 2 My 



^Vilhelm der Baden und der wald, p. 19, quoted by Marsh. 



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