418 CONSERVATION 



the conditions are much less favorable which, planted in the Pontine marshes 



to evaporation from the soil than in a near Rome in 1870 (in the vicinity of 



bare field near by, the humidity being the Convent of St. Paul of the Three 



higher and the temperature very much Fountains), have lowered the level of 



lower. Above the trees also the air is the ground water several decimeters, 



cooler and more humid than it is at In this case the transpiration from the 



the same altitude above bare ground, water-loving eucalyptus is more vigor- 



This last statement requires some ex- ous than was the evaporation from the 



planation. Sunlight passing through water and vegetation of the swamps. 



the air raises its temperature somewhat, The relation of the forest to run-off 



but a more important influence is that on level land is not an important factor, 



of the currents of air set up by contact As we have already seen, it is possible 



with the surface upon which the sun- to keep level fields in such a condition 



light falls. We all know the heat which of tilth that the amount of run-off from 



rises from bare ground, and how com- them is no greater than that from the 



paratively cool is the sward. Similarly forest. On slopes the case is different, 



the surface of the ground is hotter than The effect of the forest is, first, to 



the surface of the foliage of a forest, diminish the violence with which the 



The effect of radiation from these two rain strikes the ground and compacts 



contrasting surfaces is shown by the it, for the drops are checked by the fo- 



experience of French aeronauts to ex- liage. They fall from branch to branch 



tend to a considerable height. Accord- and reach the ground with greatly 



ing to E. Henry: 6 diminished momentum and in a finer 



The cooling effect observed by *ronauts s ^ te ' That P Ortion , which dr P s from 



in passing over forests of a certain extent the leaves comes in larger masses, but 



is indicated by a marked descent of the bal- still gently. Beneath the forest the 



loon. It is a well-established fact that this ra j n fi n( j s a mo ist soil whose capillary 



effect was experienced bv a number of ui r j i 



military balloonists above the forest of Or- water 1S capable of vigorous downward 



leans, the balloon being at an altitude of circulation. The soil is also opened by 



about a thousand meters. It appears to be the forest roots, and being protected 



demonstrated by the balloon ascensions f romt he compacting and cementing 



made up to the date of the statement that . a , , b & 



the influence of forests of similar extent influences of a free exposure is de- 



(34,000 hectares, or about twenty square cidedly porous. All these conditions 



miles) is observable up to an altitude of 1,500 militate against run-off and promote 



absorption. In consequence the run-off 



To sum up the preceding considera- from forest areas gathers slowly as 



ti^ns . The effect of these complex re- compared with bare slopes, and the 



la/ions has been made the object of in- ratio becomes increasingly favorable 



vestigation, particularly in Germany, in regard to forests as slopes become 



through the study of the amount of steeper. This fact enters into the 



moisture remaining in the soil under consideration of the value of forests in 



the various kinds of trees, and in the mountain regions as compared with 



adjacent open. Some reference to those on plains. Ebermayer, 7 from 



these results has already been made ; whose paper many of the data used in 



but the general conclusion is that the this report have been taken, says : 



effects of transpiration from luxuriant , ,, . .. 



, ,. . In view of the exact investigations which 



foliage are very materially greater than have been cited above, and which extend 



those of evaporation from the bare level through thirty years, it is no longer possible 



surface. to maintain the assumption so generally 



T>V , -i -n ,- r ,1 made, that forests increase the amount of 



The most striking illustration of this groui ; d water and constitute a storage res . 



factor is the effect of eucalyptus trees, ervoir for the supply of springs, at least not 



"Henry, E. : Sur le role de la foret dans la circulation de 1'eau a la surface des conti- 

 nents. Comptes rendus du Congress des societes savantes en 1901. 



'Ebermayer, Ernst: Einfluss der Walder auf die Bodenfenchtigkeit, auf das Sicker- 

 wasser, auf das Grundwasser, und auf die Ergiebigkeit der Quellen. 1900. p. 38. 



