4i8 



CONSERVATION 



the conditions are much less favorable 

 to evaporation from the soil than in a 

 bare field near by, the humidity being 

 higher and the temperature very much 

 lower. Above the trees also the air is 

 cooler and more humid than it is at 

 the same altitude above bare ground. 

 This last statement rec|uires some ex- 

 planation. Sunlight passing through 

 the air raises its temperature somewhat, 

 but a more important influence is that 

 of the currents of air set up by contact 

 with the surface upon which the sun- 

 light falls. We all know the heat which 

 rises from bare ground, and how com- 

 paratively cool is the sward. Similarly 

 the surface of the ground is hotter than 

 the surface of the foliage of a forest. 

 The efifect of radiation from these two 

 contrasting surfaces is shown by the 

 experience of French aeronauts to ex- 

 tend to a considerable height. Accord- 

 ing to E. Henry:" 



The cooling efifect observed by aeronauts 

 in passing over forests of a certain extent 

 is indicated by a marked descent of the bal- 

 loon. It is a well-established fact that this 

 effect was experienced by a number of 

 military balloonists above the forest of Or- 

 leans, the balloon being at an altitude of 

 about a thousand meters. It appears to be 

 demonstrated by the balloon ascensions 

 made up to the date of the statement that 

 the influence of forests of similar extent 

 (34,000 hectares, or about twenty square 

 miles) is observable up to an altitude of 1,500 

 meters. 



To sum up the preceding considera- 

 tvmi, . The efifect of these complex re- 

 la /ion's has been made the object of in- 

 vestigation, particularly in Germany, 

 through the study of the amount of 

 moisture remaining in the soil under 

 the various kinds of trees, and in the 

 adjacent open. Some reference to 

 these results has already been made ; 

 but the general conclusion is that the 

 effects of transpiration from luxuriant 

 foliage are very materially greater than 

 those of evaporation from the bare level 

 surface. 



The most striking illustration of this 

 factor is the efifect of eucalyptus trees. 



which, planted in the Pontine marshes 

 near Rome in 1870 (in the vicinity of 

 the Convent of St. Paul of the Three 

 Fountains), have lowered the level of 

 the ground water several decimeters. 

 In this case the transpiration from the 

 water-loving eucalyptus is more vigor- 

 ous than was the evaporation from the 

 water and vegetation of the swamps. 



The relation of the forest to run-ofif 

 on level land is not an important factor. 

 As we have already seen, it is possible 

 to keep level' fields in such a condition 

 of tilth that the amount of run-ofif from 

 them is no greater than that from the 

 forest. On slopes the case is dififerent. 

 The efifect of the forest is, first, to 

 diminish the violence with which the 

 rain strikes the ground and compacts 

 it, for the drops are checked by the fo- 

 liage. They fall from branch to branch 

 and reach the ground with greatly 

 diminished momentum and in a finer 

 state. That portion which drops from 

 the leaves comes in larger masses, but 

 still gently. Beneath the forest the 

 rain finds a moist soil whose capillary 

 water is capable of vigorous downward 

 circulation. The soil is also opened by 

 the forest roots, and being protected 

 from the compacting and cementing 

 influences of a free exposure is de- 

 cidedly porous. All these conditions 

 militate against run-ofif and promote 

 absorption. In consequence the run-ofif 

 from forest areas gathers slowly as 

 compared with bare slopes, and the 

 ratio becomes increasingly favorable 

 in regard to forests as slopes become 

 steeper. This fact enters into the 

 consideration of the value of forests in 

 mountain regions as compared with 

 those on plains. Ebermayer,'^ from 

 whose paper many of the data used in 

 this report have been taken, says : 



In view of the exact investigations which 

 have been cited above, and which extend 

 through thirty years, it is no longer possible 

 to maintain the assumption so generally 

 made, that forests increase the amount of 

 ground water and constitute a storage res- 

 ervoir for the supply of springs, at least not 



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

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



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

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



