FlIfTH NATIONAIv CONSERVATION CONGRESS 195 



upon precipitation of complete or even partial destruction of forests in the At- 

 lantic plains or in the Southern Appalachians? Since the mean temperature in 

 the eastern portion of the United States drops rapidly from north to south, the 

 moisture-laden air currents, upon reaching the land, would be cooled oflf and rapid- 

 ly drained of their moisture within a comparatively short distance f roih the ocean. 

 The sandy soil so characteristic of the southern pine belt of the Gulf and South 

 Atlantic States would rapidly absorb the rain, without returning much of it into 

 the atmosphere. The rain which fell upon the slopes of the mountains would 

 rapidly run off into the streams. While the remoya! of the forest might increase 

 the evaporation from the ground itself, yet the more rapid run-off and the ab- 

 sence of transpiration by the trees would reduce the total amount of water evap- 

 orated into the atmosphere. The land, even if taken up for agriculture, could 

 never return such large quantities of rain into the atmosphere as the forests did. 

 The result would be that less moisture would be carried by the prevailing winds 

 into the interior of the country, and, therefore, less precipitation would occur 

 there. 



Regarding Sweden, an eminent meteorological authority. Dr. Hamberg,^ 

 says: 



"The excess of evaporation which the forest vegetation of Sweden fur- 

 nishes to the atmosphere above what the same area would furnish if it were 

 covered only with herbaceous vegetation must, of course, be very considerable. 

 If this aqueous vapor remained in the forest and returned to the land in the form 

 of rain, it would be extremely beneficial. But winds carry it off and spread it in 

 all directions with such rapidity that its beneficial influence for our country 

 (Sweden) remains very doubtful." 



The forests of Sweden have, however, an important influence upon the pre- 

 cipitation of the countries to the east, into which the prevailing winds blow, since 

 in regions far removed from the ocean the feeding of the atmosphere by local 

 evaporation has an important bearing upon the humidity and amount of precipi- 

 tation. "On the continents in countries like central Siberia," says Hamberg, "for- 

 est vegetation must influence, of course, the humidity of the air. It returns to 

 tiie atmo-sphere in the form of vapor the water collected and conserved in the 

 forest which otherwise would run off. It lowers the temperature of the air. As a 

 result of these two causes the relative humidity of the air must increase, and with 

 it must also increase the inclination to precipitation in the form of rain or snow." 



Whether mountain forests have the same effect as forests in level countries 

 upon the precipitation of the regions into which the prevailing winds that pass 

 over them blow is difficult to determine. The problem is complicated by the fact 

 that high mountain chains themselves exert an influence upon precipitation and 

 the direction of the winds, not only by presenting a mechanical" obstruction to the 

 free passage of the air, but also on account of the difference in temperature on the 

 different slopes. A moist current of air in passing over a mountain chain under- 



1 Hamberg, H. E. De I'influence des forets sur le climat de la Suede. Stockholm, 

 1885-1897. 



