74 



SCIENCE 



[N. S. Vol. XXXVIII. No. 968 



into the interior of continents. The de- 

 struction of such forests, especially if it 

 leaves the ground bare or partly covered 

 with only weak vegetation which does not 

 transpire large quantities of water, must 

 inevitably affect the climate, not so much 

 the climate of the region in which the de- 

 struction took place but the drier regions 

 into which the prevailing air currents flow. 



I realize, of course, that direct proof of 

 this climatic influence quantitatively ex- 

 pressed is still lacking. It will take many 

 decades before direct observations of such 

 a character will be secured. If, however, 

 the premises upon which the discussion 

 rests, namely, that the precipitation of the 

 eastern half of the United States is inti- 

 mately connected with the prevailing south 

 winds, that evaporation from land contrib- 

 utes more to the precipitation over land 

 than evaporation from the ocean, that for- 

 ests evaporate more water than free water 

 surface, or any other vegetation, then for- 

 ests in the path of prevailing winds must 

 necessarily act as distributors of precipita- 

 tion over wide continents. 



What practical deductions can be made 

 from these facts? 



1. Forests must be protected not so much 

 in localities which already suffer from lack 

 of moisture as in regions which lie in the 

 path of prevailing winds and are still 

 abundantly supplied both with ground 

 water and precipitation. In the dry re- 

 gions large bodies of forests may have the 

 opposite effect upon the available water 

 supply. There only forests growing along 

 rivers may contribute to the humidity of 

 the region. There rows of trees or wind- 

 breaks surrounding fields and orchards, by 

 preventing the drifting of the snow and 

 decreasing the activity of the wind, will 

 act more as conservers of moisture in the 

 soil than solid bodies of timber. Therefore, 

 the care with which forests should be pro- 



tected in the eastern half of the United 

 States must increase from north to south 

 and from west to east. 



2. In the Atlantic plain and southern 

 Appalachians, which are the gateway for 

 the prevailing winds from the Gulf of 

 Mexico and the Atlantic Ocean, forests 

 must be especially maintained. 



(a) On moist soils, provided the excess 

 of water or the substances contained in it 

 do not prevent their development, because 

 the moister the soil on which forests grow 

 the more moisture they evaporate. For 

 this reason swamps, since they contribute 

 less to the moisture contents of the air than 

 crops or forests and lose considerable water 

 by surface run-off, must be drained, as by 

 doing this an increase of the evaporation 

 at the expense of surface run-off may be 

 secured. 



(J) On sandy soils. Forests on sandy 

 soils readily absorb water through the roots 

 and evaporate it into the atmosphere. De- 

 nuded of forest cover, sandy soils readily 

 absorb rainwater which percolates into the 

 ground and often reaches the sea by under- 

 ground channels without being returned 

 to the atmosphere. 



(c) On steep slopes and rocky places; 

 the removal of forests on such places in- 

 evitably leads to an increase in the surface 

 run-off and to a corresponding decrease in 

 local evaporation. 



3. If clearing of the forest is a necessity 

 it should be done only under condition that 

 the cleared land is to be devoted to intense 

 cultivation, as, after forests, crops contrib- 

 ute most to the moisture of the air. The 

 highest organic production, therefore, is in 

 harmony with the safeguarding of the hu- 



.midity in the regions which lie in the path 

 of the prevailing winds. Cleared land that 

 becomes waste or poor pastures or grows 

 up to weak vegetation, means so much evap- 

 oration lost to the passing air currents. 



