FACTORS AFFECTING PLANT DISTRIBUTION 21 



tion, a certain amount from the shrubs and short vines taller than 

 the plants forming the ground cover, and a still greater amount 

 from the trees and large lianas with their crowns exposed to full 

 sunlight and the more rapidly moving air currents found at this 

 distance above the soil. The forests are for this reason, therefore, 

 the greatest dessicators of the soil. Next in order of amount of 

 water evaporated would doubtless be a tall-growing cultivated crop, 

 corn, tobacco, sorghum, sunflowers, or castor beans, for example. 

 The evaporation would be increased if the weeds were allowed to 

 grow beneath these crops or if, as is commonly practiced, another 

 crop, a legume for example, were seeded in the field when tillage 

 ceased. 



The part that forests play in the control of climate, especially 

 in the amount of rainfall in the regions near great forests, has 

 long claimed the attention of intelligent observers. The influence 

 of mountains upon precipitation, as set forth in the preceding sec- 

 tion of this paper, is markedly increased if the mountains are 

 clothed with a forest cover. The effect of the forest upon local pre- 

 cipitation may not be very great, but its influence is noticed in drier 

 regions in the paths of winds coming from over forested lands. 

 Hamberg says of the forests of Sweden : "The excess of evaporation 

 which the forest vegetation furnishes to the atmosphere above what 

 the same area would furnish if it were covered with herbaceous 

 vegetation merely, must of course be very considerable. If this 

 aqueous vapor were received 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 bene- 

 ficial influence for our country (Sweden) remains very doubtful." 

 The cultivated lands in Sweden, especially its dairy farms, obtain 

 the benefit of the moisture evaporated from the forests and conti- 

 nental countries east of Sweden also feel its beneficial influence, 

 since they are in the paths of the winds blowing over Swedish 

 forests. In the United States the Ohio Valley receives the benefit 

 of the Appalachian forests when the winds are from the south. 

 Much of the increased moisture of the prairie region of the central 

 states over that of the plains states farther westward must also 

 come from this source. The digest of the present writer's opinion 

 on this point as given in a previous paper may be summarized as 

 follows: (a) The coincidence of precipitation in the United States 

 and prevailing southerly winds, (b) the amount of evaporation 



