26 TIMBER OF THE EDWARDS PLATEAU OF TEXAS. 



breeze in an open pail or in a porous earthen vessel. Over against 

 this, however, must be set the fact that the transpiration from heavy 

 grass is still. greater than that from a forest. But in those regions 

 which are too arid to support more than a scanty vegetation of other 

 than forest growth, the presence of timber would doubtless conduce 

 to some mitigation of the intense heat, and would also tend to increase 

 the moisture in the air through evaporation from the leaves. 



INFLUENCE UPON SOIL AND WATER SUPPLY. 



One of the most important services of a forest cover is the mechani- 

 cal effect which it exercises upon falling rain and upon the run-off'. 

 In this way it both checks erosion and promotes the entrance of water 

 into the earth. In the first place, the crowns of the trees, especially 

 when the foliage is on, break the force of the rain and cause it to run 

 harmlessly down the trunk, or to drip slowly through the canopy. 

 Further, the organic debris of the forest floor holds back the fallen 

 water until it has time to soak into the soil. The spreading and inter- 

 lacing network of roots serves the same purpose, and binds the soil 

 fast against erosion. Thus the rain is kept from swift discharge into 

 the streams, gulh'ing is prevented, and the run-off does not gain sud- 

 den volume and velocit}' after a downpour. The removal of timber 

 from broken or mountainous areas is pretty sure to be followed by 

 more frequent and destructive floods. 



A forest also increases the water supply from a region by increasing 

 the moisture-holding- capacitv of the soil. The undecomposed litter 

 which forms the upper layer of the forest floor will itself take up 

 much water, as well as delay its run-off. A thick mat of leaves will 

 be wet at a little distance down long after those on top have become 

 thoroughly dry. Still lower, the half -decayed rubbish is like a sponge 

 in its water-holding power. And finally, the humus, or forest soil 

 proper, with its loose texture and large proportion of organic matter, 

 is peculiarly fitted to delay percolation. Thus the forest builds up a 

 storage reservoir, the loss of which often makes necessary the con- 

 struction on a large scale of artificial lakes to conserve the water sup- 

 -plj. This work the forest does not only without expense, but while 

 itself growing wood. 



Not only does the forest store water in the soil, but it also prevents 

 its loss by evaporation. The trees themselves shield the ground from 

 the sun, and check the movement of drying winds. They also keep 

 the soil cool, and in consequence lessen the giving off of moisture. 

 This defense against evaporation is further leenforced by the under- 

 growth and the leaf litter, while the forest soil itself acts as a mulch 

 to prevent drying out. We have seen that the dry air of the Edwards 

 Plateau region is capable of removing b}^ evaporation over 50 inches 

 of free water in the course of a year. When rain falls on the bare 



