FORESTS OF POETO EICO. 37 



succession of reservoirs and a cleaning up of the channels if any 

 noticeable reduction of the eroding effects of floods is to be had. 



Forests aid in conserving the water in the soil. The trees both aid 

 the water in getting into the soil and then help the soil to hold on to 

 it. In the first place, the trees break the beating force of the rain, 

 which in the Tropics is considerable, and thus help to keep the surface 

 layers of the forest soil from being beaten down and rendered compact 

 and impervious like the soil in the open. Then the roots of the trees 

 make the soil more open and accessible to percolatmg water. The 

 roots and such ground cover and litter as there are impede the progress 

 of surface run-off and afford the soil more time to absorb the water. 

 With more water getting into the forest soil than in grassland soil, 

 both being of a retentive character, there will be more water to find 

 its way to springs and be gradually poured out into the rivers to 

 sustain them during the periods of little rain. 



The forest influences erosion in two ways: By reducing the force 

 and interrupting the passage of the surface run-off in the catchment 

 areas around the headwaters of the streams it slows up the washing 

 awky of the surface layers of the soil and greatly impedes guUying. 

 At the same time the ability of the run-off to transport eroded ma- 

 terial is very considerably lessened. A grass cover, if it forms a firm, 

 well-knit sod, is also quite effective in resisting the erosive action of 

 surface run-off. When, however, the grass grows in bunches and is 

 interspersed with patches of bare ground or with tender, succulent 

 herbage that dies out in dry weather, leaving the soil exposed, erosion 

 and run-off is little affected. This is often the condition on the 

 upper and drier slopes on the south side of the island. That these 

 open slopes are not scored more deeply than they are is undoubtedly 

 due in large measure to the tenacity of the soil. 



It is when the run-off is gathered into the streams of the island 

 and reaches the foothills country, where the character of the soil 

 changes from the heavy clays of the interior to the lighter and more 

 readily eroded coast soils, that the greatest damage is done. The 

 rivers are generally too short to choke up and overflow, as would 

 otherwise more frequently happen. Yet they are continually 

 widening and shifting their channels, cutting off islands from adjoin- 

 ing fields, and undermining their banks. Frequently it is not so 

 much the water that creates the havoc as the material which it picks 

 up and transports. Besides the finer soil particles and gravel, large 

 bowlders are dislodged and rolled along with great destructive force. 

 Thus the volume of water which comes from the hills may in the 

 course of its passage to the sea be doubled by the material trans- 

 ported by it or dumped into it from caving banks. 



A fringe of forest growth along the banks wiU materially lessen the 

 liability to this kind of erosion. Certain of the bamboos are par- 



