INFLUENCE OF FORESTS ON CLIMATE 7 



In hilly and mountainous regions there are numerous 

 springs, and the underground water is in motion. On 

 slopes the run-off water, that is nil or negligible in the 

 plain, becomes large in quantity. The action of the forest 

 is to reduce the surface run-off to percolation. This action 

 depends on the litter and humus of the floor of the forest, 

 which act like a sponge absorbing four or five times their 

 weight of water. There is practically no surface run-off 

 from wooded watersheds having an abundant leaf-mould 

 and litter. This water then sinks slowly into the ground, 

 ultimately to re-appear in the springs lower down the 

 valley. Similarly, forests preserve the snows many days 

 longer than in the open country ; their melting becomes 

 very gradual, and the run-off is stopped or diminished. On 

 a bare slope, on the contrary, the run-off is very great, so 

 that immense quantities of water reach the rivers in a short 

 time, causing often disastrous floods. Forests thus, by 

 diminishing or stopping the run-off, increase in hilly and 

 mountainous regions the quantity of water that percolates 

 into the ground and ultimately re-appears as springs. At 

 the same time the forest preserves the soil on slopes from 

 erosion. In deforested mountains torrents carry enormous 

 quantities of soil, rocks, and stones into the rivers, wliicli 

 may become silted up and useless for navigation. 



Floods, besides being very disastrous financially, as they 

 may cause much destruction of buildings, bridges, railways, 

 etc., as well as permanent injury by silting over tracts of 

 good land, are often serious to public health. There may 

 be loss of life and suffering and sickness caused by exposure 

 and privation. Damage may be done to systems of drain- 

 age, sewerage, and water supply, thus affecting injuriously 

 the health of a city or community. Not unfrequently an 

 epidemic follows a flood. 



The great diminution of the run -off water in forests is 

 due to various causes. The rain falls more gently, owing 

 to the obstruction of the foliage ; its flow is impeded by the 

 roots of the trees ; and it is absorbed as fast as it falls by 

 the deep layer of humus and litter on the soil. It must be 



