INFLUENCE OF FORESTS ON CLIMATE | 
In hilly and mountainous regions there are numerous 
springs, and the underground water is in motion. On 
slopes the run-off water, that is ni 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, which 
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 
