23 

 CHAPTER V. 



FORESTS AND CLIMATE. 



Reference has already been made to certain aspects of the indirect value of 

 forests. But l^ond these ways in which forests are indirectly of great moment, 

 there is one whose importance must be specially stressed. In every part of the 

 world where forests are great, there is a constant annual rainfall, sometimes spread 

 with fair evenness over the whole year, and sometimes occurring as seasonal rains. 

 Again, wherever forests are absent or tree-life sparse, comparatively little rain 

 falls. The presence of forests implies a sufficient rainfall; their absence often 

 means little or no rain and desert country. The world furnishes many examples 

 of this. There are no forests in Arabia, and the rainfall is of the scantiest. The 

 same may be said of North-Central Africa, where there stretches over vast arid 

 distances the Sahara Desert, Central Australia has no great forests, and such 

 trees as there are are few in number and stunted in size, and the rainfall there is 

 very small. History tells us quite clearly that the destruction of forests alters the 

 climate of a country by reducing the rainfall to a point that makes the growing 

 of food-crops impossible. Time was when Mesopotamia was a well-wooded country 

 and when grains of various kinds were produced in abundance. Many centuries 

 ago its forests were almost wholly destroyed. The rainfall diminished to such a 

 degree that agriculture could not be carried on except in the immediate neighbour- 

 hood of the two great rivers — the Euphrates and the Tigris — and such crops as were 

 grown owed their existence to irrigation. Sicily, at one time, was one of the 

 granaries of ancient Rome — that is, it grew and sent to Rome large quantities of 

 wheat every year — but its forests were almost wholly cut down and it ceased to be 

 a great wheat-growing country. The island of Cyprus is another instance in proof 

 of the fact that the wholesale destruction of forests has a disastrous effect upon 

 a country's productive power. 



But forests are in other ways of inestimable value so far as moisture is con- 

 cerned. When rain, especially heavy rain, falls upon a hard, bare country, much 

 the greater portion of it immediately runs off into the streams and rivers and on 

 into lakes or into the sea. When the country, however, is forested, the case is 

 different. Much of the surplus water runs off of course, but a great deal is re- 

 tained by the forest. In every forest there is upon the ground an accumulation 

 of what is known as "humus," made up of the leaves and twigs that have fallen 

 from the trees. This humus has to become thoroughly soaked before any water 

 begins to run off. Further, the ground within a forest is more porous than ground 

 that has no forest cover. The reason is obvious, for beneath the surface of the 

 forested ground are vast masses of roots, and these break up the sub-soil, per- 

 mitting the absorption of water. Experiments in various parts of the world have 

 proved beyond question that forests have a beneficial effect on streams and springs. 

 Rivers that are fed from forested watersheds have a more uniform discharge and 

 carry less debris than streams coming through an unforested watershed. It has 

 also been proved conclusively that extensive damage from floods occurs less fre- 

 quently in streams coming from forested watersheds than in streams rising in 

 poorly forested or treeless watersheds 



In forested country, too, streams run throughout the year, because the forest 

 gradually gives off the water it holds, while in unforested regions, the water passes 

 quickly to the streams, and, in the absence of further rain, these dry up. Springs 

 also are favourably influenced by the presence of forests, as their flow is regulated 

 and made continuous by the water conserved by the forest and given off gradually. 

 Ringbarking large areas of tree-covered land has much the same effect on streams 

 and springs as does cutting down the trees, and more particularly is this the case 

 if fire sweeps through the ringbarked country and destroys the humus. When 

 the Mundaring Weir was formed, a considerable part of the water catchment area 



