54 6 RELATION TO ENVIRONMENT. 



1030. Value of tropical forests. The forests of the tropics are 

 of vast extent, and fully one half of the area covered by them has 

 hardly been explored. Many of the woods are of very great value. 

 In regard to the Amazon basin, Agassiz says, "Nowhere in the 

 world is there finer timber, either for solid construction or for 

 works of ornament." A number of things have stood in the way 

 of the exploitation of the tropical forests, chief among them being 

 the climate and the great luxuriance of the forest itself; it is very 

 likely that in the future the work of exploiting the forests of the 

 tropics will be greatly developed. Many useful products are 

 derived from a large number of different kinds of tropical trees. 

 (See Gifford, Practical Forestry.) 



V. Relation of Forests to Rainfall. 



1031. Forests do not materially increase rainfall of a region. 



In a study of the climatic vegetation regions it is clear that the 

 forest is dependent on rainfall, and below a certain minimum 

 annual precipitation, not very definitely determined, forests will 

 not develop, and of course the rainfall must be rather evenly 

 distributed throughout the year, or at least through the growing 

 season. But that the rainfall of a region is influenced by the 

 forest to any great extent, as is often supposed, is not so evident. 

 Long-continued droughts during the growing season which occur 

 now and then, and the great accompanying forest fires, show 

 the inability of the forest to produce rainfall per se. Rainfall is 

 due to moisture-laden air-currents from warm bodies of water 

 coming in contact with the cooler air of coast lines, mountain 

 chains, or cooler air-currents from colder regions. The movement 

 of these air-currents is controlled by barometric pressures, a storm 

 center originating in an area of low barometric pressure and 

 moving to one of high pressure. But forests do account to some 

 extent for a certain per cent of the rainfall of the region. The 

 forest floor holds back a large amount of the precipitation of 

 moisture-laden air coming from a distance. Through trans- 

 piration the forest leaves as well as those of other vegetation, 

 together with evaporation from the forest floor and streams, load 



