Trees and forests may also have an important influence on the 

 drainage of a region. 



An oak, standing free in the open with 700,000 leaves will trans- 

 pire 111,225 kilograms (250,000 lbs.) of water during the five 

 months of the growing season — that is, from June to November. 

 (Even clover can give ofT twice its own weight in water in a day.) 



Cedrela and eucalypts are well known as great "pumpers." In 

 Natal they have been used for draining low, flat lands with great 

 success ; and along the Mediterranean melaleuca and eucalypts are 

 well known as "anti-fever" trees. The Roman Campagna was a 

 dismal swamp, fever infected, and almost uninhabitable, when it 

 was planted up with eucalypts by a Monastic order. The results 

 obtained were remarkable, but the credit for them was entirely mis- 

 interpreted at the time. The peculiar, pungent, emanations from the 

 foliage, and the nature of the oil found in the leaves were credited 

 with the power of counter-acting the "miasma" or evil influence 

 of the air. The real reason for the change was, as we all know 

 now, the draining of the swamps and thereby putting an end to 

 the breeding of mosquitoes ! 



In India, where trees of this kind were planted along the irriga- 

 tion ditches, they had to be cut down on account of their absorbing 

 and diverting too much of the water. 



It is true that forests tend to reduce extremes of temperature, and 

 the mean annual temperature is from .9 to 1.8 degrees F. less within 

 the forest than outside of its influence. The daily variation may 

 be as much as 5 degrees F. 



Even the temperature of the soil within the forest area is less 

 affected by changes in the seasons. In the winter it is 1.8 degrees 

 warmer and in the summer from 5.4 to 9 degrees cooler than without 

 the forest cover. The changes are slower, and never as extreme. 

 The soil never freezes to as great a depth in the woods as in the 

 open, the covering of leaf-mould tends to retain the heat. 



The relative humidity is from 4% to 12% higher in the forest in 

 the summer than without. This is due in part to the lessening of 

 wind currents, due to the mechanical interference of the trees, and 

 to the transpiration of the leaves, which has a tendency to lower the 

 temperature and bring it nearer the saturation point. 



Every farmer knows the value of a woodlot or strip of trees in 

 protecting the farm from severe zviuds and from excessive and sud- 

 den changes of temperature. 



As a windbreak for the protection of crops, woods are valuable 

 not only for their eft'ect on sidereal winds but on severe storms as 

 well. Poplars have been planted for many years in great numbers 

 in the middle west, for the protection that they afford grains and 

 other crops from the dry, hot winds that are the terror of the farmer 

 in that region; and in California and southern Florida the citrus 



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