FORESTS AND CLIMATE 3*7 



Suez Canal is said to have brought an increased rainfall; from the 

 Peninsula of Sinai, from Syria and from Algeria, in all of which de- 

 forestation is said to have changed luxuriant and fertile districts into 

 deserts. One other example, quoted by a recent writer, may, perhaps, 

 be referred to : 



In 1551 the Marquis of Northampton went from Orleans to Nantes (on the 

 river Loire), with his suite, in "five large, many-cabined boats," whereas navi- 

 gation is now impossible above Saumur, the distance of which from Nantes is 

 less than half that of Orleans. This change is ascribed to the deforestation 

 carried on extensively in the surrounding country in the seventeenth century, and 

 the consequent diminution in the volume of water in the Loire due to diminished 

 rainfall. 



There is no need to multiply these examples. They show, clearly 

 enough, why the historical method is unsafe, and why it has given but 

 meager results. 



An Essential Consideration: Why Should Forests Influence 



Climate ? 



It is a curious fact that so few of those who are firmly convinced 

 that climate is affected by forests, ever seem to ask themselves: "Why 

 should forests influence climate ? " We seem to accept it as a fact with- 

 out asking ourselves why it should be so. If we stop a moment to con- 

 sider the reasons which come to mind, we shall probably sooner or later 

 enumerate them about as follows : 



(a) Because forests must retard and obstruct air movement, favor- 

 ing calms, and causing the air to ascend slightly over the trees. Both of 

 these effects may be favorable, in a small way, to rainfall. The barrier 

 effect, by reducing the velocity of high winds, ought to moderate the 

 extremes of winter cold. 



(b) By means of their shade, trees ought to check the warming of 

 the ground, and of the air, especially in summer. 



(c) Because of the retention of moisture in the forest litter, and of 

 the decreased evaporation which may be expected to result from the 

 lessened air movement under the trees, it seems not unreasonab]e to ex- 

 pect that forest air will be somewhat damper than that outside. This, 

 under proper conditions, may also favor rainfall. 



(d) The diffusion of the water vapor transpired by and evaporated 

 from the leaves may perhaps increase the opportunity for rainfall. 



(e) We may expect the tree cover to diminish nocturnal radiation 

 from the ground underneath, and thus to maintain a slightly higher 

 temperature within the forest than outside of it at night. 



(/) Also, there may be some effect from the increased radiating 

 surface due to the presence of the leaves or needles. This must be 

 chiefly effective at night. 



(g) The heating of the leaves must be less than that of bare ground, 

 because of the evaporation of much water from the leaves, and because 



