president's address. 627 



arises can we alter the conditions on a larger scale by alternating 

 forest and field, or b}'- preserving intact large areas of forest 1 



In rejDly to this query, it seems to have been shown by the 

 forest planting at Lintzel that a considerable change in the 

 meteorological conditions resulted. 



Forest cover causes obstruction to the winds and hinders the 

 action of the sun's rays upon the soil. A difference in temperature 

 and evaporation outside and inside the forest area then arises. 

 When the areas of the different kinds are large enough, local 

 currents of air will be set up, which will cause the interchange 

 of conditions between the two areas. The size and character of 

 the forest growth, density, height, situation and composition are 

 the factors which determine its influence. It is not trees but 

 masses of foliage that do the work. 



On water supply the effect of forest is undoubted. The soil 

 acts as a sponge from which the water runs off gradually; remove 

 the forest wholesale, and the water rushes along, tearing away soil 

 and flooding and injuring growing crops. 



Irrigation is generally advocated for arid regions alone, but it 

 is also particularly serviceable in well watered regions, and here, 

 as above shown, forest cover materially assists. 



The following observations are of interest : — 



First, as to the difference of conditions within and without the 

 forest. On the average the forest is cooler than the open country 

 in summer, but about the same in winter, with a warming effect 

 in spring, and the evaporation is onl}^ one-half that in the open. 



The percentage of rainfall evaporated is about 40% in the open 

 and about 12% in the forest, taking the whole year. 



The total quantity of moisture returned into the atmosphere 

 from a forest by transpiration and evaporation from the trees and 

 the soil is about 75% of the precipitation; other forms of vegeta- 

 tion give from 70 to 90%; bare soil gives only 30%. Gauges in 

 European forests catch 75 to 85% of the rainfall, the rest runs 

 down the trees, is intercepted or evaporated. 



