1 82 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



the height-growth of trees, the rate at which stands thin out 

 with age, the progress of natural pruning, the character of the 

 living ground cover, the vigour of young tree growth, the 

 existence of several-storied forests, and many other phenomena 

 upon which the management of forests depends. A thorough 

 understanding, therefore, of the effect of light upon the life of 

 individual trees, and especially on trees in the forest, and a 

 knowledge of the methods by which the extent of this effect 

 can be determined are essential for successful cultural operations 

 in the forest. 



The aim of this Bulletin is to bring together the principal 

 facts with regard to the part which light plays in the life of 

 the forest, and the different methods of measuring it. It should 

 prove of material assistance to students of the subject, and 

 it is hoped that it may also stimulate an interest to further 

 research in determining more accurately the light requirements 

 of our forest trees, especially by actual measurements of light 



in the forest. 



Kinds of Light. 



Direct and Diffused Light. — In discussing light with reference 

 to tree growth a distinction must be made between direct 

 sunlight and diffused light. If the earth were not enveloped 

 by an atmosphere, all liglit would be direct sunlight, and its 

 intensity at every point on the earth could be calculated 

 mathematically from the position of the sun. The presence 

 of the atmosphere, however, modifies essentially the distribution 

 of light and heat on the earth's surface. Only a part of the 

 light which emanates from the sun reaches the earth as direct 

 sunlight. Another part is reflected from the small particles 

 contained in the air, such as dust and minute drops of water, 

 and forms diffused light. .Still another part is absorbed and 

 entirely disappears. The diffused light illumines the atmosphere 

 and forms the skylight. When the sky is cloudless the total 

 daylight consists of both direct and diffused light. On cloudy 

 days all the light is diffused light. 



Trees in the forest make use chiefly of the diffused skylight, 

 and for this reason it plays the most important part in their life. 

 Indeed, many plants have developed special contrivances for 

 protecting themselves from the direct rays of the sun. There 

 are, however, trees and other ]jlants which, in addition to 

 diffused light, need direct light either during their entire life 



