LIGHT IN RELATION TO TREE GROWTH. 1 97 



and later it sinks to 1/60 of the total daylight. These differences 

 in light demands are well known from common experience. 

 Thus, in a coppice forest it is often necessary, in order to 

 awaken the dormant buds of the stumps, to clear away the brush 

 around them and expose them to higher light intensities. It is 

 a well-known fact that trees in the open begin to bear seed 

 earlier than trees in a close stand. Foresters estimate the 

 average retardation due to close stand at 20 years. The 

 preparatory cuttings for natural reproduction under the shelter- 

 wood system are based on this principle of allowing more light 

 to the remaining trees in order to stimulate seed production. 



In general, the more vigorous the individual, the greater is its 

 tolerance. Any factor which reduces its vitality reduces its 

 tolerance. Trees from seed, all other conditions being equal, 

 stand shade much better than sprouts, and in artificial planta- 

 tions nursery stock is more tolerant than trees which have been 

 started by direct seeding. 



Measure of Tolerance. 



Since tolerance is affected by so many different factors, it is 

 evident that the tolerance of any particular species is necessarily 

 variable and exceedingly difficult to measure. Until recently, 

 therefore, it has been customary not to attempt to measure 

 tolerance of shade in positive, but only in relative, terms. 



The tolerance of different species is usually compared in lists 

 or scales, the most tolerant being usually placed first and the 

 least tolerant last. The position which a species occupies in 

 relation to these extremes is the expression of its tolerance. 

 Broad and rather indefinite terms have been used such as 

 " very tolerant," " tolerant," " intermediate,'' " intolerant," " very 

 intolerant." The following are examples of scales of tolerance 

 prepared by European authorities, beginning with the most 

 tolerant : — 



Gayer (1898). Buhi.er (Morozov, vol. 2 : 1286-1295). 



Taxus baccata (English yew). Taxus baccata (English yew). 



Abies pectinata (silver fir). Abies pectiiiata (silver fir). 



Fagns (beech). Fagus (beech). 



Picea (Norway spruce). Picea (Norway spruce). 

 Carpiniis bctuliis (European horn- Finns strobiis (white pine). 



beam). Uliinis tnoiitana (wych elm) 



Pintis strobiis (white pine). Tilia (linden). 



