LIGHT IN RELATION TO TREE GROWTH. 201 



and others ; Larix, Sa/ix, Popu/us, Betula, Taxodiu/ii, Magnolia, 

 Robinia, Liriodendron, and others. 



Of the secondary trees and arborescent shrubs, the evergreen 

 broad-leaf shrubs, such as Buxus, are classed as tolerant ; 

 Corylus, Cornus, Ligiistrum^ Euonymus, Lonicera, and others 

 are classed as partially tolerant; Pruuus, Spartium, Ca/Iiaia, 

 CratcBgus, Vibiirtium, and others as intolerant. 



Of forest weeds, Folyfrichini, Hypimm, Aspidium, Vaccinium, 

 Hede?-a, and others are tolerant ; A?temo/ie and Pteris^ partially 

 tolerant ; Ci/rini/i, Sik/ie, Fragaria, and most of the grass and 

 clover species, intolerant. 



Practical Value of the Scales. 



These scales have their practical use in silviculture, particularly 

 in giving a broad comparison of the different species. If one 

 is seeking a species for underplanting, he naturally concerns 

 himself with the most tolerant. If he is developing a plan for 

 reproduction cuttings, he knows that the least tolerant will 

 require a clear-cutting method, and that he can not re-establish 

 new growth under the old cover. Thus, empirical knowledge of 

 light requirements establishes the fact that young growth of a 

 tolerant species under the shade of a fully stocked stand of a 

 tolerant species begins to die off, as a rule, after from lo to 20 

 years of suppression; intolerant trees under a tolerant species 

 perish after about 5 years ; a tolerant species under an intolerant 

 species may continue to live indefinitely, because the amount of 

 light that penetrates through the thin crowns of the light- 

 demanding species is sufficient for the growth and development 

 of the tolerant species. Reproduction of an intolerant species 

 under the shade of an intolerant species is capable of enduring 

 for from 10 to 20 years, or almost as long as reproduction of 

 tolerant trees under the shade of tolerant species. Since the 

 least amount of light is admitted to the ground in a fully stocked 

 stand in its pole stage, only a tolerant species under the shade 

 of an intolerant species can live through this period without 

 injury. 



Tolerance scales based on experience are of distinct value 

 to the practitioner and to the student of forestry first learning 

 the characteristics of the diff"erent species. For accurate and 

 scientific investigation, however, they are defective, for they 

 show only that one species is more or less tolerant than another. 



VOL. XXVI. part II. o 



