196 TRANSACTIONS OK ROYAL SCOTTISH ARBORICULTUKAL SOCIETY. 



method may be applied to determine the effect of nourishment 

 upon the work of the leaves. It was found that on different 

 soils the annual production of wood in trees of the five different 

 classes (dominant, codominant, intermediate, oppressed, and 

 suppressed) for each pound of small twigs was as given in 

 Table IV. 



Table IV. — Amount of wood prodiia'd annually by diffei-ejit 

 classes of trees for each pound of small twigs. 



Classes 



Dominant 

 Codominant 

 Intermediate- 

 Oppressed 

 Suppressed 



These figures show clearly the influence of soil conditions 

 upon the assimilative activity of the leaves, and consequently 

 upon the variation in tolerance of trees according to soil fertility. 



5. Age, vigour and origin of tree. — Trees are more tolerant 

 in early youth than later in life. In fact, it may be said that 

 during the first year or two all trees are tolerant. As they 

 grow, their demands for light increase, and the distinctive light 

 requirements of different species become more and more 

 emphasised. This increase in light requirements becomes 

 especially ajjparent at the time of the most rapid height-growth, 

 after which the light requirement remains stationary for a long 

 time and increases again only late in life. White pine is a very 

 good example of a tree which in early youth is tolerant, but in 

 later life is distinctly intolerant. White pine seedlings will start 

 in dense shade ; but they cannot often live under such conditions 

 for more than ten to fifteen years, and they may perish from lack 

 of light in the first two to 'i\we years, even if abundant moisture 

 and nourishment are present. 



The amount of light needed for flowering and fruiting is 

 greater than for mere growth, so that demands for light vary 

 not only with age, but with season. Thus, early in spring at 

 the time of bursting of buds, the minimum light intensity for larch 

 is 1/2 of the total daylight, later it falls to 1/5; for beech the 

 minimum light intensity at the time of unfolding of buds is 1/4; 



