Ecological Points 25 



soils, we mean that they can still prosper under less favor- 

 able conditions, if necessary; yet they would do best on the 

 soil as described, which is, therefore, the nurseryman's 

 choice. 



As regards light supply to the foliage, we have, of course, 

 no means of increasing or modifying it in general, yet in 

 group planting, park planting, street planting, and even in 

 pruning, the knowledge that different species require differ- 

 ent light intensity for their best development is essential; 

 for here proper adaptation to hght conditions, and artificial 

 changes in light conditions can be practised. All trees 

 thrive ultimately best in full enjoyment of light and then 

 develop their characteristic form, as we see it in single trees 

 on the lawn; but, if placed in close neighborhood of each 

 other, the fact will soon appear that one species is "stronger" 

 than another; that means the one spreads its branches and 

 kills out those of its neighbor by its shade. 



In other words, the foHage of one species requires more 

 light to perform its work than that of another. This fact 

 is exhibited in the individual tree itself. The interior of 

 the crown in those which need much light, like the birch or 

 aspen, thins out, is empty of foliage, because of the shade 

 which the exterior exercises; it loses its lower branches 

 sooner on account of the shading by the upper, or else the 

 lower reach out farther to escape suppression. On the 

 other hand, the tolerant or shade-enduring kinds, like beech 

 and sugar maple, are characterized by compact and dense 

 crowns: their foliage even in the dark interior of the crown 

 can perform its office. And so a relative scale can be made 

 with reference to light requirements from the most light- 

 needing to the most shade-enduring. Birch, aspen, willow, 

 poplar, and pine are examples of those more sensitive to 

 the withdrawal of light; beech, sugar maple, holly, spruce, 



