SILVICS 5 



are stored. This surplus is greatest at the end of the growing 

 season, and upon its amount depends all growth of the next year 

 either of mother-plant or offspring in germinating seed. This 

 material is stored in the form of starch or sugars in the embryo 

 of the seed, in tubers, bulbs, roots, and medullary rays of the 

 wood. 



All trees require a certain amount of light in order to carry 

 out this work of assimilation. With regard to their ability to 

 bear shade, marked differences occur among the various species 

 of trees, so that they may be classed as: light-demanding or 

 intolerant trees, such as larch, red cedar, and gray birch; and 

 shade-enduring or tolerant species, such as beech, spruce, hem- 

 lock. Between these is a class of intermediate trees, including 

 white pine and chestnut. The demands of various species for 

 light may be gauged by the general density of the foliage of the 

 crown and the capacity of overshadowed twigs to retain life. 



Authorities 1 agree that we are far from a thorough under- 

 standing of the effect of light upon tree growth, but it has been 

 demonstrated that filtered light in the forest has little value as 

 a means of decomposition of carbon dioxid. The important 

 light in the forest is weakened white light. 2 



Until very recently it was thought by foresters that the amount 

 of light to which a tree has access determines its growth more 

 than any other factor. Recent investigations of soil moisture 

 indicate that this is an even more important factor, so that 

 while it was customary formerly to attribute increased growth, 

 after thinning a forest, to more light, it now appears that it is 

 due as much to the increased supply of water available for the 

 roots. 



The physical qualities of soil, especially as related to ability 

 to retain moisture, are more important than the chemical con- 

 stituents, for almost all soils are chemically able to bear trees. 

 The root systems of various species vary greatly with regard 



1 Light in Relation to Tree Growth, by R. Zon and H. S. Graves, U. S. Dept. 

 Agr. For. Ser. Bui. 92 (1911). 



2 G. P. Burns, Bui. 193, Vt. Agr. Exp. Sta. (1916). 



