SYLVICULTURE 29 



Between light-requiring and shade-enduring species the 

 transition is by no means sharply defined. The one class 

 gradually merges into the other. To the light-demanders 

 belong Larch, Birch, Ash, Scots Pine, and Oak ; to the 

 shade-bearers, Spruce, Hornbeam, Beech, and Silver Fir. 

 Taking only the species that are sylviculturally most im- 

 portant, the arrangement according to their capacity for 

 bearing shade is in the following order : Silver Fir, Beech, 

 Spruce, Oak, Scots Pine. 



Pure Woods and Mixed Woods 



A wood consisting of one species, with, perhaps, a few 

 stray specimens of other trees, is known as a pure wood. On 

 the other hand, when the crop is composed of several species 

 usually two, but frequently more it is said to be a mixed 

 wood. In mixed woods, the constituent species may be 

 uniformly distributed by means of single trees, or the arrange- 

 ment can be by lines, strips, groups, etc. Seldom are the 

 species found in equal numbers, one of them being usually 

 more or less predominant. The species composing a mixed 

 wood may all be planted simultaneously, or one or more may 

 be added at a subsequent period. A primary distinction, 

 then, is made between even-aged and uneven-aged mixtures. 

 Again, a mixture is not necessarily permanent. Frequently, 

 certain of the component species have only a temporary pur- 

 pose to fulfil. This may be either as shelter for the other 

 members in the mixture, or for soil protection. The former 

 is much employed where delicate species are raised upon 

 bare situations. In cases where natural shelter is absent on 

 exposed places, one may first plant a hardy species, such as 

 Scots Pine, Larch, or Birch, and under the protection thus 

 afforded, the permanent crop can be raised. The purp'ose 

 which the species first introduced fulfils is that of nursing the 

 young crop. As soon as this function is accomplished, or 

 the progress of the main crop is in any way threatened, 

 these " nurses " must be removed, 



