6 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
its future development. Consequently, where it is a question of 
providing more space for the trees that remain, such a thinning 
is absolutely useless. 
A moderate thinning is also without appreciable influence on 
the development of the trees that remain, and differs only from a 
weak thinning in enabling the forester to obtain a larger quantity 
of wood, which is also of somewhat higher quality. Such a 
degree of thinning is practised in all woods up to the end of the 
period during which height-growth is principally made. It is 
also practised right through the rotation in the case of woods of 
those species of trees—more especially the Scots fir — which 
naturally, and without any artificial assistance, lighten them- 
selves. This matter will be further dealt with later on, 
It is only when the degree of thinning is what is called 
“strong” that the operation has sufficient influence to produce a 
marked improvement in the development of the crowns of the 
remaining trees, and to induce more rapid growth. In Germany 
a strong thinning is only practised in woods where the stems 
average about 8 inches in diameter. At a later period diameter- 
growth, which is now synonymous with increase in value per 
cubic foot, may be encouraged by going even beyond a strong 
thinning ; that is to say, the wood may be “ lightened.” 
Finally, it may here be mentioned that some, especially the 
French, recommend a system that may be defined as the extension 
of the thinning to the dominant class of trees. Under this system, 
when a wood is about thirty years old, the most vigorously growing 
trees are given so much room that their crowns are practically 
isolated, and in this way the growth of such trees is encouraged 
to the greatest possible extent. Having secured this isolation in 
the dominant trees, the forester leaves the rest of the wood intact 
(beyond the removal of stems actually dead), and in this way he 
secures the cleaning of the stems of the dominant trees and the 
encouragement of their height-growth, and, at the same time, the 
ground is properly shaded. 
Until the forestal investigation stations had, within recent 
years, carried out careful researches into the growth of treeg, 
people held very indefinite ideas regarding the influence exerted 
by the different degrees of thinning and lightening, Now, how- 
ever, we possess definite information on the subject, and I shall, 
as shortly as possible, give a brief resumé of the results of recent 
research. 
