8 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY, 
further supply of light is practically without effect. The only 
trees which are capable of responding satisfactorily to the new 
conditions are those of medium size. If, however, one thins the 
wood to such an extent that these medium-sized trees have the 
opportunity of making materially accelerated growth, one finds 
that, even with a light-felling only slightly in excess of a strong 
thinning, one soon reaches the limit where the gross production 
begins to decline in consequence of the reduction of the number 
of trees. The result is that one obtains a less aggregate yield of 
timber than would be got from a close wood, in spite of the fact 
that the growth of a certain number of individual trees has been 
stimulated. 
Besides the considerations affecting quantity and quality of 
timber, the question of thinning is intimately connected with the 
conservation of the soil fertility, on which the productive capacity 
of the ground and the maintenance of the yield so largely depend. 
We know from experience that if woods are over-thinned on light 
sands or on steep declivities, there is a danger of sand-drift in the 
one case, and of the fertile surface soil being washed away in 
the other. 
But apart from these cases, recent advances in our knowledge 
of the chemistry of soils have shown us that if woods are too 
thin, that is to say, if the surface of the ground is insufficiently 
shaded, or if the close leafy canopy is suddenly and strongly 
interrupted, the results may be most prejudicial to the productive 
capacity of the soil. On the other hand—apart from certain 
exceptional conditions, which may here be neglected—there is 
nothing to be feared in this connection from having woods too 
dense. 
Amongst the undesirable effects that follow insufficient stocking 
of the ground, I may mention the destruction of that porous or 
crumbly character of the soil which is so intimately associated 
with fertility, and the encouragement of the formation of sour raw 
humus. If the force of falling rain is not mitigated by a canopy 
of branches and a mantle of good humus on the surface of the 
ground, it will be found that the soluble plant-food is washed 
out of the upper strata of sandy soil, while in the case of a loam 
the porous or crumbly character of the soil is destroyed by the 
battering and consolidation to which it is subjected. In the 
case of calcareous ground, the soil undergoes very undesirable 
changes in structure if allowed to become too dry, as will 
