THE IMPORTANCE OF DENSITY IN SYLVICULTURE. a 
happen if the direct rays of the sun are not effectively 
excluded. 
Simultaneously with these undesirable changes, the covering of 
the ground also alters for the worse. Sandy soils soon become 
covered by a dense turf of fine-leaved and shallow-rooted grasses, 
and this felted mass of roots not only exhausts the moisture in 
the soil, but also prevents the entrance of additional supplies in 
the form of rain-water. On more fertile soils the herbage which 
appears on the surface of the ground is composed of better species 
of plants, but in this case also the development of the trees is 
materially interfered with. 
Hardly anything exerts such a prejudicial influence upon the 
fertility of soil as the accumulation thereon of large quantities of 
undecomposed vegetable matter (raw humus). In a wood of 
normal density, raw humus is either not formed at all, or only to 
a very limited extent, and the soil-covering is loose and favour- 
able for tree-growth. 
If a wood is not sufficiently close, or if its canopy has been in- 
cautiously interrupted, the soil—especially where the situation is 
poor—generally becomes covered with raw humus. Simultane- 
ously with the formation of this material, heath usually appears 
on the ground, and this rapidly increases the stock of raw humus, 
and especially so under the unfavourable climatic conditions of a 
high rainfall and a low temperature, such as prevail in moun- 
tainous districts or near the coast. Under such circumstances the 
area may gradually be changed into a bog, as has happened in 
the case of the large boggy moors in North Germany, which were 
at one time covered by forests. 
In other cases the rain washes the organic acids out of the 
humus, and carries them into the upper strata of the mineral soil, 
where they render a large part of the plant-food soluble, and in 
this condition it is carried into the subsoil, and precipitated in 
the form of a moor-pan, a formation which is often met with on 
a large scale, and which is extremely prejudicial to all kinds of 
plant life. 
The principles that should guide forestal practice, so far as they 
are concerned with the maintenance of the factors of production 
in the soil, are thus intimately bound up in the preservation of a 
porous or crambly consistency in the soil, and the prevention of 
the formation of raw humus. 
Keeping in view the production of high-class timber and the 
