THE FORMATION OF FORESTS 97 
opening the natural ditches which drain it. In the 
north woods the small streams are used in winter for 
log-roads. They are filled with corduroy, brush, and 
other débris to such extent that their flow is inter- 
rupted. Many acres may be drained by a few hours’ 
work in removing débris. When ditches are dug for 
the purpose of drainage they should be left open. 
Underdrains soon become clogged with roots and 
are soon, of course, utterly useless. After the 
trees are once well established, they will help 
drain the land. ‘Trees are often planted for this 
purpose. 
In many places where there is an excessive 
amount of vegetable matter, such as peat or duff, 
burning may be a benefit. Plants must have some 
mineral soil. On the coast of Alaska, for instance, 
this material is often many feet thick. It must be 
removed before a crop of anything but moss and 
bushes can be produced. There are vast accumula- 
tions of this sort of material in northern Europe in 
the form of deep peat or “ torf ” beds. 
In the case of shifting sand it may be necessary 
to build brush fences or wattles to check the force 
of the wind and cut off the supply of sand. Brush 
may be piled upon the sand to hold it in place, and to 
protect the young plants from the sun and wind-blast. 
In some places it may be necessary to plant shel- 
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