32 FARM FORESTRY 



The decomposition of the leaves and twigs is brought about 

 by bacteria and fungi. Air, moisture and warmth in certain 

 amounts are essential for their action. Where one or more 

 of these factors are not favorable to the decomposition of the 

 leaves and twigs, humus will accumulate rapidly. In the cool 

 north woods under a dense canopy, the humus or duff is 

 often found over a foot in thickness. It accumulates rapidly 

 in cold, damp situations as on a northern slope, in a ravine 

 or beneath the dense shade of the forest. It does not accu- 

 mulate in an open stand of trees where the sun has a chance 

 to warm and dry the soil, bringing about conditions favorable 

 for the growth of the fungi and bacteria. It is the decompo- 

 sition of the leaves and twigs and the formation of the black, 

 powdery humus and the setting free of soluble salts that is 

 of advantage to trees not the accumulation of the partially 

 decayed leaves in thick layers. Too thick an accumulation of 

 humus tends to make the soil sour because of the vegetable 

 acids set free. Opening up the forest canopy by the removal 

 of a few trees, allowing the sunlight to reach the floor, will 

 bring about a rapid reduction of the humus. 



The Region of Root Development. This comprises the 

 soil and the subsoil and the root systems of the trees. The 

 subsoil is of more importance in forestry than in agriculture. 

 It often determines the kind of tree that will grow. There 

 is scarcely any soil that will not support some kind of tree 

 growth. The depth and character are of great importance. 

 Where the soil is not of sufficient depth to allow the develop- 

 ment of the characteristic root form of trees, the growth of 

 the trees will suffer. This is especially true where trees that 

 naturally produce a tap root are grown on shallow soil. The 

 roots of a tree extend out in all directions as far as or farther 

 than the branches. In a woodlot the root systems of the differ- 

 ent trees interlace, until the soil and often the subsoil down to 

 the water table is completely filled with roots. Trees growing 

 too closely together constantly compete with each other for 



