THE GROWTH OF SPRUCE. 39 
While the Spruce grows more rapidly and produces straighter and 
clearer timber in pure forests, the danger from windfall is much greater 
than where there is a scattering of Birch or other hardwood timber. 
The best results are obtained where the hardwoods form about one- 
quarter of the crop. 
As is explained on page 36, the reproduction of Spruce is better in 
pure than in mixed forests, because there are no seed trees of other 
species in the immediate neighborhood, and in consequence there is 
but little competition for the occupancy of the ground, and also because 
the soil cover is more favorable to the germination of Spruce seed. 
THE GROWTH OF SPRUCE. 
EFFECT OF THINNING. 
It has been found that when a crowded stand is thinned the trees 
which remain grow more rapidly than before. ‘This accelerated growth 
is caused, first, by the more rapid disintegration of the humus and 
the consequent liberation of an increased amount of available food 
material, and, second, by the increased spread and efficiency of the 
roots and crowns. 
In the Adirondacks, where the forest is dense and the climate cool, 
a deep layer of humus accumulates. When the forest is thinned the 
humus disintegrates more rapidly on account of the admission of the 
sun’s rays and the freer circulation of air, and an increased amount of 
food material is made available for the growth of the trees. The 
immediate increase in growth is probably due to this cause. How long 
it will last depends upon the length of time before the humus disap- 
pears. Professor Hartig, of Munich, estimates this period, under 
favorable conditions, at about 10 years. 
The effect of openings in the forest on vigorous and suppressed trees 
alike is to give them more room for development, a larger and better 
apparatus of roots and leaves for gathering and digesting food, and so 
to increase their rate of growth in diameter and height. 
The practice of thinning is based on this capacity for increased 
growth on the part of trees which have been more or less vigorously 
set free, or, in other words, on the part of the members of a piece of 
forest which has been thinned. The removal of a certain number of 
trees from overcrowded woods increases the final product, instead 
of decreasing it, and an additional product is obtained from the wood 
cut in the thinning. In this way the total output of a piece of forest 
in final cuttings and thinnings together is greater than it would be 
without sylvicultural attention. 
INCREASED GROWTH AFTER LUMBERING. 
In the Adirondacks the forest to be dealt with does not cousist of 
one species, but is a mixture of deciduous and coniferous trees of all - 
