FOREST PLANTING IN THE EASTERN UNITED STATES. 15 
~The presence of dead or dying trees in the stand, a very dense 
crown cover, or an apparent stagnation in the growth of the hying 
trees indicates that a thinning is needed. The usual practice is to 
thin when the product is of sufficient size to pay for the operation 
and to repeat the process thereafter as often as the material has 
accumulated in sufficient quantity to again pay for the cost. Many 
plantations, however, need their first thinning before they reach this 
state. Silver maple, black locust, and other species have a decided 
tendency to grow toward openings in the crown canopy, and in 
their efforts to reach these the trunks become crooked. Under such 
conditions a thinning should be made whether the operation will pay 
for itself or not. The first thinning may be needed by the time the 
stand is 10 years old. 
As a rule, trees of the least potential value should be the ones 
removed in a thinning. In the early life of a stand the trees range 
themselves into several crown classes—dominant, codominant, in- 
termediate, suppressed, and dead. The dominant trees are the 
tallest ones, whose crowns receive almost complete sunlight; co- 
dominant trees are those of slightly less height, with relatively narrow 
crowns which are not fully exposed to sunlight; intermediate trees 
are considerably smaller than those of the first two classes, but 
still healthy, because their crowns continue to occupy open spaces 
in the canopy; suppressed trees are those hopelessly behind in height 
growth, and which will eventually be killed by the shade of the other 
trees. The trees which remain after a thinning should, as a rule, 
be those which are most vigorous, of the best form, and presumably 
of the highest final market value. This does not mean that no 
codominant or dominant trees should ever be cut, or that no interme- 
diate and suppressed trees be allowed to remain. High-grade trees 
must sometimes be cut to obtain the proper opening of the crown 
canopy, and inferior trees may serve the useful purpose of shading 
the soil, thus tending to retard evaporation and prevent the growth 
of harmful vegetation on the forest floor. Except where needed for 
soil shading, however, suppressed and intermediate trees should 
generally be thinned in preference to the larger trees of the first two 
classes. When it can be done cheaply dead trees should be removed 
in order to rid the stand of material likely to increase the danger 
irom fire. 
The extent to which the crown of a stand may be opened depends 
largely upon the rate of growth of the species and their demand for 
light. In general, openings should not be so large that they will not 
close again within from three to five years by the growth of the remain- 
ing crowns. Rapid-growing trees, such as cottonwood or silver maple, 
should have their crowns opened to a much greater extent than 
