The care of the forester, then, should be to maintain the smallest
number of individuals on the ground which will secure the greatest
amount of wood growth in the most desirable form of which the soil and
climate are capable, without deteriorating the soil conditions. He
tries to secure the most advantageous individual development of single
trees without suffering the disadvantages resulting from too open
stand. The solution of this problem requires the greatest skill and
judgment, and rules can hardly be formulated with precision, since for
every species or combination of species and conditions these rules must
be modified.
In a well-established young crop the number of seedlings per acre
varies greatly, from 3,000 to 100,000, according to soil, species, and
the manner in which it originated, whether planted, sown, or seeded
naturally.[3] Left to themselves, the seedlings, as they develop, begin
to crowd each other. At first this crowding results only in increasing
the height growth and in preventing the spread and full development
of side branches; by and by the lower branches failing to receive
sufficient light finally die and break off--the shaft "clears itself."
Then a distinct development of definite crowns takes place, and after
some years a difference of height growth in different individuals
becomes marked. Not a few trees fail to reach the general upper crown
surface, and, being more or less overtopped, we can readily classify
them according to height and development of crown, the superior or
"dominating" ones growing more and more vigorously, the inferior or
"dominated" trees falling more and more behind, and finally dying for
lack of light, and thus a natural reduction in numbers, or thinning,
takes place. This natural thinning goes on with varying rates at
different ages continuing through the entire life of the crop, so
that, while only 4,000 trees per acre may be required in the tenth
year to make a dense crown cover or normally close stand, untouched by
man, in the fortieth year 1,200 would suffice to make the same dense
cover, in the eightieth year 350 would be a full stand, and in the one
hundredth not more than 250, according to soil and species, more or
less. As we can discern three stages in the development of a single
tree--the juvenile, adolescent, and mature--so, in the development
of a forest growth, we may distinguish three corresponding stages,
namely, the "thicket" or brushwood, the "pole-wood" or sapling, and
the "timber" stage. During The thicket stage, in which the trees have
a bushy appearance, allowing hardly any distinction of stem and crown,
the height growth is most rapid. This period may last, according to
conditions and species, from 5 or 10 to 30 and even 40 years--longer on
poor soils and with shade-enduring species, shorter with light-needing
species on good soils--and, while it lasts, it is in the interest of
the wood grower to maintain the close stand, which produces the long
shaft, clear of branches, on which at a later period the wood that
makes valuable, clear timber, may accumulate. Form development is now
most important. The lower branches are to die and break off before they
become too large. (See illustrations of the progress of "clearing,"
on pp. 15 and 16.) With light-needing species and with deciduous
trees generally this dying off is accomplished more easily than with
conifers. The spruces and even the white pine require very dense
shading to "clear" the shaft. During this period it is only necessary
to weed out the undesirable kinds, such as trees infested by insect
and fungus, shrubs, sickly, stunted, or bushy trees which are apt to
overtop and prevent the development of their better neighbors. In
short, our attention is now devoted mainly to improving the composition
of the crop.
[3] If the crop does not, at 3 to 5 years of age, shade the ground
well, with a complete crown cover, or canopy, it can not be said to be
well established and should be filled out by planting.