From the soil trees take mainly water, which enters through the roots
and is carried through the younger part of the tree to the leaves, to
be used in part on its passage for food and wood formation and in part
to be given up to the air by transpiration.
In a vigorously growing tree the solid wood substance itself will
contain half its weight in the form of water chemically combined, and
the tree, in addition, will contain from 40 to 65 per cent and more of
its dry weight in water mechanically or "hygroscopically" held. This
last, when the tree is cut, very largely evaporates; yet well-seasoned
wood still contains 10 to 12 per cent of such water. The weight of a
green tree, a pine, for instance, is made up, in round numbers, of
about 30 per cent of carbon and 70 per cent or water, either chemically
or hygroscopically held, while a birch contains a still larger
percentage of water.
The largest part of the water which passes through the tree is
transpired--i. e., given off to the air in vapor. The amounts thus
transpired during the season vary greatly with the species of tree,
its age, the amount of foliage at work, the amount of light at its
disposal, the climatic conditions (rain, temperature, winds, relative
humidity), and the season. These amounts are, however, very large when
compared with the quantity retained; so that while an acre of forest
may store in its trees, say, 1,000 pounds of carbon, 15 to 20 pounds
of mineral substances, and 5,000 pounds of water in a year, it will
have transpired--taken up from, the soil and returned to the air--from
500,000 to 1,500,000 pounds of water (one-quarter to one-half as much
as agricultural crops).
Mineral substances are taken up only in very small quantities, and
these are mostly the commoner sorts, such as lime, potash, magnesia,
and nitrogen. These are carried in solution to the leaves, where they
are used (as perhaps also on their passage through the tree), with a
part of the water, in food preparation. The main part of the mineral
substances taken up remains, however, as the water transpires, in the
leaves and young twigs, and is returned to the soil when the leaves are
shed or when the tree is cut and the brush left to decompose and make
humus.
Hence the improvement of the fertility of the soil by wood crops is
explained, the minerals being returned in more soluble form to the
soil; as also the fact that wood crops do not exhaust the soil of its
minerals, provided the leaves and litter are allowed to remain on the
ground.
For this reason there is no necessity of alternating wood crops, as far
as their mineral needs are concerned; the same kind of trees can be
grown on the same soil continuously, provided the soil is not allowed
to deteriorate from other causes.
As the foliage can perform its work of food assimilation only when
sufficient water is at its disposal, the amount of growth is also
dependent not only on the presence of sufficient sources of supply, but
also on the opportunity had by the roots to utilize the supply, and
this opportunity is dependent upon the condition of the soil. If the
soil is compact, so that the rain water can not penetrate readily, and
runs off superficially, or if it is of coarse grain and so deep that
the water rapidly sinks out of reach of the roots and can not be drawn
up by capillary action, the water supply is of no avail to the plants;
but if the soil is porous and moderately deep (depth being the distance
from the surface to the impenetrable subsoil, rock, or ground water)
the water not only can penetrate but also can readily be reached and
taken up by the roots.
The moisture of the soil being the most important element in it for
tree growth, the greatest attention must be given to its conservation
and most advantageous distribution through the soil.
No trees grow to the best advantage in very dry or very wet soil,
although some can live and almost thrive in such unfavorable
situations. A moderately but evenly moist soil, porous and deep enough
or fissured enough to be well drained, and yet of such a structure that
the water supplies from the depths can readily be drawn up and become
available to the roots--that is the soil on which all trees grow most
thriftily.
The agriculturist procures this condition of the soil as far as
possible by plowing, drainage, and irrigation, and he tries by
cultivating to keep the soil from compacting again, as it does under
the influence of the beating rain and of the drying out of the upper
layers by sun and wind.
The forest grower can not rely upon such methods, because they are
either too expensive or entirely impracticable. He may, indeed, plow
for his first planting, and cultivate the young trees, but in a few
years this last operation will become impossible and the effects of the
first operation will be lost. He must, therefore, attain his object in
another manner, namely, by shading and mulching the soil. The shading
is done at first by planting very closely, so that the ground may be
protected as soon as possible from sun and wind, and by maintaining the
shade well throughout the period of growth. This shade is maintained,
if necessary, by more planting, and in case the main crop in later life
thins out inordinately in the crowns or tops, or by the accidental
death of trees, it may even become desirable to introduce an underbrush.
The mulching is done by allowing the fallen leaves and twigs to remain
and decay, and form a cover of rich mold or humus. This protective
cover permits the rain and snow waters to penetrate without at the same
time compacting the soil, keeping it granular and in best condition for
conducting water, and at the same time preventing evaporation at the
surface.
The soil moisture, therefore, is best maintained by proper soil cover,
which, however, is needful only in naturally dry soils. Wet soils,
although supporting tree growth, do not, if constantly wet, produce
satisfactory wood crops, the growth being very slow. Hence they must be
drained and their water level sunk below the depth of the root system.
Irrigation is generally too expensive to be applied to wood crops,
except perhaps in the arid regions, where the benefit of the shelter
belt may warrant the expense.
Attention to favorable moisture conditions in the soil requires the
selection of such kinds of trees as shade well for a long time, to
plant closely, to protect the woody undergrowth (but not weeds), and to
leave the litter on the ground as a mulch.
Different species, to be sure, adapt themselves to different degrees of
soil moisture, and the crop should therefore be selected with reference
to its adaptation to available moisture supplies.
While, as stated, all trees thrive best with a moderate and even supply
of moisture, some can get along with very little, like the conifers,
especially pines; others can exist even with an excessive supply, as
the bald cypress, honey locust, some oaks, etc. The climate, however,
must also be considered in this connection, for a tree species,
although succeeding well enough on a dry soil in an atmosphere which
does not require much transpiration, may not do so in a drier climate
on the same soil.
In the selection of different kinds of trees for different soils, the
water conditions of the soil should, therefore, determine the choice.