I have spoken of trees as the purifiers and renovators of the
atmosphere, as regulators of its humidity, equalizers of the electric
fluid, and as safeguards against both drought and inundations; but I
have not yet alluded to the fact that they are, in dense assemblages,
the actual creators, in many places, of the soil upon which they stand.
The trees by means of their foliage are direct fertilizers of the ground
they cover, causing it to increase in bulk as long as they stand upon
it. But the leaves of trees are not the only source of this increase of
bulk and fertility. The lichens and mosses, and various incrustations
upon their bark, and the offal of birds, insects, and quadrupeds, all
contribute to the same end. Hence the most barren situations will
produce good crops for several years after the removal of their wood;
and from these facts we may learn why a forest is still vigorous, though
it has remained for centuries upon the same ground. If it were
fertilized only by the decayed foliage of the trees, it would gradually
lose its fitness to promote the health and growth of the timber. But the
foreign matters I have enumerated, the decayed cryptogamous plants, and
the relics and deposits of animals which have lived and died there,
supply the soil with nitrogenous ingredients in which decomposed leaves
are wanting.
But what are the sources of all the matters which are furnished by the
trees alone? They are chiefly the atmosphere and the deeper strata of
the soil. The roots of the trees, penetrating to a considerable depth,
draw up from the subsoil certain nutritive salts that enter into the
substance of all parts of the tree. This is restored to the surface by
every tree or branch that falls and moulders upon it, and the leaves
increase its bulk still more by their annual decay. According to
Vaupell, “the carbonic acid given out by decaying leaves, when taken up
by water, serves to dissolve the mineral constituents of the soil, and
it is particularly active in disintegrating feldspar and the clay
derived from its decomposition.” These facts explain why the surface
soil in a forest may constantly increase in bulk, without communication
with any foreign sources of supply.
If a wood be situated in a valley or on a level plain, it retains all
these substances for its own benefit. But if it stand upon a declivity,
a part of the _débris_ will be washed down by floods into the fields
below. Hence, by preserving a growth of wood upon all barren slopes and
elevations, the farmer derives benefit from it, both as a fertilizer and
as a source of irrigation to the lower part of the slopes or the base of
the hill. For some days after a rain, thousands of little rills are
constantly oozing from the spongy bed of the wood, that cannot
immediately become dry like an open surface. Hills, when either very
barren or steep, are unprofitable alike for tillage or pasture. They
require more manure than other grounds, and more labor in its
distribution. Hence, if divested of wood, as I have often repeated, they
are almost useless; while, if densely wooded, they fertilize and
irrigate the lands below, protect them from winds, and afford a certain
annual amount of fuel.
When I am journeying through the country and behold the rocky hills,
sometimes for miles in extent, entirely bare of trees, and affording too
little sustenance to support even a crop of whortleberry-bushes, where
an acre would hardly pasture a single sheep, I am informed by the older
inhabitants that these barren fields were since their childhood covered
with forest. This wood cannot be restored, because the soil has been
washed down from the surface into the plains below, and nothing remains
to support a new growth of trees. And then I think, if our predecessors,
instead of wrangling about theology, had left its mysteries to be
explained by their pastors, and studied some of the plain laws of
meteorology, this devastation had not taken place!
If these rising grounds, like most of the hills in New England, have a
granite foundation and a comparatively barren surface soil, forests are
the only means which can be used by nature to render them productive or
useful in any way to the prosperity of agriculture. Were they stripped
of trees, they could not long maintain their original fertility; for
there is nothing to prevent the soil from washing down their sides,
nothing to prevent inundations from copious rains, nothing to prevent
their becoming rapidly parched by drought during a great part of every
summer. Hence a mountain that is covered with a dense forest, how thin
and meagre soever the soil may be from which the trees derive their
support, is a source of perpetual fertilization to the lands below.
Millions of living creatures, which are harbored in these woods,
annually perish, leaving their remains upon the ground to fertilize it
and increase its bulk. During their lifetime also, besides various
substances which they have manipulated, they are constantly leaving
deposits of many kinds upon the surface; and if the quantity thus spread
upon a single acre of woodland could be measured, we should be
astonished at the amount.
By means of forests, therefore, in favorable situations, a farmer
obtains something apparently out of nothing, and makes the barren rocks
and hills the sources of a part of the substances with which he
fertilizes his grounds.
But I have said nothing of the pasturage afforded to cattle on the
borders of woods. Out of every two or three tons of leaves which are
cast upon the ground, a hundredweight at least is but a solidification
of the gases of the atmosphere. All this would be lost to the farmer, if
the upper parts of his barren elevations and the sides of his steep
declivities were despoiled of their wood and shrubbery. Without this
forest, tons of compost produced by the annual decay of leaves would
never have been created. All that proceeds from living creatures would
also be lost, because they would either have never come into existence,
or they would have lived and died in another place and benefited some
other region.