THE FOREST ORGANISM.
The forest is much more than an assemblage of different trees, it is
an organism; that is, the trees that compose it have a vital relation
to each other. It may almost be said to have a life of its own, since
it has a soil and a climate, largely of its own making.
Without these conditions, and without the help and hindrance which
forest trees give to each other, these trees would not have their
present characteristics, either in shape, habits of growth or nature
of wood grain. Indeed, some of them could not live at all.
Since by far the greater number of timber trees grow in the forest, in
order to understand the facts about trees and woods, it is necessary
to know something about the conditions of forest life.
A tree is made up of three distinct parts: (1) the roots which anchor
it in the ground, and draw its nourishment from the moist soil; (2)
the trunk, or bole, or stem, which carries the weight of the branches
and leaves, and conveys the nourishment to and from the leaves; (3)
the crown, composed of the leaves, the branches on which they hang,
and the buds at the ends of the branches. As trees stand together in
the forest, their united crowns make a sort of canopy or cover, Fig.
55, which, more than anything, determines the factors affecting
forest life, viz., the soil, the temperature, the moisture, and most
important of all, the light.
[Illustration: Fig. 55. The Forest Cover. Spruce Forest, Bavaria,
Germany. _U. S. Forest Service._]
On the other hand, every species of tree has its own requirements
in respect to these very factors of temperature,--moisture, soil and
light. These are called its _silvical characteristics_.
SOIL.
Some trees, as black walnut, flourish on good soil, supplanting others
because they are better able to make use of the richness of the soil;
while some trees occupy poor soil because they alone are able to live
there at all. Spruce, Fig. 56, will grow in the north woods on such
poor soil that it has no competitors, and birches, too, will grow
anywhere in the north woods. In general, it is true that mixed
forests, Fig. 57, _i.e._, those having a variety of species, grow on
good loamy soil. The great central, deciduous Atlantic Forest grew on
such soil until it was removed to make room for farms. On the other
hand, pure stands--_i.e._, forests made up of single varieties--of
pine occupy poor sandy soil. Within a distance of a few yards in the
midst of a pure stand of pine in the south, a change in the soil will
produce a dense mixed growth of broad-leaves and conifers.
[Illustration: Fig. 56. Virgin Stand of Red Spruce. White Mountains,
New Hampshire. _U. S. Forest Service._]
[Illustration: Fig. 57. Typical Mixed Forest,--Red Spruce, Hemlock,
White Ash, Yellow Birch, Balsam Fir, and Red Maple. Raquette Lake, New
York. _U. S. Forest Service._]
The soil in the forest is largely determined by the forest itself. In
addition to the earth, it is composed of the fallen and decayed leaves
and twigs and tree trunks, altogether called the _forest floor_. It is
spongy and hence has the ability to retain moisture, a fact of great
importance to the forest.
MOISTURE.
Some trees, as black ash and cypress, Fig. 58, and cotton gum, Fig.
59, grow naturally only in moist places; some, as the pinon and
mesquite, a kind of locust, grow only in dry places; while others,
as the juniper and Douglas fir, adapt themselves to either. Both
excessively wet and dry soils tend to diminish the number of kinds
of trees. In many instances the demand for water controls the
distribution altogether. In the Puget Sound region, where there is a
heavy rain-fall, the densest forests in the world are found, whereas
on the eastern slopes of the same mountains, altho the soil is not
essentially different, there are very few trees, because of the
constant drouth.
[Illustration: Fig. 58. Cypress and Cypress "Knees." Jasper Co.,
Texas. _U. S. Forest Service._]
[Illustration: Fig. 59. Cotton Gums, Showing Buttresses. St. Francis
River, Arkansas. _U. S. Forest Service._]
TEMPERATURE.
The fact that some trees, as paper birch and white spruce, grow only
in cold regions, and some, as rubber trees and cypress, only in the
tropics, is commonplace; but a fact not so well known is that it is
not the average temperature, but the extremes which largely determine
the habitat of trees of different kinds. Trees which would not live
at all where there is frost, might flourish well in a region where
the average temperature was considerably lower. On the other hand,
provided the growing season is long enough for the species, there is
no place on earth too cold for trees to live. Fig. 60.
[Illustration: Fig. 60. Northern Forest,--Young Spruce Growing Under
Yellow Birch. Santa Clara, New York. _U. S. Forest Service._]
In general, cold affects the forest just as poor soil and drought do,
simplifying its composition and stunting its growth. In Canada there
are only a few kinds of trees, of which the hardwoods are stunted;
south of the Great Lakes, there is a great variety of large trees;
farther south in the southern Appalachian region, there is a still
greater variety, and the trees are just as large; and still farther
south in tropical Florida, there is the greatest variety of all. The
slopes of a high mountain furnish an illustration of the effect of
temperature. In ascending it, one may pass from a tropical forest at
the base, thru a belt of evergreen, broad-leaved trees, then thru a
belt of deciduous broad-leaved trees, then thru a belt of conifers and
up to the timber line where tree life ceases. Figs. 61, and 62.
[Illustration: Fig. 61. Mixed Hardwoods on Lower Levels. Spruce
and Balsam Dominate on Higher Elevations. Mt. McIntyre, Adirondack
Mountains, New York. _U. S. Forest Service._]
[Illustration: Fig. 62. Scrub Growth on Mountain Top. Mt. Webster, New
Hampshire. _U. S. Forest Service._]
LIGHT.
More than by any other factor, the growth of trees in a forest is
determined by the effect of light. All trees need light sooner or
later, but some trees have much more ability than others to grow
in the shade when young. Such trees, of which maple and spruce are
examples, are called _tolerant_, while others, for instance, larch,
which will endure only a comparatively thin cover or none at all, are
called _intolerant_. The leaves of tolerant trees endure shade well,
so that their inner and lower leaves flourish under the shadow of
their upper and outer leaves, with the result that the whole tree,
as beech and maple, makes a dense shadow; whereas the leaves of
intolerant trees are either sparse, as in the larch, or are so hung
that the light sifts thru them, as in poplar and oak. The spruces and
balsam fir have the remarkable power of growing slowly under heavy
shade for many years, and then of growing vigorously when the light is
let in by the fall of their overshadowing neighbors. This can plainly
be seen in the cross-section of balsam fir, Fig. 63, where the narrow
annual rings of the early growth, are followed by the wider ones of
later growth. A common sight in the dense woods is the maple sending
up a long, spindly stem thru the trees about it and having at its top
a little tuft of leaves, Fig. 64. By so doing it survives. The fact
that a tree can grow without shade often determines its possession
of a burnt-over tract. The order in the North Woods after a fire
is commonly, first, a growth of fire weed, then raspberries or
blackberries, then aspen, a very intolerant tree whose light shade
in turn permits under it the growth of the spruce, to which it is a
"nurse," Fig. 65. In general it may be said that all seedling conifers
require some shade the first two years, while hardwoods in temperate
climates, as a rule, do not.
[Illustration: Fig. 63. Cross section of Balsam Fir, Showing Fast
Growth After Years of Suppression. Notice the width of the annual
rings in later age compared with early. _U. S. Forest Service._]
[Illustration: Fig. 64. Tolerant Maple. The trees are too slender to
stand alone. _U. S. Forest Service._]
[Illustration: Fig. 65. Intolerant Aspen, a "nurse" of Tolerant
Spruce._ U. S. Forest Service._]
This matter of tolerance has also much to do with the branching of
trees. The leaves on the lower branches of an intolerant tree will not
thrive, with the result that those branches die and later drop off.
This is called "cleaning," or natural pruning. Intolerant trees, like
aspen and tulip, Fig. 66, clean themselves well and hence grow with
long, straight boles, while tolerant trees, like spruce and fir,
retain their branches longer.
[Illustration: Fig. 66. Intolerant Tulip. Notice the long, straight
boles. _U. S. Forest Service._]
The distribution of a species may also be determined by geographical
barriers, like mountain ranges and oceans. This is why the western
forests differ radically from the eastern forests and why the forest
of Australasia is sharply distinct from any other forest in the world.
Any one or several of these factors, soil, moisture, heat, and light,
may be the determining factor in the make-up of a forest, or it may
be that a particular tree may survive, because of a faster rate of
growth, thus enabling it to overtop its fellows and cut off their
light. The struggle for survival is constant, and that tree survives
which can take the best advantage of the existent conditions.
Besides these topographical and climatic factors which help determine
the distribution of trees, a very important factor is the historical
one. For example, the only reason by which the location of the few
isolated groves of big trees in California can be accounted for is the
rise and fall of glacial sheets, which left them, as it were, islands
stranded in a sea of ice. As the glaciers retreated, the region
gradually became re-forested, those trees coming up first which were
best able to take advantage of the conditions, whether due to the
character of their seeds, their tolerance, their endurance of moisture
or whatever. This process is still going on and hardwoods are probably
gaining ground.
Besides these external factors which determine the composition and
organic life of the forest, the trees themselves furnish an important
factor in their methods of reproduction. These, in general, are two,
(1) by sprouts, and (2) by seeds.
(1) Most conifers have no power of sprouting. The chief exceptions
are pitch pine and, to a remarkable degree, the redwood, Fig. 67. This
power, however, is common in broad-leaved trees, as may be seen after
a fire has swept thru second growth, hardwood timber. Altho all the
young trees are killed down to the ground, the young sprouts spring
up from the still living roots. This may happen repeatedly. Coppice
woods, as of chestnut and oak, which sprout with great freedom, are
the result of this ability. The wood is poor so that it is chiefly
used for fuel.
[Illustration: Fig. 67. Sprouting Redwood Stumps. Glen Blair, Calif.
_U. S. Forest Service._]
(2) Most trees, however, are reproduced by seeds. Trees yield these
in great abundance, to provide for waste,--nature's method. Many seeds
never ripen, many perish, many are eaten by animals, many fall on
barren ground or rocks, and many sprout, only to die. The weight of
seeds has much to do with their distribution. Heavy seeds like acorns,
chestnuts, hickory and other nuts, grow where they fall, unless
carried down hill by gravity or by water, or scattered by birds and
squirrels.
Trees with winged seeds, however, Fig. 68, as bass, maple and pine,
or with light seeds, as poplar, often have their seeds carried by the
wind to great distances.
[Illustration: Fig. 68. Winged Seeds. 1, Basswood; 2, Box-elder; 3,
Elm; 4, Fir; 5, 6, 7, 8, Pines. _U. S. Forest Service._]
Again some trees, as spruce, are very fertile, while others, like
beech, have only occasional seed-bearing seasons, once in three or
four years. Willow seeds lose their power of germination in a few
days, and hence, unless they soon reach ground where there is plenty
of moisture, they die. This is why they grow mostly along water
courses. On the other hand, black locust pods and the cones of some
pines keep their seeds perfect for many years, often until a
fire bursts them open, and so they live at the expense of their
competitors.
It is such facts as these that help to account for some of the acts of
forest composition,--why in one place at one time there is a growth of
aspens, at another time pines, at still another oaks; and why beeches
spring up one year and not another. That red cedars grow in avenues
along fences, is explained by the fact that the seeds are dropped
there by birds, Fig. 69.
[Illustration: Fig. 69. Red Cedar Avenue. Seeds dropped by birds which
perched on the fences. Indiana. _U. S. Forest Service._]
The fact that conifers, as the longleaf pine, Fig. 46, p. 200, and
spruce, Fig. 55, p. 212, are more apt to grow in pure stands than
broad-leaved trees, is largely accounted for by their winged seeds;
whereas the broad-leaved trees grow mostly in mixed stands because
their heavy seeds are not plentifully and widely scattered. This is
a rule not without exceptions, for beech sometimes covers a whole
mountain side, as Slide Mountain in the Catskills, and aspens come in
over a wide area after a fire; but later other trees creep in until at
length it becomes a mixed forest.
The essential facts of the relation of trees to each other in the
forest has been clearly stated by Gifford Pinchot thus:[1]
The history of the life of a forest is a story of the help and
harm which trees receive from one another. On one side every
tree is engaged in a relentless struggle against its neighbors
for light, water and food, the three things trees need most.
On the other side each tree is constantly working with all its
neighbors, even those which stand at some distance, to bring
about the best condition of the soil and air for the growth
and fighting power of every other tree.
The trees in a forest help each other by enriching the soil in which
they stand with their fallen leaves and twigs, which are not quickly
blown or washed away as are those under a tree in the open. This
collection of "duff" or "the forest floor" retains the moisture about
their roots, and this moist mass tends to keep the temperature of the
forest warmer in winter and cooler in summer. The forest cover, Fig.
55, p. 212, consisting largely of foliage, has the same effect, and in
addition protects the bark, the roots, and the seedlings of the trees
from the direct and continuous hot rays of the sun. Without the shade
of the leaves, many trees, as white pine, would quickly die, as
may readily be seen by transplanting them to the open. The mass of
standing trees tempers the force of the wind, which might overthrow
some of them, and hinders the drying up of the duff.
[Illustration: Fig. 70. Shallow Roots of Hemlock. Bronx Park, New
York, N. Y.]
But trees hinder as well as help each other. There is a constant
struggle between them for nourishment and light. To get food and
water, some trees, as spruces and hemlocks, Fig. 70, spread their
roots out flat; others, as oak and pine, send down a deep tap root.
Those succeed in any environment that find the nourishment they need.
Still more evident is the struggle for light and air. However well a
tree is nourished thru its roots, unless its leaves have an abundance
of light and air it will not thrive and make wood.
[Illustration: Fig. 71. Long-bodied White Oak of the Forest. _U. S.
Forest Service._]
Even the trees most tolerant of shade in youth, like spruce, must have
light later or perish, and hence in a forest there is the constant
upward reach. This produces the characteristic "long-bodied" trunk of
the forest tree, Fig. 71, in contrast to the "short-bodied" tree of
the open, where the branches reach out in all directions, Fig. 72.
In this constant struggle for existence is involved the persistent
attempt of scattered seeds to sprout whenever there is an opening. The
result is that a typical forest is one in which all sizes and ages
of trees grow together. Scattered among these are bushes and scrubby
trees, called "forest weeds," such as mountain maple and dogwood, Fig.
80, p. 234, which do not produce timber.
[Illustration: Fig. 72. Short-bodied White Oak of the Open. Fort Lee,
N. J.]
By foresters the trees themselves are classified according to their
size into:
Seedlings, less than 3' high,
Saplings,
Small, 3'-10' high.
Large, 4" in diameter, at breast height (4' 6").
Poles,
Small, 4"-8" in diameter, at breast height.
Large, 8"-12" in diameter, at breast height.
Standards, 1'-2' in diameter, at breast height.
Veterans, over 2' in diameter, at breast height.
Every age has its own dangers. Many seeds never germinate, many
seedlings perish because they do not reach soil, or are killed by
too much or too little moisture, or by heat or cold, or shade. At the
sapling age, the side branches begin to interfere with those of other
saplings. Buds are bruised and lower branches broken by thrashing in
the wind, and their leaves have less light. Only the upper branches
have room and light, and they flourish at the expense of lower ones,
which gradually die and are thus pruned off. Some trees naturally
grow faster than others, and they attain additional light and room
to spread laterally, thus overtopping others which are suppressed and
finally killed, beaten in the race for life.
If the growth should remain about even so that the trees grew densely
packed together, the whole group would be likely to be of a poorer
quality, but ordinarily the few outgrow the many and they are called
dominant trees. Even then, they still have to struggle against their
neighbors, and at this, the large sapling stage, many perish, and of
those that survive there are great differences in size. Trees make
their most rapid growth in height, and lay on the widest yearly
"rings," at the large sapling and small pole age, Fig. 114, p. 263. It
is at this stage, too, if the growth is at all dense, that the young
trees (poles) clean themselves most thoroly of their branches. The
growth in diameter continues to the end of the tree's life, long after
the height growth has ceased.
When trees become "standards," and reach the limit of height growth,
thru their inability to raise water to their tops, their branches
must perforce grow sidewise, or not at all. The struggle for life thus
takes a new form.
How trees are able to raise water as high as they do is still
unexplained, but we know that the chief reason why some trees grow
taller than others, is due to their ability to raise water. The most
remarkable in this respect are the California redwoods, the big trees,
and certain eucalypts in Australia. This inability of trees to grow
above a certain height results in a flattening of the crown, Fig. 73,
and at this stage, the trees struggle against each other by crowding
at the side.
[Illustration: Fig. 73. Flattened Crown of Red Pine. _U.S. Forest
Service._]
Inasmuch as trees grow more sensitive to shade with advancing age, the
taller trees have the advantage. Each survivor is one of a thousand,
and has outlived the others because it is best fitted for the place.
This fact has its effect upon the next generation, because it is these
dominant surviving trees which bear seed most abundantly. After the
tree has finished growing in height and diameter most vigorously--the
pole stage--and proved to be fitted for the place, its energy is
largely spent in raising seed. As this process goes on generation
after generation, only the best coming to maturity in each, the poorer
sorts are sifted out, and each region and continent has those species
best fitted to meet the conditions of life there.
This is the reason why exotics are very likely to be sensitive and
perhaps succumb to influences to which native trees are immune.
Standards and veterans are the survivors of all the lower stages,
each of which has had its especial dangers. If left alone, the tree
gradually dies and at last falls and decays, adding somewhat to the
fertility of the forest soil. From the point of view of human use, it
would far better have been cut when ripe and turned into lumber. It
is a mistake to suppose that the natural virgin forest is the best
possible forest, and that it should therefore be left alone. In the
National Forests the ripe lumber is sold and a considerable revenue
is thus available. But nature's way with the dead tree is to use it
to produce more life. How she does so will be explained in the next
chapter, on the enemies of the forest.
[Footnote 1: Gifford Pinchot, _Primer of Forestry_, p. 44.]
THE FOREST ORGANISM.
REFERENCES:[A]
Pinchot, _For. Bull._ No. 24, I, pp. 25-66.
Bruncken, pp. 13-31.
_For. Circ._ No. 36, p. 8.
Fernow, _Economics_, pp. 140-164.
[Footnote A: For general bibliography, see p. 4]