THINNING PLANTATIONS
In the successful rearing of timber trees for profit there is, perhaps,
no other branch of more importance than a good knowledge of the art of
thinning, and, at the same time, one on which so great a diversity of
opinion exists.
Thinning plantations, be they old or young, must always be subject to
great modification, according to the nature of the trees and soil, or
ultimate design of the plantation being operated upon, and is a matter
requiring great tact, forethought, and discrimination, and this can
only be acquired by long experience and by those having a good insight
into the peculiarities and properties of our forest trees.
The same rule as regards thinning will not apply to, say, fir,
hardwood, and mixed plantations; and far less will it hold good in the
case of an ornamental and a profitable wood. No universal rule can,
however, be laid down for thinning, but general principles can be given
that will be sufficient for the guidance of those who have to undertake
such work.
In thinning any plantation two important points are to be borne in
mind—first, cut away all diseased, dead and dying trees; second,
study the relationship of trees and soil, and act accordingly. To be
more explicit, we might say that in the first case, the removal of
all dead and dying trees is a necessity, and that being done, one can
proceed with the disposal of the standards to be left. In the second
case, by suiting the trees to the soil, at least as far as possible,
great benefit results, and an instance of this that came under our
own observation lately will be given as an example. A plantation
130 acres in extent was composed of oak, larch and Spanish chestnut,
placed at regular distances apart throughout the whole extent of the
wood. Now, the soil was gravelly for a considerable distance down, and,
consequently, fairly well suited for the chestnut, but just the reverse
for the larch, which, on such a soil, generally becomes “pumped,” or
rotten at the core. In thinning this particular plantation, it would
have been very unwise to cut away the chestnut and the oak and leave
the larch, and this had happened to some extent before the proper
system of management and adaptability of soil to tree was thought
of. Larch may look healthy enough, and show but small indications of
disease, even when growing on gravel up to twenty or thirty years of
age, so that it is with difficulty that the inexperienced become aware
of the pending doom that usually awaits this tree when planted on
such a soil. This example is merely given to point out how carefully
thinning should be gone about, and that in all cases it is a wise
policy to study soil in relation to the future crop of timber before an
axe is laid to the tree.
At the outset of these remarks on thinning plantations it should be
distinctly borne in mind that there are two chief objects for which
trees may be grown, each requiring a special mode of management, in
order that the best results may be obtained.
The first is their management in a purely economical sense or with a
view to profit; the second, their management with a view to ornament.
There is also a third object that is well worthy of consideration,
and that is the growing of timber in one and the same wood, both for
ornament and utility combined, and this is very frequently the case
with home woodlands that are visible from roads and drives, with strips
bounding parks or pleasure-grounds, and on small properties.
To produce ornamental trees of natural appearance is by no means
difficult, as by allowing the individual specimens ample room for
branch development, the desired effect is gradually brought about. Far
greater difficulty, however, attends the production of the greatest
quantity of the most valuable trees on a given space of ground. Here
many questions of the greatest moment, on which difference of opinion
exists, crop up: such as at what age thinning should be commenced, to
what extent should it be engaged in, what time should elapse between
each thinning, and which trees should be removed.
[Illustration: FELLING TOOLS.]
Any one at all interested in the management of our woods and
plantations must have noticed that trees having an abundance of room
and light on all sides make comparatively short and thick trunks that
are well furnished with branches; whereas such as are grown up in a
circumscribed space and amongst others are tall and straight, with
clean, well-formed stems destitute of branches for the greater part
of their height. All species, or nearly all, are governed by the same
laws, that is to say, those that have the least room laterally within
certain prescribed limits, which will be described hereafter, produce
the tallest, cleanest and straightest trunks, and vice versa.
The influence of light has not, in this country at least, been
sufficiently taken into account in the rearing of timber, but it has
everything to do in directing the growth of trees, and should be
reckoned as a most important factor by the forester. Thus, if it be
allowed in excess, as when the trees stand far apart, the growth of
lateral shoots and large branches will be greatly induced, the result
being short and thick boles, that are rough and knotty, and ill-adapted
for constructive purposes. On the other hand, by keeping the trees
thick on the ground, light is, to a greater or less extent, excluded,
and the trees grow tall, straight and branchless for the greater part
of their height, and are of the greatest economic value. But here
another and very important question crops up. To what extent in a wood,
managed solely for the value of the timber it produces, will it be
profitable to thin? In dealing with this, two distinct bearings should
be kept in mind—the first, that too small a quantity of branches and
consequently of leaves, must, to a greater or less extent, check the
growth of the trees, and so diminish the production of timber; and,
second, that by having too large a quantity, the value of the timber
is greatly reduced in consequence, and the number of trees to the acre
much diminished as well.
There is, therefore, a medium between these two, by adopting which,
the greatest quantity of the most valuable timber will be produced;
although, at the same time, it is astonishing, when looked at from
a physiological point of view, what a small quantity of foliage is
required to keep a plantation tree in a healthy, growing condition,
and to produce a trunk of useful dimensions. This may, however, be
explained by the fact that the almost entire absence of large limbs and
branches, which in some cases would amount to one-fifth the total bulk
of the tree, renders the amount of sap, and consequently of leaves,
required proportionately less, the nutriment being mainly elaborated in
the building of the trunk.
Independent altogether of the forests of northern Europe, Canada,
etc., which have grown up naturally, examples might be given in our
own country—the native Fir forests in the Highlands of Scotland, the
valuable Beech woods on the Chiltern Hills, and a few Larch plantations
in Yorkshire and Bedfordshire—where trees averaging 70 ft. in height,
growing at from 4 to 6 yards apart, and with only a tuft of foliage
atop, may be seen. It is only, however, by long and careful study and
attention to different trees at different stages of their growth that
any idea can be formed of the amount of branches and foliage required
for preparing the sap that will be necessary for the profitable yearly
increase of the trunk until maturity is arrived at.
The questions now to be dealt with in growing timber for purely
economical purposes are: (1) At what age should thinning be commenced?
(2) To what extent should it be engaged in? (3) What time should elapse
between each thinning? And (4) Which trees should be removed?
At the outset, it may be well to mention, that in so far as any of
the above questions are concerned, no hard and fast line can be laid
down as to the universal management of plantations, each tract of
wood, and even different parts of the same wood, according to the
particular species of tree, quality of soil, altitude, and exposure,
being dealt with on its own peculiar footing. Many serious mistakes
have been committed, and irreparable damage done, by treating every
plantation alike and according to the rule-of-thumb method. Upon the
manner in which thinning is performed, much of the ultimate success
of a plantation will depend, and it is a matter requiring great tact,
forethought, and discrimination, which can only be acquired by long
experience, and an intimate knowledge of trees and their surroundings.
The quality as well as quantity of timber produced should be carefully
considered in the management of every plantation. In dealing with the
following questions regarding thinning, it may be well to state that
the average plantation will alone form the basis of remarks.
=1. At what age should thinning be commenced?=—The necessity of
thinning, few practical arboriculturists will care to deny, and that
whether the plantation be composed of coniferous or hard-wooded trees.
It is the abuse of the practice that, in this country at least, is so
to be condemned, and I entirely dissent from those who consider that
a coniferous plantation should be left to Nature, or, in other words,
should thin itself. The argument that the natural forests of America,
the source from which we derive the finest and best quality of timber,
thinned themselves, is often quoted; but it should also be remembered
that the conditions of such forests are widely different from those
of our own, and that the waste of timber is immense, great quantities
being destroyed in procuring what is required. Although, however,
we cannot adopt all the details of Nature’s practice, we can and we
should admit the correctness of the principle on which she acts, and by
studying this we learn much, and it is by acquiring a knowledge of her
economy, and where and when to apply assistance, that the point of the
whole argument rests. Natural regeneration, unless it be in a limited
number of cases, and particularly with our least valuable classes of
timber, is never, in this country, likely to supersede artificial
planting, and it has been conclusively proved in the Forest of Dean,
that with our most valuable timber-producing tree, the Oak, the latter
system is most to be encouraged.
No thinning of young trees should take place until a complete ground
shade has been established; and it is most important for the welfare of
the plantation, that a complete overhead foliage covering be brought
about at as early a date as possible after planting. At what age this
may take place will depend mainly on the size of the plants used and
the distance apart at which they were inserted in the ground, and
to a lesser extent on the quality of soil and other considerations.
At a short period after a complete leaf canopy has been established,
the individual trees begin to press against each other, and later on
a struggle for existence commences, the stronger specimens gaining
the supremacy over the weaker. Here it must be borne in mind that
hard-wooded trees require proportionately more room for their healthy
development than coniferous kinds, while length and clearness of
stem, produced by a due proportion of shade, is an object of prime
importance. It might, as some suppose, do little harm, to let the
struggle for existence go on unchecked, but there can be no question
that for several reasons it is wise policy to allow the stronger trees
every chance of succeeding, and to cut away the weaker. The object
should be to provide for the trees left standing that amount of room or
growing space best suited for bringing about the particular conditions
aimed at, and in thinning, it must be the aim of the forester to arrive
at the happy mean—neither over nor underdoing the work, the former in
particular. But while overcrowding is not to be tolerated, the danger
of suddenly exposing the trees to currents of air, to which they have
hitherto been unaccustomed, must be carefully guarded against, and it
should be distinctly borne in mind that to thin trees in an abrupt
manner is one of the greatest mistakes that it is possible to make.
Statistics compiled from a healthy Larch plantation, growing on fair
soil, and in a moderately sheltered position in southern England, will
now be given, from which a good idea can be formed as to the age and
size when thinning should be commenced. The trees when planted were 2
ft. high, and pitted at 3 ft. apart. In four years the outer branches
began to touch each other, and in six years from time of planting the
average height of trees was 7 ft. 10 in. and the shade occasioned had
killed out most of the grassy undergrowth. At this period of growth,
the disproportion in the size of the trees was, as is usually the case,
considerable, and left no doubt about which to remove when the first
thinning took place. Two years afterwards, or in eight years from time
of planting, thinning was first engaged in, the taller trees at that
time averaging nearly 11 ft. in height, but many of the suppressed and
weakly were little more than half that size. Another thinning took
place during the twelfth year, and in 1911 the trees average 40 ft. in
height and were standing at a distance of 9 ft. apart. The plantation
referred to is on the Earl of Derby’s property of Holwood, in Kent.
=2. To what extent should thinning be engaged in?=—In following up
the latter case, the tallest and healthiest trees were reserved; all
distorted, sickly, and stunted specimens being removed; but in the case
of two or more proportionately small trees growing in close proximity,
the most promising was left, and the others cut away, thus avoiding
gaps in the plantation. At the first thinning it will be found quite
impossible to leave the trees anything like regular over the ground,
although this should be studied as much as possible, and bare spaces
are to be carefully guarded against. With the vigorous growth of the
trees, and the interlacing of branches previous to the first thinning,
no great openings will require to be made, and nothing more than will
be canopied over during the next two years.
=3. What time should elapse between each thinning?=—This can only be
correctly decided after a careful inspection of the particular wood.
Generally speaking, after the first thinning, when the trees were
eight years old, the lower branches gradually began to give way, and
as the trees increased in height, this became more and more apparent,
and formed a good guide as to the time which should elapse before the
second thinning might profitably be engaged in. The intervals between
the various thinnings should for the first thirty years in the case of
Larch—and, indeed, most other trees—be comparatively short, but become
longer with advance of age; but heavy thinnings must be carefully
avoided, especially if the best class of coniferous timber is to be
produced.
A well-managed Larch plantation of twenty years’ growth should have
the trees branchless for about one-half of their height, which is, of
course, brought about by crowding, and at forty years three-fourths of
the trunk should be clear of branches.
I have purposely abstained from giving the number of feet apart at
which trees ought to stand at various stages of their growth, and the
number of thinnings they require, as being likely to prove misleading.
=4. The question as to which trees should be removed= need cause little
or no anxiety, as the dead, dying, diseased, and badly-formed will
naturally receive first attention.
In thinning, the following short rules should be observed:—
1. Thin not at all until the undergrowing vegetation has been
completely killed out by the overhead foliage, the golden rule of
sylviculture being to keep the sunshine off the ground until near the
end of the tree’s career.
2. Thinning should be performed in such a manner as not to reduce the
value of the crop, but so as to tend to the production of the greatest
quantity of the most valuable timber in the shortest possible space of
time.
3. By thinning allow of sufficient light and air to prevent the trees
becoming drawn up and lanky, but avoid too much space, which induces
the growth of side branches and detracts from the value of the timber.
4. Keep up the number of trees to the highest possible pitch until they
are tall, straight and clean, and thin gradually.
5. The danger of suddenly exposing the trees to currents of cold air to
which they have hitherto been unaccustomed must be carefully guarded
against and it is a serious mistake to thin in an abrupt or unequal
manner.
6. In thinning remove all dead and dying trees first, then the
distorted and stunted, the best grown and healthiest being preserved
for the permanent crop.
7. Hard-wooded trees require proportionately more space for development
than conifers, and the annual rings in the timber of the latter should
be narrow in proportion to those of the former, good quality of
hard-wooded timber being indicated by broad annual rings, but with
coniferous wood the reverse is the case.
8. Thinning should be commenced at the central or most sheltered point
of a plantation, or section of same, so that the outer intact boundary
may continue to form a barrier to cold winds, which might prove
injurious to trees that had hitherto stood in close order.
9. In economic forestry, timber trees should be cut down when they have
arrived at maturity or have ceased to grow. With coniferous trees this
generally occurs at from seventy to ninety years; but with hardwoods,
excepting the ash and chestnut, which are of most value, say, up to
fifty years’ growth, the period may be greatly extended.
10. Thinning is usually performed in autumn and early winter, but
it may be well to remember that at the latter period the lowest
percentage of moisture (about 47 per cent.) is present, and the timber
consequently of the greatest value for constructive purposes.