THE WOODS.
THE HABITABLE EARTH ORIGINALLY WOODED--THE FOREST DOES NOT FURNISH
FOOD FOR MAN--FIRST REMOVAL OF THE WOODS--EFFECTS OF FIRE ON FOREST
SOIL--EFFECTS OF THE DESTRUCTION OF THE FOREST--ELECTRICAL INFLUENCE
OF TREES--CHEMICAL INFLUENCE OF THE FOREST.
INFLUENCE OF THE FOREST, CONSIDERED AS INORGANIC MATTER, ON TEMPERATURE:
_a_, ABSORBING AND EMITTING SURFACE; _b_, TREES AS CONDUCTORS OF HEAT;
_c_, TREES IN SUMMER AND IN WINTER; _d_, DEAD PRODUCTS OF TREES; _e_,
TREES AS A SHELTER TO GROUNDS TO THE LEEWARD OF THEM; _f_, TREES AS A
PROTECTION AGAINST MALARIA--THE FOREST, AS INORGANIC MATTER, TENDS TO
MITIGATE EXTREMES.
TREES AS ORGANISMS: SPECIFIC TEMPERATURE--TOTAL INFLUENCE OF THE FOREST
ON TEMPERATURE.
INFLUENCE OF FORESTS ON THE HUMIDITY OF THE AIR AND THE EARTH:
_a_, AS INORGANIC MATTER; _b_, AS ORGANIC--WOOD MOSSES AND FUNGI--
FLOW OF SAP--ABSORPTION AND EXHALATION OF MOISTURE BY TREES--BALANCE
OF CONFLICTING INFLUENCES--INFLUENCE OF THE FOREST ON TEMPERATURE AND
PRECIPITATION--INFLUENCE OF THE FOREST ON THE HUMIDITY OF THE SOIL--
ITS INFLUENCE ON THE FLOW OF SPRINGS--GENERAL CONSEQUENCES OF THE
DESTRUCTION OF THE WOODS--LITERATURE AND CONDITION OF THE FOREST IN
DIFFERENT COUNTRIES--THE INFLUENCE OF THE FOREST ON INUNDATIONS--
DESTRUCTIVE ACTION OF TORRENTS--THE PO AND ITS DEPOSITS--MOUNTAIN
SLIDES--PROTECTION AGAINST THE FALL OF ROCKS AND AVALANCHES BY
TREES--PRINCIPAL CAUSES OF THE DESTRUCTION OF THE FOREST--AMERICAN
FOREST TREES--SPECIAL CAUSES OF THE DESTRUCTION OF EUROPEAN WOODS--
ROYAL FORESTS AND GAME LAWS--SMALL FOREST PLANTS, VITALITY OF SEEDS--
UTILITY OF THE FOREST--THE FORESTS OF EUROPE--FORESTS OF THE UNITED
STATES AND CANADA--THE ECONOMY OF THE FOREST--EUROPEAN AND AMERICAN
TREES COMPARED--SYLVICULTURE--INSTABILITY OF AMERICAN LIFE.
_The Habitable Earth Originally Wooded._
There is good reason to believe that the surface of the habitable earth,
in all the climates and regions which have been the abodes of dense and
civilized populations, was, with few exceptions, already covered with a
forest growth when it first became the home of man. This we infer from
the extensive vegetable remains--trunks, branches, roots, fruits, seeds,
and leaves of trees--so often found in conjunction with works of
primitive art, in the boggy soil of districts where no forests appear to
have existed within the eras through which written annals reach; from
ancient historical records, which prove that large provinces, where the
earth has long been wholly bare of trees, were clothed with vast and
almost unbroken woods when first made known to Greek and Roman
civilization;[112] and from the state of much of North and of South
America when they were discovered and colonized by the European
race.[113]
These evidences are strengthened by observation of the natural economy
of our own time; for, whenever a tract of country, once inhabited and
cultivated by man, is abandoned by him and by domestic animals,[114] and
surrendered to the undisturbed influences of spontaneous nature, its
soil sooner or later clothes itself with herbaceous and arborescent
plants, and at no long interval, with a dense forest growth. Indeed,
upon surfaces of a certain stability, and not absolutely precipitous
inclination, the special conditions required for the spontaneous
propagation of trees may all be negatively expressed and reduced to
these three: exemption from defect or excess of moisture, from perpetual
frost, and from the depredations of man and browsing quadrupeds. Where
these requisites are secured, the hardest rock is as certain to be
overgrown with wood as the most fertile plain, though, for obvious
reasons, the process is slower in the former than in the latter case.
Lichens and mosses first prepare the way for a more highly organized
vegetation. They retain the moisture of rains and dews, and bring it to
act, in combination with the gases evolved by their organic processes,
in decomposing the surface of the rocks they cover; they arrest and
confine the dust which the wind scatters over them, and their final
decay adds new material to the soil already half formed beneath and upon
them. A very thin stratum of mould is sufficient for the germination of
seeds of the hardy evergreens and birches, the roots of which are often
found in immediate contact with the rock, supplying their trees with
nourishment from a soil derived from the decomposition of their own
foliage, or sending out long rootlets into the surrounding earth in
search of juices to feed them.
The eruptive matter of volcanoes, forbidding as is its aspect, does not
refuse nutriment to the woods. The refractory lava of Etna, it is true,
remains long barren, and that of the great eruption of 1669 is still
almost wholly devoid of vegetation.[115] But the cactus is making
inroads even here, while the volcanic sand and molten rock thrown out by
Vesuvius soon becomes productive. George Sandys, who visited this
latter mountain in 1611, after it had reposed for several centuries,
found the throat of the volcano at the bottom of the crater "almost
choked with broken rocks and _trees_ that are falne therein." "Next to
this," he continues, "the matter thrown up is ruddy, light, and soft:
more removed, blacke and ponderous: the uttermost brow, that declineth
like the seates in a theater, flourishing with trees and excellent
pasturage. The midst of the hill is shaded with chestnut trees, and
others bearing sundry fruits."[116]
I am convinced that forests would soon cover many parts of the Arabian
and African deserts, if man and domestic animals, especially the goat
and the camel, were banished from them. The hard palate and tongue and
strong teeth and jaws of this latter quadruped enable him to break off
and masticate tough and thorny branches as large as the finger. He is
particularly fond of the smaller twigs, leaves, and seedpods of the
_sont_ and other acacias, which, like the American Robinia, thrive well
on dry and sandy soils, and he spares no tree the branches of which are
within his reach, except, if I remember right, the tamarisk that
produces manna. Young trees sprout plentifully around the springs and
along the winter watercourses of the desert, and these are just the
halting stations of the caravans and their routes of travel. In the
shade of these trees, annual grasses and perennial shrubs shoot up, but
are mown down by the hungry cattle of the Bedouin, as fast as they grow.
A few years of undisturbed vegetation would suffice to cover such points
with groves, and these would gradually extend themselves over soils
where now scarcely any green thing but the bitter colocynth and the
poisonous foxglove is ever seen.
_The Forest does not Furnish Food for Man._
In a region absolutely covered with trees, human life could not long be
sustained, for want of animal and vegetable food. The depths of the
forest seldom furnish either bulb or fruit suited to the nourishment of
man; and the fowls and beasts on which he feeds are scarcely seen except
upon the margin of the wood, for here only grow the shrubs and grasses,
and here only are found the seeds and insects, which form the sustenance
of the non-carnivorous birds and quadrupeds.[117]
_First Removal of the Forest._
As soon as multiplying man had filled the open grounds along the margin
of the rivers, the lakes, and the sea, and sufficiently peopled the
natural meadows and savannas of the interior, where such existed,[118]
he could find room for expansion and further growth, only by the
removal of a portion of the forest that hemmed him in. The destruction
of the woods, then, was man's first geographical conquest, his first
violation of the harmonies of inanimate nature.
Primitive man had little occasion to fell trees for fuel, or, for the
construction of dwellings, boats, and the implements of his rude
agriculture and handicrafts. Windfalls would furnish a thin population
with a sufficient supply of such material, and if occasionally a growing
tree was cut, the injury to the forest would be too insignificant to be
at all appreciable.
The accidental escape and spread of fire, or, possibly, the combustion
of forests by lightning, must have first suggested the advantages to be
derived from the removal of too abundant and extensive woods, and, at
the same time, have pointed out a means by which a large tract of
surface could readily be cleared of much of this natural incumbrance. As
soon as agriculture had commenced at all, it would be observed that the
growth of cultivated plants, as well as of many species of wild
vegetation, was particularly rapid and luxuriant on soils which had been
burned over, and thus a new stimulus would be given to the practice of
destroying the woods by fire, as a means of both extending the open
grounds, and making the acquisition of a yet more productive soil. After
a few harvests had exhausted the first rank fertility of the virgin
mould, or when weeds and briers and the sprouting roots of the trees had
begun to choke the crops of the half-subdued soil, the ground would be
abandoned for new fields won from the forest by the same means, and the
deserted plain or hillock would soon clothe itself anew with shrubs and
trees, to be again subjected to the same destructive process, and again
surrendered to the restorative powers of vegetable nature.[119] This
rude economy would be continued for generations, and wasteful as it is,
is still largely pursued in Northern Sweden, Swedish Lapland, and
sometimes even in France and the United States.[120]
_Effects of Fire on Forest Soil._
Aside from the mechanical and chemical effects of the disturbance of the
soil by agricultural operations, and of the freer admission of sun,
rain, and air to the ground, the fire of itself exerts an important
influence on its texture and condition. It consumes a portion of the
half-decayed vegetable mould which served to hold its mineral particles
together and to retain the water of precipitation, and thus loosens,
pulverizes, and dries the earth; it destroys reptiles, insects, and
worms, with their eggs, and the seeds of trees and of smaller plants; it
supplies, in the ashes which it deposits on the surface, important
elements for the growth of a new forest clothing, as well as of the
usual objects of agricultural industry; and by the changes thus
produced, it fits the ground for the reception of a vegetation different
in character from that which had spontaneously covered it. These new
conditions help to explain the natural succession of forest crops, so
generally observed in all woods cleared by fire and then abandoned.
There is no doubt, however, that other influences contribute to the same
result, because effects more or less analogous follow when the trees are
destroyed by other causes, as by high winds, by the woodman's axe, and
even by natural decay.[121]
_Effects of Destruction of the Forest._
The physico-geographical effects of the destruction of the forests may
be divided into two great classes, each having an important influence on
vegetable and on animal life in all their manifestations, as well as on
every branch of rural economy and productive industry, and, therefore,
on all the material interests of man. The first respects the meteorology
of the countries exposed to the action of these influences; the second,
their superficial geography, or, in other words, configuration,
consistence, and clothing of surface.
For reasons assigned in the first chapter, the meteorological or
climatic branch of the subject is the most obscure, and the conclusions
of physicists respecting it are, in a great degree, inferential only,
not founded on experiment or direct observation. They are, as might be
expected, somewhat discordant, though certain general results are almost
universally accepted, and seem indeed too well supported to admit of
serious question.
_Electrical Influence of Trees._
The properties of trees, singly and in groups, as exciters or conductors
of electricity, and their consequent influence upon the electrical state
of the atmosphere, do not appear to have been much investigated; and the
conditions of the forest itself are so variable and so complicated, that
the solution of any general problem respecting its electrical influence
would be a matter of extreme difficulty. It is, indeed, impossible to
suppose that a dense cloud, a sea of vapor, can pass over miles of
surface bristling with good conductors, without undergoing some change
of electrical condition. Hypothetical cases may be put in which the
character of the change could be deduced from the known laws of
electrical action. But in actual nature, the elements are too numerous
for us to seize. The true electrical condition of neither cloud nor
forest could be known, and it could seldom be predicted whether the
vapors would be dissolved as they floated over the wood, or discharged
upon it in a deluge of rain. With regard to possible electrical
influences of the forest, wider still in their range of action, the
uncertainty is even greater. The data which alone could lead to certain,
or even probable, conclusions are wanting, and we should, therefore,
only embarrass our argument by any attempt to discuss this
meteorological element, important as it may be, in its relations of
cause and effect to more familiar and better understood meteoric
phenomena. It may, however, be observed that hail storms--which were
once generally supposed, and are still held by many, to be produced by a
specific electrical action, and which, at least, are always accompanied
by electrical disturbances--are believed, in all countries particularly
exposed to that scourge, to have become more frequent and destructive in
proportion as the forests have been cleared. Caimi observes: "When the
chains of the Alps and the Apennines had not yet been stripped of their
magnificent crown of woods, the May hail, which now desolates the
fertile plains of Lombardy, was much less frequent; but since the
general prostration of the forest, these tempests are laying waste even
the mountain soils whose older inhabitants scarcely knew this
plague.[122] The _paragrandini_,[123] which the learned curate of
Rivolta advised to erect, with sheaves of straw set up vertically, over
a great extent of cultivated country, are but a Liliputian image of the
vast paragrandini, pines, larches, firs, which nature had planted by
millions on the crests and ridges of the Alps and the Apennines."[124]
"Electrical action being diminished," says Meguscher, "and the rapid
congelation of vapors by the abstraction of heat being impeded by the
influence of the woods, it is rare that hail or waterspouts are
produced, within the precincts of a large forest when it is assailed by
the tempest."[125] Arthur Young was told that since the forests which
covered the mountains between the Riviera and the county of Montferrat
had disappeared, hail had become more destructive in the district of
Acqui,[126] and it appears upon good authority, that a similar increase
in the frequency and violence of hail storms in the neighborhood of
Saluzzo and Mondovi, the lower part of the Valtelline, and the territory
of Verona and Vicenza, is probably to be ascribed to a similar
cause.[127]
_Chemical Influence of the Forest._
We know that the air in a close apartment is appreciably affected
through the inspiration and expiration of gases by plants growing in it.
The same operations are performed on a gigantic scale by the forest, and
it has even been supposed that the absorption of carbon, by the rank
vegetation of earlier geological periods, occasioned a permanent change
in the constitution of the terrestrial atmosphere.[128] To the effects
thus produced, are to be added those of the ultimate gaseous
decomposition of the vast vegetable mass annually shed by trees, and of
their trunks and branches when they fall a prey to time. But the
quantity of gases thus abstracted from and restored to the atmosphere is
inconsiderable--infinitesimal, one might almost say--in comparison with
the ocean of air from which they are drawn and to which they return; and
though the exhalations from bogs, and other low grounds covered with
decaying vegetable matter, are highly deleterious to human health, yet,
in general, the air of the forest is hardly chemically distinguishable
from that of the sand plains, and we can as little trace the influence
of the woods in the analysis of the atmosphere, as we can prove that the
mineral ingredients of land springs sensibly affect the chemistry of
the sea. I may, then, properly dismiss the chemical, as I have done the
electrical influences of the forest, and treat them both alike, if not
as unimportant agencies, at least as quantities of unknown value in our
meteorological equation.[129] Our inquiries upon this branch of the
subject will accordingly be limited to the thermometrical and
hygrometrical influences of the woods.
_Influence of the Forest, considered as Inorganic Matter, on
Temperature._
The evaporation of fluids, and the condensation and expansion of vapors
and gases, are attended with changes of temperature; and the quantity of
moisture which the air is capable of containing, and, of course, the
evaporation, rise and fall with the thermometer. The hygroscopical and
the thermoscopical conditions of the atmosphere are, therefore,
inseparably connected as reciprocally dependent quantities, and neither
can be fully discussed without taking notice of the other. But the
forest, regarded purely as inorganic matter, and without reference to
its living processes of absorption and exhalation of water and gases,
has, as an absorbent, a radiator and a conductor of heat, and as a mere
covering of the ground, an influence on the temperature of the air and
the earth, which may be considered by itself.
a. _Absorbing and Emitting Surface._
A given area of ground, as estimated by the every-day rule of
measurement in yards or acres, presents always the same apparent
quantity of absorbing, radiating, and reflecting surface; but the real
extent of that surface is very variable, depending, as it does, upon its
configuration, and the bulk and form of the adventitious objects it
bears upon it; and, besides, the true superficies remaining the same,
its power of absorption, radiation, reflection, and conduction of heat
will be much affected by its consistence, its greater or less humidity,
and its color, as well as by its inclination of plane and
exposure.[130] An acre of chalk, rolled hard and smooth, would have
great reflecting power, but its radiation would be much increased by
breaking it up into clods, because the actually exposed surface would be
greater, though the outline of the field remained the same. The area of
a triangle being equal to its base multiplied by half the length of a
perpendicular let fall from its apex, it follows that the entire
superficies of the triangular faces of a quadrangular pyramid, the
perpendicular of whose sides should be twice the length of the base,
would be four times the area of the ground it covered, and would add to
the field on which it stood so much surface capable of receiving and
emitting heat, though, in consequence of obliquity and direction of
plane, its actual absorption and emission of heat might not be so great
as that of an additional quantity of level ground containing four times
the area of its base. The lesser inequalities which always occur in the
surface of ordinary earth affect in the same way its quantity of
superficies acting upon the temperature of the atmosphere, and acted on
by it, though the amount of this action and reaction is not susceptible
of measurement.
Analogous effects are produced by other objects, of whatever form or
character, standing or lying upon the earth, and no solid can be placed
upon a flat piece of ground, without itself exposing a greater surface
than it covers. This applies, of course, to forest trees and their
leaves, and indeed to all vegetables, as well as to other prominent
bodies. If we suppose forty trees to be planted on an acre, one being
situated in the centre of every square of two rods the side, and to grow
until their branches and leaves everywhere meet, it is evident that,
when in full foliage, the trunks, branches, and leaves would present an
amount of thermoscopic surface much greater than that of an acre of bare
earth; and besides this, the fallen leaves lying scattered on the
ground, would somewhat augment the sum total.[131] On the other hand,
the growing leaves of trees generally form a succession of stages, or,
loosely speaking, layers, corresponding to the animal growth of the
branches, and more or less overlying each other. This disposition of the
foliage interferes with that free communication between sun and sky
above, and leaf surface below, on which the amount of radiation and
absorption of heat depends. From all these considerations, it appears
that though the effective thermoscopic surface of a forest in full leaf
does not exceed that of bare ground in the same proportion as does its
measured superficies, yet the actual quantity of area capable of
receiving and emitting heat must be greater in the former than in the
latter case.[132]
It must further be remembered that the form and texture of a given
surface are important elements in determining its thermoscopic
character. Leaves are porous, and admit air and light more or less
freely into their substance; they are generally smooth and even glazed
on one surface; they are usually covered on one or both sides with
spiculae, and they very commonly present one or more acuminated points in
their outline--all circumstances which tend to augment their power of
emitting heat by reflection or radiation. Direct experiment on growing
trees is very difficult, nor is it in any case practicable to
distinguish how far a reduction of temperature produced by vegetation is
due to radiation, and how far to exhalation of the fluids of the plant
in a gaseous form; for both processes usually go on together. But the
frigorific effect of leafy structure is well observed in the deposit of
dew and the occurrence of hoarfrost on the foliage of grasses and other
small vegetables, and on other objects of similar form and consistence,
when the temperature of the air a few yards above has not been brought
down to the dew point, still less to 32 deg., the degree of cold required to
congeal dew to frost.[133]
b. _Trees as Conductors of Heat._
We are also to take into account the action of the forest as a conductor
of heat between the atmosphere and the earth. In the most important
countries of America and Europe, and especially in those which have
suffered most from the destruction of the woods, the superficial strata
of the earth are colder in winter, and warmer in summer than those a few
inches lower, and their shifting temperature approximates to the
atmospheric mean of the respective seasons. The roots of large trees
penetrate beneath the superficial strata, and reach earth of a nearly
constant temperature, corresponding to the mean for the entire year. As
conductors, they convey the heat of the atmosphere to the earth when the
earth is colder than the air, and transmit it in the contrary direction
when the temperature of the earth is higher than that of the atmosphere.
Of course, then, as conductors, they tend to equalize the temperature of
the earth and the air.
c. _Trees in Summer and Winter._
In countries where the questions I am considering have the greatest
practical importance, a very large proportion, if not a majority, of the
trees are of deciduous foliage, and their radiating as well as their
shading surface is very much greater in summer than in winter. In the
latter season, they little obstruct the reception of heat by the ground
or the radiation from it; whereas, in the former, they often interpose a
complete canopy between the ground and the sky, and materially
interfere with both processes.
d. _Dead Products of Trees._
Besides this various action of standing trees considered as inorganic
matter, the forest exercises, by the annual moulting of its foliage,
still another influence on the temperature of the earth, and,
consequently, of the atmosphere which rests upon it. If you examine the
constitution of the superficial soil in a primitive or an old and
undisturbed artificially planted wood, you find, first, a deposit of
undecayed leaves, twigs, and seeds, lying in loose layers on the
surface; then, more compact beds of the same materials in incipient,
and, as you descend, more and more advanced stages of decomposition;
then, a mass of black mould, in which traces of organic structure are
hardly discoverable except by microscopic examination; then, a stratum
of mineral soil, more or less mixed with vegetable matter carried down
into it by water, or resulting from the decay of roots; and, finally,
the inorganic earth or rock itself. Without this deposit of the dead
products of trees, this latter would be the superficial stratum, and as
its powers of absorption, radiation, and conduction of heat would differ
essentially from those of the layers with which it has been covered by
the droppings of the forest, it would act upon the temperature of the
atmosphere, and be acted on by it, in a very different way from the
leaves and mould which rest upon it. Leaves, still entire, or partially
decayed, are very indifferent conductors of heat, and, therefore, though
they diminish the warming influence of the summer sun on the soil below
them, they, on the other hand, prevent the escape of heat from that soil
in winter, and, consequently, in cold climates, even when the ground is
not covered by a protecting mantle of snow, the earth does not freeze to
as great a depth in the wood as in the open field.
e. _Trees as a Shelter to Ground to the Leeward._
The action of the forest, considered merely as a mechanical shelter to
grounds lying to the leeward of it, would seem to be an influence of too
restricted a character to deserve much notice; but many facts concur to
show that it is an important element in local climate, and that it is
often a valuable means of defence against the spread of miasmatic
effluvia, though, in this last case, it may exercise a chemical as well
as a mechanical agency. In the report of a committee appointed in 1836
to examine an article of the forest code of France, Arago observes: "If
a curtain of forest on the coasts of Normandy and of Brittany were
destroyed, these two provinces would become accessible to the winds from
the west, to the mild breezes of the sea. Hence a decrease of the cold
of winter. If a similar forest were to be cleared on the eastern border
of France, the glacial east wind would prevail with greater strength,
and the winters would become more severe. Thus the removal of a belt of
wood would produce opposite effects in the two regions."[134]
This opinion receives confirmation from an observation of Dr. Dwight,
who remarks, in reference to the woods of New England: "Another effect
of removing the forest will be the free passage of the winds, and among
them of the southern winds, over the surface. This, I think, has been an
increasing fact within my own remembrance. As the cultivation of the
country has extended farther to the north, the winds from the south have
reached distances more remote from the ocean, and imparted their warmth
frequently, and in such degrees as, forty years since, were in the same
places very little known. This fact, also, contributes to lengthen the
summer, and to shorten the winter-half of the year."[135]
It is thought in Italy that the clearing of the Apennines has very
materially affected the climate of the valley of the Po. It is asserted
in Le Alpi che cingono l'Italia that: "In consequence of the felling of
the woods on the Apennines, the sirocco prevails greatly on the right
bank of the Po, in the Parmesan territory, and in a part of Lombardy; it
injures the harvests and the vineyards, and sometimes ruins the crops of
the season. To the same cause many ascribe the meteorological changes in
the precincts of Modena and of Reggio. In the communes of these
districts, where formerly straw roofs resisted the force of the winds,
tiles are now hardly sufficient; in others, where tiles answered for
roofs, large slabs of stone are now ineffectual; and in many neighboring
communes the grapes and the grain are swept off by the blasts of the
south and southwest winds."
On the other hand, according to the same authority, the pinery of Porto,
near Ravenna--which is 33 kilometres long, and is one of the oldest pine
woods in Italy--having been replanted with resinous trees after it was
unfortunately cut, has relieved the city from the sirocco to which it
had become exposed, and in a great degree restored its ancient
climate.[136]
The felling of the woods on the Atlantic coast of Jutland has exposed
the soil not only to drifting sands, but to sharp sea winds, that have
exerted a sensible deteriorating effect on the climate of that
peninsula, which has no mountains to serve at once as a barrier to the
force of the winds, and as a storehouse of moisture received by
precipitation or condensed from atmospheric vapors.[137]
It is evident that the effect of the forest, as a mechanical impediment
to the passage of the wind, would extend to a very considerable distance
above its own height, and hence protect while standing, or lay open when
felled, a much larger surface than might at first thought be supposed.
The atmosphere, movable as are its particles, and light and elastic as
are its masses, is nevertheless held together as a continuous whole by
the gravitation of its atoms and their consequent pressure on each
other, if not by attraction between them, and, therefore, an obstruction
which mechanically impedes the movement of a given stratum of air, will
retard the passage of the strata above and below it. To this effect may
often be added that of an ascending current from the forest itself,
which must always exist when the atmosphere within the wood is warmer
than the stratum of air above it, and must be of almost constant
occurrence in the case of cold winds, from whatever quarter, because the
still air in the forest is slow in taking up the temperature of the
moving columns and currents around and above it. Experience, in fact,
has shown that mere rows of trees, and even much lower obstructions, are
of essential service in defending vegetation against the action of the
wind. Hardy proposes planting, in Algeria, belts of trees at the
distance of one hundred metres from each other, as a shelter which
experience had proved to be useful in France.[138] "In the valley of the
Rhone," says Becquerel, "a simple hedge, two metres in height, is a
sufficient protection for a distance of twenty-two metres."[139] The
mechanical shelter acts, no doubt, chiefly as a defence against the
mechanical force of the wind, but its uses are by no means limited to
this effect. If the current of air which it resists moves horizontally,
it would prevent the access of cold or parching blasts to the ground for
a great distance; and did the wind even descend at a large angle with
the surface, still a considerable extent of ground would be protected by
a forest to the windward of it. If we suppose the trees of a wood to
have a mean height of only twenty yards, they would often beneficially
affect the temperature or the moisture of a belt of land two or three
hundred yards in width, and thus perhaps rescue valuable crops from
destruction.[140]
The local retardation of spring so much complained of in Italy, France,
and Switzerland, and the increased frequency of late frosts at that
season, appear to be ascribable to the admission of cold blasts to the
surface, by the felling of the forests which formerly both screened it
as by a wall, and communicated the warmth of their soil to the air and
earth to the leeward. Caimi states that since the cutting down of the
woods of the Apennines, the cold winds destroy or stunt the vegetation,
and that, in consequence of "the usurpation of winter on the domain of
spring," the district of Mugello has lost all its mulberries, except the
few which find in the lee of buildings a protection like that once
furnished by the forest.[141]
"It is proved," says Clave, "Etudes," p. 44, "that the department of
Ardeche, which now contains not a single considerable wood, has
experienced within thirty years a climatic disturbance, of which the
late frosts, formerly unknown in the country, are one of the most
melancholy effects. Similar results have been observed in the plain of
Alsace, in consequence of the denudation of several of the crests of the
Vosges."
Dussard, as quoted by Ribbe,[142] maintains that even the _mistral_, or
northwest wind, whose chilling blasts are so fatal to tender vegetation
in the spring, "is the child of man, the result of his devastations."
"Under the reign of Augustus," continues he, "the forests which
protected the Cevennes were felled, or destroyed by fire, in mass. A
vast country, before covered with impenetrable woods--powerful obstacles
to the movement and even to the formation of hurricanes--was suddenly
denuded, swept bare, stripped, and soon after, a scourge hitherto
unknown struck terror over the land from Avignon to the Bouches du
Rhone, thence to Marseilles, and then extended its ravages, diminished
indeed by a long career which had partially exhausted its force, over
the whole maritime frontier. The people thought this wind a curse sent
of God. They raised altars to it and offered sacrifices to appease its
rage." It seems, however, that this plague was less destructive than at
present, until the close of the sixteenth century, when further
clearings had removed most of the remaining barriers to its course. Up
to that time, the northwest wind appears not to have attained to the
maximum of specific effect which now characterizes it as a local
phenomenon. Extensive districts, from which the rigor of the seasons has
now banished valuable crops, were not then exposed to the loss of their
harvests by tempests, cold, or drought. The deterioration was rapid in
its progress. Under the Consulate, the clearings had exerted so
injurious an effect upon the climate, that the cultivation of the olive
had retreated several leagues, and since the winters and springs of 1820
and 1836, this branch of rural industry has been abandoned in a great
number of localities where it was advantageously pursued before. The
orange now flourishes only at a few sheltered points of the coast, and
it is threatened even at Ilyeres, where the clearing of the hills near
the town has proved very prejudicial to this valuable tree.
Marchand informs us that, since the felling of the woods, late spring
frosts are more frequent in many localities north of the Alps; that
fruit trees thrive well no longer, and that it is difficult to raise
young trees.[143]
f. _Trees as a Protection against Malaria._
The influence of forests in preventing the diffusion of miasmatic vapors
is a matter of less familiar observation, and perhaps does not come
strictly within the sphere of the present inquiry, but its importance
will justify me in devoting some space to the subject. "It has been
observed" (I quote again from Becquerel) "that humid air, charged with
miasmata, is deprived of them in passing through the forest. Rigaud de
Lille observed localities in Italy where the interposition of a screen
of trees preserved everything beyond it, while the unprotected grounds
were subject to fevers."[144] Few European countries present better
opportunities for observation on this point than Italy, because in that
kingdom the localities exposed to miasmatic exhalations are numerous,
and belts of trees, if not forests, are of so frequent occurrence that
their efficacy in this respect can be easily tested. The belief that
rows of trees afford an important protection against malarious
influences is very general among Italians best qualified by intelligence
and professional experience to judge upon the subject. The commissioners
appointed to report on the measures to be adopted for the improvement of
the Tuscan Maremme advised the planting of three or four rows of
poplars, _Populus alba_, in such directions as to obstruct the currents
of air from malarious localities, and thus intercept a great proportion
of the pernicious exhalations."[145] Lieutenant Maury even believed that
a few rows of sunflowers, planted between the Washington Observatory and
the marshy banks of the Potomac, had saved the inmates of that
establishment from the intermittent fevers to which they had been
formerly liable. Maury's experiments have been repeated in Italy. Large
plantations of sunflowers have been made upon the alluvial deposits of
the Oglio, above its entrance into the Lake of Iseo near Pisogne, and it
is said with favorable results to the health of the neighborhood.[146]
In fact, the generally beneficial effects of a forest wall or other
vegetable screen, as a protection against noxious exhalations from
marshes or other sources of disease situated to the windward of them,
are very commonly admitted.
It is argued that, in these cases, the foliage of trees and of other
vegetables exercises a chemical as well as a mechanical effect upon the
atmosphere, and some, who allow that forests may intercept the
circulation of the miasmatic effluvia of swampy soils, or even render
them harmless by decomposing them, contend, nevertheless, that they are
themselves active causes of the production of malaria. The subject has
been a good deal discussed in Italy, and there is some reason to think
that under special circumstances the influence of the forest in this
respect may be prejudicial rather than salutary, though this does not
appear to be generally the case.[147] It is, at all events, well known
that the great swamps of Virginia and the Carolinas, in climates nearly
similar to that of Italy, are healthy even to the white man, so long as
the forests in and around them remain, but become very insalubrious when
the woods are felled.[148]
_The Forest, as Inorganic Matter, tends to mitigate Extremes._
The surface which trees and leaves present augments the general
superficies of the earth exposed to the absorption of heat, and
increases the radiating and reflecting area in the same proportion. It
is impossible to measure the relative value of these two
elements--increase of absorbing and increase of emitting surface--as
thermometrical influences, because they exert themselves under
infinitely varied conditions; and it is equally impossible to make a
quantitative estimate of any partial, still more of the total effect of
the forest, considered as dead matter, on the temperature of the
atmosphere, and of the portion of the earth's surface acted on by it.
But it seems probable that its greatest influence in this respect is due
to its character of a screen, or mechanical obstacle to the transmission
of heat between the earth and the air; and this is equally true of the
standing tree and of the dead foliage which it deposits in successive
layers at its foot.
The complicated action of trees and their products, as dead absorbents,
radiators, reflectors, and conductors of heat, and as interceptors of
its transmission, is so intimately connected with their effects upon the
humidity of the air and the earth, and with all their living processes,
that it is difficult to separate the former from the latter class of
influences; but upon the whole, the forest must thus far be regarded as
tending to mitigate extremes, and, therefore, as an equalizer of
temperature.
TREES AS ORGANISMS.
_Specific Heat._
Trees, considered as organisms, produce in themselves, or in the air, a
certain amount of heat, by absorbing and condensing atmospheric vapor,
and they exert an opposite influence by absorbing water and exhaling it
in the form of vapor; but there is still another mode by which their
living processes may warm the air around them, independently of the
thermometric effects of condensation and evaporation. The vital heat of
a dozen persons raises the temperature of a room. If trees possess a
specific temperature of their own, an organic power of generating heat,
like that with which the warm-blooded animals are gifted, though by a
different process, a certain amount of weight is to be ascribed to this
element, in estimating the action of the forest upon atmospheric
temperature.
"Observation shows," says Meguscher, "that the wood of a living tree
maintains a temperature of +12 deg. or 13 deg. Cent. [= 54 deg., 56 deg. Fahr.] when the
temperature of the air stands at 3 deg., 7 deg., and 8 deg. [=37 deg., 46 deg., 47 deg. F.]
above zero, and that the internal warmth of the tree does not rise and
fall in proportion to that of the atmosphere. So long as the latter is
below 18 deg. [= 67 deg. Fahr.], that of the tree is always the highest; but if
the temperature of the air rises to 18 deg., that of the vegetable growth is
the lowest. Since, then, trees maintain at all seasons a constant mean
temperature of 12 deg. [= 54 deg. Fahr.], it is easy to see why the air in
contact with the forest must be warmer in winter, cooler in summer, than
in situations where it is deprived of that influence."[149]
Boussingault remarks: "In many flowers there has been observed a very
considerable evolution of heat, at the approach of fecundation. In
certain _arums_ the temperature rises to 40 deg. or 50 deg. Cent. [= 104 deg. or
122 deg. Fahr.]. It is very probable that this phenomenon is general, and
varies only in the intensity with which it is manifested."[150]
If we suppose the fecundation of the flowers of forest trees to be
attended with a tenth only of this calorific power, they could not fail
to exert an important influence on the warmth of the atmospheric strata
in contact with them.
In a paper on Meteorology by Professor Henry, published in the United
States Patent Office Report for 1857, p. 504, that distinguished
physicist observes: "As a general deduction from chemical and mechanical
principles, we think no change of temperature is ever produced where the
actions belonging to one or both of these principles are not present.
Hence, in midwinter, when all vegetable functions are dormant, we do not
believe that any heat is developed by a tree, or that its interior
differs in temperature from its exterior further than it is protected
from the external air. The experiments which have been made on this
point, we think, have been directed by a false analogy. During the
active circulation of the sap and the production of new tissue,
variations of temperature belonging exclusively to the plant may be
observed; but it is inconsistent with general principles that heat
should be generated where no change is taking place."
There can be no doubt that moisture is given out by trees and evaporated
in extremely cold winter-weather, and unless new fluid were supplied
from the roots, the tree would be exhausted of its juices before winter
was over. But this is not observed to be the fact, and, though the point
is disputed, respectable authorities declare that "wood felled in the
depth of winter is the heaviest and fullest of sap."[151] Warm weather
in winter, of too short continuance to affect the temperature of the
ground sensibly, stimulates a free flow of sap in the maple. Thus, in
the last week of December, 1862, and the first week of January, 1863,
sugar was made from that tree, in various parts of New England. "A
single branch of a tree, admitted into a warm room in winter through an
aperture in a window, opened its buds and developed its leaves while the
rest of the tree in the external air remained in its winter sleep."[152]
The roots of forest trees in temperate climates, remain, for the most
part, in a moist soil, of a temperature not much below the annual mean,
through the whole winter; and we cannot account for the uninterrupted
moisture of the tree, unless we suppose that the roots furnish a
constant supply of water.
Atkinson describes a ravine in a valley in Siberia, which was filled
with ice to the depth of twenty-five feet. Poplars were growing in this
ice, which was thawed to the distance of some inches from the stem. But
the surface of the soil beneath it must have remained still frozen, for
the holes around the trees were full of water resulting from its
melting, and this would have escaped below if the ground had been
thawed. In this case, although the roots had not thawed the thick
covering of earth above them, the trunks must have melted the ice in
contact with them. The trees, when observed by Atkinson, were in full
leaf, but it does not appear at what period the ice around their stems
had melted.
From these facts, and others of the like sort, it would seem that "all
vegetable functions are" not absolutely "dormant" in winter, and,
therefore, that trees may give out _some_ heat at that season. But,
however this may be, the "circulation of the sap" commences at a very
early period in the spring, and the temperature of the air in contact
with trees may then be sufficiently affected by heat evolved in the
vital processes of vegetation, to raise the thermometric mean of wooded
countries for that season, and, of course, for the year.[153]
_Total Influence of the Forest on Temperature._
It has not yet been found practicable to measure, sum up, and equate the
total influence of the forest, its processes and its products, dead and
living, upon temperature, and investigators differ much in their
conclusions on this subject. It seems probable that in every particular
case the result is, if not determined, at least so much modified by
local conditions which are infinitely varied, that no general formula is
applicable to the question.
In the report to which I referred on page 149, Gay-Lussac says: "In my
opinion we have not yet any positive proof that the forest has, in
itself, any real influence on the climate of a great country, or of a
particular locality. By closely examining the effects of clearing off
the woods, we should perhaps find that, far from being an evil, it is an
advantage; but these questions are so complicated when they are examined
in a climatological point of view, that the solution of them is very
difficult, not to say impossible."
Becquerel, on the other hand, considers it certain that in tropical
climates, the destruction of the forests is accompanied with an
elevation of the mean temperature, and he thinks it highly probable that
it has the same effect in the temperate zones. The following is the
substance of his remarks on this subject:--
"Forests act as frigorific causes in three ways:
"1. They shelter the ground against solar irradiation and maintain a
greater humidity.
"2. They produce a cutaneous transpiration by the leaves.
"3. They multiply, by the expansion of their branches, the surfaces
which are cooled by radiation.
"These three causes acting with greater or less force, we must, in the
study of the climatology of a country, take into account the proportion
between the area of the forests and the surface which is bared of trees
and covered with herbs and grasses.
"We should be inclined to believe _a priori_, according to the foregoing
considerations, that the clearing of the woods, by raising the
temperature and increasing the dryness of the air, ought to react on
climate. There is no doubt that, if the vast desert of the Sahara were
to become wooded in the course of ages, the sands would cease to be
heated as much as at the present epoch, when the mean temperature is
twenty-nine degrees [centigrade, = 85 deg. Fahr.]. In that case, the
ascending currents of warm air would cease, or be less warm, and would
not contribute, by descending in our latitudes, to soften the climate of
Western Europe. Thus the clearing of a great country may react on the
climates of regions more or less remote from it.
"The observations by Boussingault leave no doubt on this point. This
writer determined the mean temperature of wooded and of cleared points,
under the same latitude, and at the same elevation above the sea, in
localities comprised between the eleventh degree of north and the fifth
degree of south latitude, that is to say, in the portion of the tropics
nearest to the equator, and where radiation tends powerfully during the
night to lower the temperature under a sky without clouds."[154]
The result of these observations, which has been pretty generally
adopted by physicists, is that the mean temperature of cleared land in
the tropics appears to be about one degree centigrade, or a little less
than two degrees of Fahrenheit, above that of the forest. On page 147 of
the volume just cited, Becquerel argues that, inasmuch as the same and
sometimes a greater difference is found in favor of the open ground, at
points within the tropics so elevated as to have a temperate or even a
polar climate, we must conclude that the forests in Northern America
exert a refrigerating influence equally powerful. But the conditions of
the soil are so different in the two regions compared, that I think we
cannot, with entire confidence, reason from the one to the other, and it
is much to be desired that observations be made on the summer and winter
temperature of both the air and the ground in the depths of the North
American forests, before it is too late.[155]
INFLUENCE OF FORESTS ON THE HUMIDITY OF THE AIR AND THE EARTH.
a. _As Inorganic Matter._
The most important influence of the forest on climate is, no doubt, that
which it exercises on the humidity of the air and the earth, and this
climatic action it exerts partly as dead, partly as living matter. By
its interposition as a curtain between the sky and the ground, it
intercepts a large proportion of the dew and the lighter showers, which
would otherwise moisten the surface of the soil, and restores it to the
atmosphere by evaporation; while in heavier rains, the large drops which
fall upon the leaves and branches are broken into smaller ones, and
consequently strike the ground with less mechanical force, or are
perhaps even dispersed into vapor without reaching it.[156] As a screen,
it prevents the access of the sun's rays to the earth, and, of course,
an elevation of temperature which would occasion a great increase of
evaporation. As a mechanical obstruction, it impedes the passage of air
currents over the ground, which, as is well known, is one of the most
efficient agents in promoting evaporation and the refrigeration
resulting from it.[157] In the forest, the air is almost quiescent, and
moves only as local changes of temperature affect the specific gravity
of its particles. Hence there is often a dead calm in the woods when a
furious blast is raging in the open country at a few yards' distance.
The denser the forest--as for example, where it consists of spike-leaved
trees, or is thickly intermixed with them--the more obvious is its
effect, and no one can have passed from the field to the wood in cold,
windy weather, without having remarked it.[158]
The vegetable mould, resulting from the decomposition of leaves and of
wood, carpets the ground with a spongy covering which obstructs the
evaporation from the mineral earth below, drinks up the rains and
melting snows that would otherwise flow rapidly over the surface and
perhaps be conveyed to the distant sea, and then slowly gives out, by
evaporation, infiltration, and percolation, the moisture thus imbibed.
The roots, too, penetrate far below the superficial soil, conduct the
water along their surface to the lower depths to which they reach, and
thus serve to drain the superior strata and remove the moisture out of
the reach of evaporation.
b. _The Forest as Organic._
These are the principal modes in which the humidity of the atmosphere is
affected by the forest regarded as lifeless matter. Let us inquire how
its organic processes act upon this meteorological element.
The commonest observation shows that the wood and bark of living trees
are always more or less pervaded with watery and other fluids, one of
which, the sap, is very abundant in trees of deciduous foliage when the
buds begin to swell and the leaves to develop themselves in the spring.
The outer bark of most trees is of a corky character, not admitting the
absorption of much moisture from the atmosphere through its pores, and
we can hardly suppose that the buds are able to extract from the air a
much larger supply. The obvious conclusion as to the source from which
the extraordinary quantity of sap at this season is derived, is that to
which scientific investigation leads us, namely, that it is absorbed
from the earth by the roots, and thence distributed to all parts of the
plant. Popular opinion, indeed, supposes that all the vegetable fluids,
during the entire period of growth, are thus drawn from the bosom of the
earth, and that the wood and other products of the tree are wholly
formed from matter held in solution in the water abstracted by the roots
from the ground. This is an error, for, not only is the solid matter of
the tree, in a certain proportion not important to our present inquiry,
received from the atmosphere in a gaseous form, through the pores of the
leaves and of the young shoots, but water in the state of vapor is
absorbed and contributed to the circulation, by the same organs.[159]
The amount of water taken up by the roots, however, is vastly greater
than that imbibed through the leaves, especially at the season when the
juices are most abundant, and when, as we have seen, the leaves are yet
in embryo. The quantity of water thus received from the air and the
earth, in a single year, by a wood of even a hundred acres, is very
great, though experiments are wanting to furnish the data for even an
approximate estimate of its measure; for only the vaguest conclusions
can be drawn from the observations which have been made on the
imbibition and exhalation of water by trees and other plants reared in
artificial conditions diverse from those of the natural forest.[160]
_Wood Mosses and Fungi._
Besides the water drawn by the roots from the earth and the vapor
absorbed by the leaves from the air, the wood mosses and fungi, which
abound in all dense forests, take up a great quantity of moisture from
the atmosphere when it is charged with humidity, and exhale it again
when the air is dry. These humble organizations, which play a more
important part in regulating the humidity of the air than writers on the
forest have usually assigned to them, perish with the trees they grow
on; but, in many situations, nature provides a compensation for the tree
mosses in ground species, which, on cold soils, especially those with a
northern exposure, spring up abundantly both before the woods are
felled, and when the land is cleared and employed for pasturage, or
deserted. These mosses discharge a portion of the functions appropriated
to the wood, and while they render the soil of improved lands much less
fit for agricultural use, they, at the same time, prepare it for the
growth of a new harvest of trees, when the infertility they produce
shall have driven man to abandon it and suffer it to relapse into the
hands of nature.[161]
_Flow of Sap._
The amount of sap which can be withdrawn from living trees furnishes,
not indeed a measure of the quantity of water sucked up by their roots
from the ground--for we cannot extract from a tree its whole
moisture--but numerical data which may aid the imagination to form a
general notion of the powerful action of the forest as an absorbent of
humidity from the earth.
The only forest tree known to Europe and North America, the sap of which
is largely enough applied to economical uses to have made the amount of
its flow a matter of practical importance and popular observation, is
the sugar maple, _Acer saccharinum_, of the Anglo-American Provinces and
States. In the course of a single "sugar season," which lasts ordinarily
from twenty-five to thirty days, a sugar maple two feet in diameter will
yield not less than twenty gallons of sap, and sometimes much more.[162]
This, however, is but a trifling proportion of the water abstracted
from the earth by the roots during this season, when the yet undeveloped
leaves can hardly absorb an appreciable quantity of vapor from the
atmosphere;[163] for all this fluid runs from two or three incisions or
auger holes, so narrow as to intercept the current of comparatively few
sap vessels, and besides, experience shows that large as is the quantity
withdrawn from the circulation, it is relatively too small to affect
very sensibly the growth of the tree.[164] The number of large maple
trees on an acre is frequently not less than fifty,[165] and of course
the quantity of moisture abstracted from the soil by this tree alone is
measured by thousands of gallons to the acre. The sugar orchards, as
they are called, contain also many young maples too small for tapping,
and numerous other trees--two of which, at least, the black birch,
_Betula lenta_, and yellow birch, _Betula excelsa_, both very common in
the same climate, are far more abundant in sap than the maple[166]--are
scattered among the sugar trees; for the North American native forests
are remarkable for the mixture of their crops.
The sap of the maple, and of other trees with deciduous leaves which
grow in the same climate, flows most freely in the early spring, and
especially in clear weather, when the nights are frosty and the days
warm; for it is then that the melting snows supply the earth with
moisture in the justest proportion, and that the absorbent power of the
roots is stimulated to its highest activity.[167]
When the buds are ready to burst, and the green leaves begin to show
themselves beneath their scaly covering, the ground has become drier,
the thirst of the roots is quenched, and the flow of sap from them to
the stem is greatly diminished.[168]
_Absorption and Exhalation of Moisture._
The leaves now commence the process of absorption, and imbibe both
uncombined gases and an unascertained but perhaps considerable quantity
of watery vapor from the humid atmosphere of spring which bathes them.
The organic action of the tree, as thus far described, tends to the
desiccation of air and earth; but when we consider what volumes of water
are daily absorbed by a large tree, and how small a proportion of the
weight of this fluid consists of matter which enters into new
combinations, and becomes a part of the solid framework of the
vegetable, or a component of its deciduous products, it is evident that
the superfluous moisture must somehow be carried off almost as rapidly
as it flows into the tree.[169] At the very commencement of vegetation
in spring, some of this fluid certainly escapes through the buds, the
nascent foliage, and the pores of the barb, and vegetable physiology
tells us that there is a current of sap toward the roots as well as from
them.[170] I do not know that the exudation of water into the earth,
through the bark or at the extremities of these latter organs, has been
directly proved, but the other known modes of carrying off the surplus
do not seem adequate to dispose of it at the almost leafless period when
it is most abundantly received, and it is therefore difficult to believe
that the roots do not, to some extent, drain as well as flood the
watercourses of their stem. Later in the season the roots absorb less,
and the now developed leaves exhale a vastly increased quantity of
moisture into the air. In any event, all the water derived by the
growing tree from the atmosphere and the ground is returned again by
transpiration or exudation, after having surrendered to the plant the
small proportion of matter required for vegetable growth which it held
in solution or suspension.[171] The hygrometrical equilibrium is then
restored, so far as this: the tree yields up again the moisture it had
drawn from the earth and the air, though it does not return it each to
each; for the vapor carried off by transpiration greatly exceeds the
quantity of water absorbed by the foliage from the atmosphere, and the
amount, if any, carried back to the ground by the roots.
The evaporation of the juices of the plant, by whatever process
effected, takes up atmospheric heat and produces refrigeration. This
effect is not less real, though much less sensible, in the forest than
in meadow or pasture land, and it cannot be doubted that the local
temperature is considerably affected by it. But the evaporation that
cools the air diffuses through it, at the same time, a medium which
powerfully resists the escape of heat from the earth by radiation.
Visible vapors or clouds, it is well known, prevent frosts by
obstructing radiation, or rather by reflecting back again the heat
radiated by the earth, just as any mechanical screen would do. On the
other hand, clouds intercept the rays of the sun also, and hinder its
heat from reaching the earth. The invisible vapors given out by leaves
impede the passage of heat reflected and radiated by the earth and by
all terrestrial objects, but oppose much less resistance to the
transmission of direct solar heat, and indeed the beams of the sun seem
more scorching when received through clear air charged with uncondensed
moisture than after passing through a dry atmosphere. Hence the
reduction of temperature by the evaporation of moisture from vegetation,
though sensible, is less than it would be if water in the gaseous state
were as impervious to heat given out by the sun as to that emitted by
terrestrial objects.
The hygroscopicity of vegetable mould is much greater than that of any
mineral earth, and therefore the soil of the forest absorbs more
atmospheric moisture than the open ground. The condensation of the vapor
by absorption disengages heat, and consequently raises the temperature
of the soil which absorbs it. Von Babo found the temperature of sandy
earth thus elevated from 20 deg. to 27 deg. centigrade, making a difference of
nearly thirteen degrees of Fahrenheit, and that of soil rich in humus
from 20 deg. to 31 deg. centigrade, a difference of almost twenty degrees of
Fahrenheit.[172]
_Balance of Conflicting Influences._
We have shown that the forest, considered as dead matter, tends to
diminish the moisture of the air, by preventing the sun's rays from
reaching the ground and evaporating the water that falls upon the
surface, and also by spreading over the earth a spongy mantle which
sucks up and retains the humidity it receives from the atmosphere,
while, at the same time, this covering acts in the contrary direction by
accumulating, in a reservoir not wholly inaccessible to vaporizing
influences, the water of precipitation which might otherwise suddenly
sink deep into the bowels of the earth, or flow by superficial channels
to other climatic regions. We now see that, as a living organism, it
tends, on the one hand, to diminish the humidity of the air by absorbing
moisture from it, and, on the other, to increase that humidity by
pouring out into the atmosphere, in a vaporous form, the water it draws
up through its roots. This last operation, at the same time, lowers the
temperature of the air in contact with or proximity to the wood, by the
same law as in other cases of the conversion of water into vapor.
As I have repeatedly said, we cannot measure the value of any one of
these elements of climatic disturbance, raising or lowering of
temperature, increase or diminution of humidity, nor can we say that in
any one season, any one year, or any one fixed cycle, however long or
short, they balance and compensate each other. They are sometimes, but
certainly not always, contemporaneous in their action, whether their
tendency is in the same or in opposite directions, and, therefore, their
influence is sometimes cumulative, sometimes conflicting; but, upon the
whole, their general effect seems to be to mitigate extremes of
atmospheric heat and cold, moisture and drought. They serve as
equalizers of temperature and humidity, and it is highly probable that,
in analogy with most other works and workings of nature, they, at
certain or uncertain periods, restore the equilibrium which, whether as
lifeless masses or as living organisms, they may have temporarily
disturbed.
When, therefore, man destroyed these natural harmonizers of climatic
discords, he sacrificed an important conservative power, though it is
far from certain that he has thereby affected the mean, however much he
may have exaggerated the extremes of atmospheric temperature and
humidity, or, in other words, may have increased the range and
lengthened the scale of thermometric and hygrometric variation.
_Influence of the Forest on Temperature and Precipitation._
Aside from the question of compensation, it does not seem probable that
the forests sensibly affect the total quantity of precipitation, or the
general mean of atmospheric temperature of the globe, or even that they
had this influence when their extent was vastly greater than at present.
The waters cover about three fourths of the face of the earth,[173] and
if we deduct the frozen zones, the peaks and crests of lofty mountains
and their craggy slopes, the Sahara and other great African and Asiatic
deserts, and all such other portions of the solid surface as are
permanently unfit for the growth of wood, we shall find that probably
not one tenth of the total superficies of our planet was ever, at any
one time in the present geological epoch, covered with forests. Besides
this, the distribution of forest land, of desert, and of water, is such
as to reduce the possible influence of the former to a low expression;
for the forests are, in large proportion, situated in cold or temperate
climates, where the action of the sun is comparatively feeble both in
elevating temperature and in promoting evaporation; while, in the torrid
zone, the desert and the sea--the latter of which always presents an
evaporable surface--enormously preponderate. It is, upon the whole, not
probable that so small an extent of forest, so situated, could produce
an appreciable influence on the _general_ climate of the globe, though
it might appreciably affect the local action of all climatic elements.
The total annual amount of solar heat absorbed and radiated by the
earth, and the sum of terrestrial evaporation and atmospheric
precipitation must be supposed constant; but the distribution of heat
and of humidity is exposed to disturbance in both time and place, by a
multitude of local causes, among which the presence or absence of the
forest is doubtless one.
So far as we are able to sum up the general results, it would appear
that, in countries in the temperate zone still chiefly covered with
wood, the summers would be cooler, moister, shorter, the winters milder,
drier, longer, than in the same regions after the removal of the forest.
The slender historical evidence we possess seems to point to the same
conclusion, though there is some conflict of testimony and of opinion on
this point, and some apparently well-established exceptions to
particular branches of what appears to be the general law.
One of these occurs both in climates where the cold of winter is severe
enough to freeze the ground to a considerable depth, as in Sweden and
the Northern States of the American Union, and in milder zones, where
the face of the earth is exposed to cold mountain winds, as in some
parts of Italy and of France; for there, as we have seen, the winter is
believed to extend itself into the months which belong to the spring,
later than at periods when the forest covered the greater part of the
ground.[174] More causes than one doubtless contribute to this result;
but in the case of Sweden and the United States, the most obvious
explanation of the fact is to be found in the loss of the shelter
afforded to the ground by the thick coating of leaves which the forest
sheds upon it, and the snow which the woods protect from blowing away,
or from melting in the brief thaws of winter. I have already remarked
that bare ground freezes much deeper than that which is covered by beds
of leaves, and when the earth is thickly coated with snow, the strata
frozen before it fell begin to thaw. It is not uncommon to find the
ground in the woods, where the snow lies two or three feet deep,
entirely free from frost, when the atmospheric temperature has been for
several weeks below the freezing point, and for some days even below the
zero of Fahrenheit. When the ground is cleared and brought under
cultivation, the leaves are ploughed into the soil and decomposed, and
the snow, especially upon knolls and eminences, is blown off, or
perhaps half thawed, several times during the winter. The water from the
melting snow runs into the depressions, and when, after a day or two of
warm sunshine or tepid rain, the cold returns, it is consolidated to
ice, and the bared ridges and swells of earth are deeply frozen.[175] It
requires many days of mild weather to raise the temperature of soil in
this condition, and of the air in contact with it, to that of the earth
in the forests of the same climatic region. Flora is already plaiting
her sylvan wreath before the corn flowers which are to deck the garland
of Ceres have waked from their winter's sleep; and it is not a popular
error to believe that, where man has substituted his artificial crops
for the spontaneous harvest of nature, spring delays her coming.
In many cases, the apparent change in the period of the seasons is a
purely local phenomenon, which is probably compensated by a higher
temperature in other months, without any real disturbance of the average
thermometrical equilibrium. We may easily suppose that there are
analogous partial deviations from the general law of precipitation; and,
without insisting that the removal of the forest has diminished the sum
total of snow and rain, we may well admit that it has lessened the
quantity which annually falls within particular limits. Various
theoretical considerations make this probable, the most obvious
argument, perhaps, being that drawn from the generally admitted fact,
that the summer and even the mean temperature of the forest is below
that of the open country in the same latitude. If the air in a wood is
cooler than that around it, it must reduce the temperature of the
atmospheric stratum immediately above it, and, of course, whenever a
saturated current sweeps over it, it must produce precipitation which
would fall upon or near it.
But the subject is so exceedingly complex and difficult, that it is
safer to regard it as a historical problem, or at least as what lawyers
call a mixed question of law and fact, than to attempt to decide it upon
_a priori_ grounds. Unfortunately the evidence is conflicting in
tendency, and sometimes equivocal in interpretation, but I believe that
a majority of the foresters and physicists who have studied the question
are of opinion that in many, if not in all cases, the destruction of the
woods has been followed by a diminution in the annual quantity of rain
and dew. Indeed, it has long been a popularly settled belief that
vegetation and the condensation and fall of atmospheric moisture are
reciprocally necessary to each other, and even the poets sing of
Afric's barren sand,
Where nought can grow, because it raineth not,
And where no rain can fall to bless the land,
Because nought grows there.[176]
Before stating the evidence on the general question and citing the
judgments of the learned upon it, however, it is well to remark that the
comparative variety or frequency of inundations in earlier and later
centuries is not necessarily, in most cases not probably, entitled to
any weight whatever, as a proof that more or less rain fell formerly
than now; because the accumulation of water in the channel of a river
depends far less upon the quantity of precipitation in its valley, than
upon the rapidity with which it is conducted, on or under the surface of
the ground, to the central artery that drains the basin. But this point
will be more fully discussed in a subsequent chapter.
There is another important observation which may properly be introduced
here. It is not universally, or even generally true, that the atmosphere
returns its humidity to the local source from which it receives it. The
air is constantly in motion,
----howling tempests scour amain
From sea to land, from land to sea;[177]
and, therefore, it is always probable that the evaporation drawn up by
the atmosphere from a given river, or sea, or forest, or meadow, will be
discharged by precipitation, not at or near the point where it rose, but
at a distance of miles, leagues, or even degrees. The currents of the
upper air are invisible, and they leave behind them no landmark to
record their track. We know not whence they come, or whither they go. We
have a certain rapidly increasing acquaintance with the laws of general
atmospheric motion, but of the origin and limits, the beginning and end
of that motion, as it manifests itself at any particular time and place,
we know nothing. We cannot say where or when the vapor, exhaled to-day
from the lake on which we float, will be condensed and fall; whether it
will waste itself on a barren desert, refresh upland pastures, descend
in snow on Alpine heights, or contribute to swell a distant torrent
which shall lay waste square miles of fertile corn land; nor do we know
whether the rain which feeds our brooklets is due to the transpiration
from a neighboring forest, or to the evaporation from a far-off sea. If,
therefore, it were proved that the annual quantity of rain and dew is
now as great on the plains of Castile, for example, as it was when they
were covered with the native forest, it would by no means follow that
those woods did not augment the amount of precipitation elsewhere.
But I return to the question. Beginning with the latest authorities, I
cite a passage from Clave.[178] After arguing that we cannot reason from
the climatic effects of the forest in tropical and sub-tropical
countries as to its influence in temperate latitudes, the author
proceeds: "The action of the forests on rain, a consequence of that
which they exercise on temperature, is difficult to estimate in our
climate, but is very pronounced in hot countries, and is established by
numerous examples. M. Boussingault states that in the region comprised
between the Bay of Cupica and the Gulf of Guayaquil, which is covered
with immense forests, the rains are almost continual, and that the mean
temperature of this humid country rises hardly to twenty-six degrees (=
80 deg. Fahr.). M. Blanqui, in his 'Travels in Bulgaria,' informs us that at
Malta rain has become so rare, since the woods were cleared to make room
for the growth of cotton, that at the time of his visit in October,
1841, not a drop of rain had fallen for three years.[179] The terrible
droughts which desolate the Cape Verd Islands must also be attributed to
the destruction of the forests. In the Island of St. Helena, where the
wooded surface has considerably extended within a few years, it has been
observed that the rain has increased in the same proportion. It is now
in quantity double what it was during the residence of Napoleon. In
Egypt, recent plantations have caused rains, which hitherto were almost
unknown."
Schacht[180] observes: "In wooded countries, the atmosphere is generally
humid, and rain and dew fertilize the soil. As the lightning rod
abstracts the electric fluid from the stormy sky, so the forest attracts
to itself the rain from the clouds, which, in falling, refreshes not it
alone, but extends its benefits to the neighboring fields. * * The
forest, presenting a considerable surface for evaporation, gives to its
own soil and to all the adjacent ground an abundant and enlivening dew.
There falls, it is true, less dew on a tall and thick wood than on the
surrounding meadows, which, being more highly heated during the day by
the influence of insolation, cool with greater rapidity by radiation.
But it must be remarked, that this increased deposition of dew on the
neighboring fields is partly due to the forests themselves; for the
dense, saturated strata of air which hover over the woods descend in
cool, calm evenings, like clouds, to the valley, and in the morning,
beads of dew sparkle on the leaves of the grass and the flowers of the
field. Forests, in a word, exert, in the interior of continents, an
influence like that of the sea on the climate of islands and of coasts:
both water the soil and thereby insure its fertility." In a note upon
this passage, quoting as authority the _Historia de la Conquista de las
siete islas de Gran Canaria, de Juan de Abreu Galindo_, 1632, p. 47, he
adds: "Old historians relate that a celebrated laurel in Ferro formerly
furnished drinkable water to the inhabitants of the island. The water
flowed from its foliage, uninterruptedly, drop by drop, and was
collected in cisterns. Every morning the sea breeze drove a cloud toward
the wonderful tree, which attracted it to its huge top," where it was
condensed to a liquid form.
In a number of the _Missionary Herald_, published at Boston, the date of
which I have mislaid, the Rev. Mr. Van Lennep, well known as a competent
observer, gives the following remarkable account of a similar fact
witnessed by him in an excursion to the east of Tocat in Asia Minor:
"In this region, some 3,000 feet above the sea, the trees are mostly
oak, and attain a large size. I noticed an illustration of the influence
of trees in general in collecting moisture. Despite the fog, of a week's
duration, the ground was everywhere perfectly dry. The dry oak leaves,
however, had gathered the water, and the branches and trunks of the
trees were more or less wet. In many cases the water had run down the
trunk and moistened the soil around the roots of the tree. In two
places, several trees had each furnished a small stream of water, and
these, uniting, had run upon the road, so that travellers had to pass
through the mud; although, as I said, everywhere else the ground was
perfectly dry. Moreover, the collected moisture was not sufficient to
drop directly from the leaves, but in every case it ran down the
branches and trunk to the ground. Farther on we found a grove, and at
the foot of each tree, on the north side, was a lump of ice, the water
having frozen as it reached the ground. This is a most striking
illustration of the acknowledged influence of trees in collecting
moisture; and one cannot for a moment doubt, that the parched regions
which commence at Sivas, and extend in one direction to the Persian
Gulf, and in another to the Red Sea, were once a fertile garden, teeming
with a prosperous population, before the forests which covered the
hillsides were cut down--before the cedar and the fir tree were rooted
up from the sides of Lebanon.
"As we now descended the northern side of the watershed, we passed
through the grove of walnut, oak, and black mulberry trees, which shade
the village of Oktab, whose houses, cattle, and ruddy children were
indicative of prosperity."
Coultas thus argues: "The ocean, winds, and woods may be regarded as the
several parts of a grand distillatory apparatus. The sea is the boiler
in which vapor is raised by the solar heat, the winds are the guiding
tubes which carry the vapor with them to the forests where a lower
temperature prevails. This naturally condenses the vapor, and showers of
rain are thus distilled from the cloud masses which float in the
atmosphere, by the woods beneath them."[181]
Sir John F. W. Herschel enumerates among "the influences unfavorable to
rain," "absence of vegetation in warm climates, and especially of trees.
This is, no doubt," continues he, "one of the reasons of the extreme
aridity of Spain. The hatred of a Spaniard toward a tree is proverbial.
Many districts in France have been materially injured by denudation
(Earl of Lovelace on Climate, etc.), and, on the other hand, rain has
become more frequent in Egypt since the more vigorous cultivation of the
palm tree."
Hohenstein remarks: "With respect to the temperature in the forest, I
have already observed that, at certain times of the day and of the year,
it is less than in the open field. Hence the woods may, in the daytime,
in summer and toward the end of winter, tend to increase the fall of
rain; but it is otherwise in summer nights and at the beginning of
winter, when there is a higher temperature in the forest, which is not
favorable to that effect. * * * The wood is, further, like the mountain,
a mechanical obstruction to the motion of rain clouds, and, as it checks
them in their course, it gives them occasion to deposit their water.
These considerations render it probable that the forest increases the
quantity of rain; but they do not establish the certainty of this
conclusion, because we have no positive numerical data to produce on the
depression of temperature, and the humidity of the air in the
woods."[182]
Barth presents the following view of the subject: "The ground in the
forest, as well as the atmospheric stratum over it, continues humid
after the woodless districts have lost their moisture; and the air,
charged with the humidity drawn from them, is usually carried away by
the winds before it has deposited itself in a condensed form on the
earth. Trees constantly transpire through their leaves a great quantity
of moisture, which they partly absorb again by the same organs, while
the greatest part of their supply is pumped up through their widely
ramifying roots from considerable depths in the ground. Thus a constant
evaporation is produced, which keeps the forest atmosphere moist even in
long droughts, when all other sources of humidity in the forest itself
are dried up. * * * Little is required to compel the stratum of air
resting upon a wood to give up its moisture, which thus, as rain, fog,
or dew, is returned to the forest. * * * The warm, moist currents of
air which come from other regions are cooled as they approach the wood
by its less heated atmosphere, and obliged to let fall the humidity with
which they are charged. The woods contribute to the same effect by
mechanically impeding the motion of fog and rain cloud, whose particles
are thus accumulated and condensed to rain. The forest thus has a
greater power than the open ground to retain within its own limits
already existing humidity, and to preserve it, and it attracts and
collects that which the wind brings it from elsewhere, and forces it to
deposit itself as rain or other precipitation. * * * In consequence of
these relations of the forest to humidity, it follows that wooded
districts have both more frequent and more abundant rain, and in general
are more humid, than woodless regions; for what is true of the woods
themselves, in this respect, is true also of their treeless
neighborhood, which, in consequence of the ready mobility of the air and
its constant changes, receives a share of the characteristics of the
forest atmosphere, coolness and moisture. * * * When the districts
stripped of trees have long been deprived of rain and dew, * * * and the
grass and the fruits of the field are ready to wither, the grounds which
are surrounded by woods are green and flourishing. By night they are
refreshed with dew, which is never wanting in the moist air of the
forest, and in due season they are watered by a beneficent shower, or a
mist which rolls slowly over them."[183]
Asbjoernsen, after adducing the familiar theoretical arguments on this
point, adds: "The rainless territories in Peru and North Africa
establish this conclusion, and numerous other examples show that woods
exert an influence in producing rain, and that rain fails where they are
wanting; for many countries have, by the destruction of the forests,
been deprived of rain, moisture, springs, and watercourses, which are
necessary for vegetable growth. * * * The narratives of travellers show
the deplorable consequences of felling the woods in the Island of
Trinidad, Martinique, San Domingo, and indeed, in almost the entire
West Indian group. * * * In Palestine and many other parts of Asia and
Northern Africa, which in ancient times were the granaries of Europe,
fertile and populous, similar consequences have been experienced. These
lands are now deserts, and it is the destruction of the forests alone
which has produced this desolation. * * * In Southern France, many
districts have, from the same cause, become barren wastes of stone, and
the cultivation of the vine and the olive has suffered severely since
the baring of the neighboring mountains. Since the extensive clearings
between the Spree and the Oder, the inhabitants complain that the clover
crop is much less productive than before. On the other hand, examples of
the beneficial influence of planting and restoring the woods are not
wanting. In Scotland, where many miles square have been planted with
trees, this effect has been manifest, and similar observations have been
made in several places in Southern France. In Lower Egypt, both at Cairo
and near Alexandria, rain rarely fell in considerable quantity--for
example, during the French occupation of Egypt, about 1798, it did not
rain for sixteen months--but since Mehemet Aali and Ibrahim Pacha
executed their vast plantations (the former alone having planted more
than twenty millions of olive and fig trees, cottonwood, oranges,
acacias, planes, &c.), there now falls a good deal of rain, especially
along the coast, in the months of November, December, and January; and
even at Cairo it rains both oftener and more abundantly, so that real
showers are no rarity."[184]
Babinet, in one of his lectures,[185] cites the supposed fact of the
increase of rain in Egypt in consequence of the planting of trees, and
thus remarks upon it: "A few years ago it never rained in Lower Egypt.
The constant north winds, which almost exclusively prevail there, passed
without obstruction over a surface bare of vegetation. Grain was kept
on the roofs in Alexandria, without being covered or otherwise
protected from injury by the atmosphere; but since the making of
plantations, an obstacle has been created which retards the current of
air from the north. The air thus checked, accumulates, dilates, cools,
and yields rain.[186] The forests of the Vosges and Ardennes produce
the same effects in the north east of France, and send us a great river,
the Meuse, which is as remarkable for its volume as for the small extent
of its basin. With respect to the retardation of the atmospheric
currents, and the effects of that retardation, one of my illustrious
colleagues, M. Mignet, who is not less a profound thinker than an
eloquent writer, suggested to me that, to produce rain, a forest was as
good as a mountain, and this is literally true."
Monestier-Savignat arrives at this conclusion: "Forests on the one hand
diminish evaporation; on the other, they act on the atmosphere as
refrigerating causes. The second scale of the balance predominates over
the other, for it is established that in wooded countries it rains
oftener, and that, the quantity of rain being equal, they are more
humid."[187]
Boussingault--whose observations on the drying up of lakes and springs,
from the destruction of the woods, in tropical America, have often been
cited as a conclusive proof that the quantity of rain was thereby
diminished--after examining the question with much care, remarks: "In my
judgment it is settled that very large clearings must diminish the
annual fall of rain in a country;" and on a subsequent page, he
concludes that, "arguing from meteorological facts collected in the
equinoctial regions, there is reason to presume that clearings diminish
the annual fall of rain."[188]
The same eminent author proposes series of observations on the level of
natural lakes, especially on those without outlet, as a means of
determining the increase or diminution of precipitation in their basins,
and, of course, of measuring the effect of clearing when such
operations take place within those basins. But it must be observed that
lakes without a visible outlet are of very rare occurrence, and besides,
where no superficial conduit for the discharge of lacustrine waters
exists, we can seldom or never be sure that nature has not provided
subterranean channels for their escape. Indeed, when we consider that
most earths, and even some rocks under great hydrostatic pressure, are
freely permeable by water, and that fissures are frequent in almost all
rocky strata, it is evident that we cannot know in what proportion the
depression of the level of a lake is to be ascribed to infiltration, to
percolation, or to evaporation.[189] Further, we are, in general, as
little able to affirm that a given lake derives all its water from the
fall of rain within its geographical basin, or that it receives all the
water that falls in that basin except what evaporates from the ground,
as we are to show that all its superfluous water is carried off by
visible channels and by evaporation.
Suppose the strata of the mountains on two sides of a lake, east and
west, to be tilted in the same direction, and that those of the hill on
the east side incline toward the lake, those of that on the west side
from it. In this case a large proportion of the rain which falls on the
eastern slope of the eastern hill may find its way between the strata to
the lake, and an equally large proportion of the precipitation upon the
eastern slope of the western ridge may escape out of the basin by
similar channels. In such case the clearing of the _outer_ slopes of
either or both mountains, while the forests of the _inner_ declivities
remained intact, might affect the quantity of water received by the
lake, and it would always be impossible to know to what territorial
extent influences thus affecting the level of a lake might reach.
Boussingault admits that extensive clearing _below_ an alpine lake, even
at a considerable distance, might affect the level of its waters. How it
would produce this influence he does not inform us, but, as he says
nothing of the natural subterranean drainage of surface waters, it is to
be presumed that he refers to the supposed diminution of the quantity of
rain from the removal of the forest, which might manifest itself at a
point more elevated than the cause which occasioned it. The elevation or
depression of the level of natural lakes, then, cannot be relied upon as
a proof, still less as a measure of an increase or diminution in the
fall of rain within their geographical basins, resulting from the
felling of the woods which covered them; though such phenomena afford
very strong presumptive evidence that the supply of water is somehow
augmented or lessened. The supply is, in most cases, derived much less
from the precipitation which falls directly upon the surface of lakes,
than from waters which flow above or under the ground around them, and
which, in the latter case, often come from districts not comprised
within what superficial geography would regard as belonging to the lake
basins.
It is, upon the whole, evident that the question can hardly be
determined except by the comparison of pluviometrical observations made
at a given station before and after the destruction of the woods. Such
observations, unhappily, are scarcely to be found, and the opportunity
for making them is rapidly passing away, except so far as a converse
series might be collected in countries--France, for example--where
forest plantation is now going on upon a large scale. The Smithsonian
Institution at Washington is well situated for directing the attention
of observers in the newer territory of the United States to this
subject, and it is to be hoped that it will not fail to avail itself of
its facilities for this purpose.
Numerous other authorities might be cited in support of the proposition
that forests tend, at least in certain latitudes and at certain seasons,
to produce rain; but though the arguments of the advocates of this
doctrine are very plausible, not to say convincing, their opinions are
rather _a priori_ conclusions from general meteorological laws, than
deductions from facts of observation, and it is remarkable that there is
so little direct evidence on the subject.
On the other hand, Foissac expresses the opinion that forests have no
influence on precipitation, beyond that of promoting the deposit of dew
in their vicinity, and he states, as a fact of experience, that the
planting of large vegetables, and especially of trees, is a very
efficient means of drying morasses, because the plants draw from the
earth a quantity of water larger than the average annual fall of
rain.[190] Kloeden, admitting that the rivers Oder and Elbe have
diminished in quantity of water, the former since 1778, the latter since
1828, denies that the diminution of volume is to be ascribed to a
decrease of precipitation in consequence of the felling of the forests,
and states, what other physicists confirm, that, during the same period,
meteorological records in various parts of Europe show rather an
augmentation than a reduction of rain.[191]
The observations of Belgrand tend to show, contrary to the general
opinion, that less rain falls in wooded than in denuded districts. He
compared the precipitation for the year 1852, at Vezelay in the valley
of the Bouchat, and at Avallon in the valley of the Grenetiere. At the
first of these places it was 881 millimetres, at the latter 581
millimetres. The two cities are not more than eight miles apart. They
are at the same altitude, and it is stated that the only difference in
their geographical conditions consists in the different proportions of
forest and cultivated country around them, the basin of the Bouchat
being entirely bare, while that of the Grenetiere is well wooded.[192]
Observations in the same valleys, considered with reference to the
seasons, show the following pluviometric results:
FOR LA GRENETIERE.
February, 1852, 42.2 millimetres precipitation.
November, " 23.8 " "
January, 1853, 35.4 " "
-----
Total, 106.4 in three cold months.
September, 1851, 27.1 millimetres precipitation.
May, 1852, 20.9 " "
June, " 56.3 " "
July, " 22.8 " "
September, " 22.8 " "
-----
Total, 149.9 in five warm months.
FOR LE BOUCHAT.
February, 1852, 51.3 millimetres precipitation.
November, " 36.6 " "
January, 1853, 92.0 " "
-----
Total, 179.9 in three cold months.
September, 1851, 43.8 millimetres precipitation.
May, 1852, 13.2 " "
June, " 55.5 " "
July, " 19.5 " "
September, " 26.5 " "
-----
Total, 158.5 in five warm months.
These observations, so far as they go, seem to show that more rain falls
in cleared than in wooded countries, but this result is so contrary to
what has been generally accepted as a theoretical conclusion, that
further experiment is required to determine the question.
Becquerel--whose treatise on the climatic effects of the destruction of
the forest is the fullest general discussion of that subject known to
me--does not examine this particular point, and as, in the summary of
the results of his investigations, he does not ascribe to the forest any
influence upon precipitation, the presumption is that he rejects the
doctrine of its importance as an agent in producing the fall of rain.