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Transactions of the Highland and Agricultural Society of Scotland
On the effects produced on Trees and Shrubs by Smoke from Public Works


By Robert Hutchison of Carlowrie, Kirkliston
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The rapid expansion of the commercial and manufacturing activity of the United Kingdom, during the past quarter of a century, in nearly every part of the country, and the consequent-erection and enlargement of public works of various descriptions, —while of paramount importance to the social well-being and industry of teeming thousands of the population,—are, nevertheless, inseparably connected with the origin of latent evils which affect in a greater or less degree, according to circumstances, in the vicinity of such manufactories, the amenity and salubrity of the district, the growth and health of the adjacent crops within their influence, and the vitality of vegetation generally from the tiniest blade of grass to the tree of giant dimensions.

These pernicious influences are of various kinds and degrees, affecting the atmosphere, the soil, the water supply, or, it may be, all these together. For simplification they may be classed under the following heads:—

1st, Pollution of water-supply, and food of plants by chemical discharges from public works into open drains, water courses, or rivers.

2nd, Saturation of the soil from deleterious matters filtrating into it, and its consequent destruction for plant-life.

3rd, Atmospheric pollution by smoke, and other noxious vapours or gaseous discharges, whereby the pure natural air is vitiated, and rendered injurious to the respiratory organs of vegetable life, and an artificial atmosphere, as it were, by long continuance of constant smoke exhalations, is created, inimical to growth of plants, from its containing in excess large amounts of sulphurous and acid vapours.

Sometimes the effects so produced are individually sufficient to constitute a serious sanitary objection to the continuance of the works which cause them; but in many instances, where the intelligent practical application of chemical science and engineering has been called in to aid in mitigating or entirely removing the evils complained of, success has been achieved, without injuriously interfering with the industrial pursuits of those engaged in the particular manufacture; and it is to be hoped that in many other cases, where no remedial measures have as yet been attempted,—manufacturers will yet see it to be their duty,—while carrying on advantageously to themselves their commercial enterprizes,—to use every effort which science and experience may discover or suggest, to mitigate evils inseparably connected with their processes, but which are capable of amelioration in the interests of the amenities of nature and their neighbourhood generally.

With the clamant evils of river pollution, the government, last session, endeavoured (unfortunately without success) to grapple, but the subject is so important, that once fairly roused, as it has been, it will probably again form matter for legislation, until successfully settled; and in that way an immense source of destruction to vegetable life will be put an end to. With the gaseous vapours and smoke discharges from large public works polluting the air for miles around, the question is a more difficult one, and a problem of greater intricacy, and it is with the effects produced by this species of destruction to trees and shrubs that we have in this paper more particularly to deal at present.

Without specifying in detail the various noxious chemical ingredients discharged from large public works, and which differ in the degree and extent of injury which they create, according to the nature of their chemical composition, and vary according to the special description of work or process whence they emanate, it may be generally stated that the primary and chief destruction to vegetation in the neighbourhood of such works, or large towns, is caused by the smoke discharges from them, arising from the combustion of coal in furnaces, and by the gases and vapours that are produced during metallurgical processes of the chemical arts and manufactures. Vegetable life, in general, is very susceptible of impurities existing in the air around it, and even slight modifications in the proportions of its constituent parts in their natural forms, more or less injuriously affect all growths, and chiefly those in shady or sheltered spots where there may be, to some extent, want of light, or defective circulation of air. The wonder, therefore, rather should be, that in the vicinity of large cities and manufacturing centres of industry, and sometimes even in their very midst, we see trees and shrubs existing as they do, and sustaining life under such abnormal conditions, with comparative hardihood. But, as will be afterwards explained, nature placed in circumstances so uncongenial may be artificially assisted in many ways, and induced, with a little attention and care to her subjects, to cheer and brighten with her freshness and greenery many a dismal town square or city walk.

In the immediate neighbourhood of chemical works, from which, by day and by night in ceaseless volume, large quantities of poisonous exhalations are poured forth into the atmosphere, the living functions of vegetation and tree-life are much more seriously crippled than by the mere smoke from coal-consumption near towns, and in such situations herbage of all kinds is stunted and browned, the very grass lingers on a feeble existence, trees are leafless and withered, and in a very few years cease to live. The same remarks apply to plants and trees in close proximity to the calcining hearths of ironstone pits and blast furnaces. The discharge from these of smoke strongly impregnated with the sulphate of alumina is highly deleterious to all life. Blown by a strong and steadily prevailing wind across any district, however fertile and highly farmed and cultivated it may be, the pernicious effects to cereals and green crops is most apparent, and cause great deterioration and damages annually to many farmers. In many parts of Lanarkshire, Fife, and Stirlingshire, the losses caused by these effects are severely felt, and although compensation by pecuniary payment be made in most cases where the damage has been proved, the indirect deterioration to the farmer, from lack of straw, and head, and bulb, from their more stinted growth, caused by the agency of this nuisance, is not compensated for by any pecuniary allowance. Again, in some districts in Ayrshire and West Lothian, as well as in Lanarkshire, the damage caused to dairy farms from the destruction to their produce by the deposit of soot-flakes, from these and other works, such as the shale oil manufactories, is very great; and it can easily be understood that the permanent injury to the perennial occupants of the ground, such as trees and shrubs, must be also very marked. If it were not for the presence of the chemicals already referred to, in the smoke issuing from these and kindred works, the mischief would not be nearly so serious, as the winter's snow and rain bleach the merely smoke-begrimmed trunk and branches, and thus clear out the plugged-up pores to a great extent periodically.

Probably the most fatal enemy to plant-life is an acid vapour termed hydrochloric acid (a mixture of hydrogen and chlorine). It is evolved among the dense white fumes issuing from most chemical works, and in a very short time causes absolute sterility to all land within its influence. Its poisonous effects upon a tree or shrub are first observed by the shrivelling up and drooping of the leaves, which in a few days are shed. The young wood buds lose their plump, healthy aspect, becoming-scaly and falling off; the twigs then present a dead appearance, which soon spreads itself down the branches, the bark cracks and shrivels off, here and there on the stem, a young bud may in spring be seen struggling in the vain effort to develop itself into a new branch to sustain the failing vitality, but very soon these last attempts at self-resuscitation cease, and the tree dies. The effects produced upon tree and shrub life by smoke from public works are similar in operation to those now described, and when the functions of the leaves in vegetable economy are considered, it will be easily understood how the health of plants is interfered with by these effects.

As the function mainly of the foliage of a tree or shrub, is to expose the secretions of the plant to light and air, and so to assist in the formation of wood, if the leaves are coated over by dense deposits of carbon and other substances, a film is formed sufficiently opaque to prevent the free action of light and air upon the leaves, and hence the secretions necessary for the formation of wood-buds are impeded in their development. Hence we see how stunted in stature, and in thickness of trunk, are trees in the vicinity of towns as contrasted with others of similar age planted in the open country, or in rural districts with an uncontaminated atmosphere.

Again, leaves have another most important function to perform in the healthy economy of trees and shrubs. They daily give off into the atmosphere large quantities of watery fluid by the process which they perform, called in technical terms exhalation, The importance of this function in relation to smoke deposit on the leaves, must be at once apparent. The moisture on the surface of the leaf, created by its healthy action, at once attracts the minute particles of carbon and other injurious ingredients which thereby adhere to the leaves, and, coating them over, at once arrest the healthy process of exhalation over the whole system of the plant; for as the leaf surface cannot find vent for the secretions pumped up from the root action, this latter function is next impaired, and a drag, so to speak, is put upon its healthy flow. The humidity of air which trees promote in their immediate proximity, by the process of exhalation, is very materially lessened by its cessation, and the moist and cool condition of soil, which is so essential to their welfare, is in its turn affected, and the direct action of the sun's rays has more power and opportunity to cause undue evaporation. And with a decreased healthy action of foliage, or with a scant crop of leaves induced by the effects of such substances as have been referred to, another very important agency in sanitary economy is interfered with; for while leaves, doubtless, in the first instance, by the healthy action of their various functions, act for their own benefit; they, at the same time materially influence and conduce to the purity of the atmosphere itself, by their absorption of carbonic acid gas, which in excess is so injurious and fatal to animal life. Under the influence of light, leaves in a healthy state, and young green twigs and shoots of shrubs and trees, decompose this deleterious gas, and so contribute largely to the preservation of the purity of the air for man's own breathing.

But not only upon the leaves of trees and shrubs are the effects of smoke and other deleterious substances apparent, nor is it through their medium alone that the life of the plant is destroyed. The lateral air-vessels which penetrate the bark and stems of trees horizontally for the purpose of "oxygenating," as it may be called, the juices and secretions, form another channel by which the injurious and subtle poisons are conveyed into the system of the plant. The plugging-up of the mouths of these surface-absorbents prevents the inhalation by the bark of the moisture of the atmosphere, and in this way, acts prejudicially upon the growth and development of the alburnum or sap-wood, as well as upon the expansion of the new buds, and consequently upon the ultimate growth of the tree itself. A familiar instance may be given of the injury which accrues to a tree by the adherence of a foreign substance to the surface of the bark, if we notice the effects produced by smearing the stem with coal-tar, oil, or paint, so as to completely stop up the spiracula or pores of the bark for some considerable length up the trunk from the ground. The result will be found to correspond precisely with what has been stated with regard to smoke clogging the bark from atmospheric causes. The new buds will first be destroyed, and finally the tree itself.

Upon the various descriptions of trees and shrubs the effects of smoke or a polluted atmosphere are not always equally severe or rapid. Doubtless plants, like animals, may individually vary in regard to the delicacy of their constitution, and while some are much more susceptible than others of the alterations in the temperature and humidity of the atmosphere, it is quite possible that some plants may be injuriously, if not even fatally, affected by the presence of foreign inimical matter in the air, or soil, or food, from which others of the same species, of a hardier and more robust constitution, would scarcely suffer. But independently of this, there are species of trees and shrubs which are found to thrive better than other species do when placed by the force of circumstances in situations inimical to their nature, or under conditions which injuriously affect plant life generally.

Coniferous trees and shrubs will not thrive—the phrase must be taken as comparative—in any polluted atmosphere; for as we have shown that trees and shrubs breathe through both the bark and the leaf, such species as exude or secrete gum or resin in any quantity are peculiarly liable to suffer from polluted air. In the case of such trees, soot and such-like substances cling to them, and when their resinous exudations become coated with carbon, the rain has no effect in removing it, but rather the reverse, as it washes down aqueous volumes of soot-impregnated water upon their branches and bark. In like manner, hard-wooded trees, possessing gummy or sticky buds and leaf sheaths, suffer in a similar way; and should not be planted in the near proximity of large public works, or in the open spaces in the midst of densely populated towns.

The varieties of trees and shrubs which will be found to thrive best (again using the phrase in a comparative sense) in smoky atmospheres, are deciduous hardwoods with smooth leaf-surfaces and scaly barks ; as for example, trees like the plane or the maple. From the smooth upper surface of the leaf, much of the coating deposit which settles there is easily washed off by rain, while the constantly peeling off habit of the scales of the outer bark keeps up a recuperative process in the cells and pores communicating with the alburnum and inner bark surface for the oxygenation of the sap.

Many evergreens also possess remarkable vitality and power of resisting the baneful influences of atmospheric pollution. As a rule, such plants as possess a thick leathery leaf with smooth surface endure the effects of smoke with greatest impunity; such, for example, as the common ivy and Aucuba japonica, whose glossy leaves are easily cleansed, and consequently the epidermis is kept in a more healthy condition from the action of the rain, than is the case with those plants and bushes with a downy or hairy foliage. But although some evergreens may succeed well in smoky atmospheres, there seems no doubt that, owing to the complete renewal annually of their foliage, deciduous trees and shrubs have a marked advantage in those situations over the evergreen species; and where it is intended to plant under such trying circumstances, deciduous trees and shrubs should be preferred, and such varieties of them as we have indicated as possessing special qualifications for withstanding the injurious effects of an impure atmosphere should be selected.

The following list comprises such trees and shrubs as seem most adapted for culture in these situations:—Platanus occidentalis, Acer eriocarpum, Populus balsamifera, Populus fastigiata, Populus alba, Quercus ilex, Tilia europrea, Fraxinus, Robinia, Cytisus Laburnum, Syringa, Ulmus, Ligustrum, Vinca, Viburnum Tinus, Philadelphus, Crataegus, Ampelopsis hederacea, Clematis, Aucuba japonica, Ailanthus glandulosa, Ficus Carica, Cydonia japonica, Hedera helix, Jasminum officinale, Rhamnus alaternus, Ribes sanguinea, Sophora japonica, Ilex aquifolium, Sambucus, and Leycesteria formosa.

Of course, in naming these it must be quite understood that their success in such situations is merely comparative, and such of them as may be classed as flowering shrubs cannot be expected to blossom at all freely under the disadvantageous circumstances of their situation.

But it may be said, are there no remedies for the pollution of the atmosphere by public works and smoke-creating nuisances? This is a very delicate question, but one which will stand discussion, inasmuch as the Public Health Acts, the Smoke Consumption Acts, the Alkali Acts (in England), all seem more or less to have been framed for the suppression of such noxious discharges as poison air and destroy the amenity of whole neighbourhoods. A more stringent application of their enactments and enforcement of their provisions would doubtless in many places tend greatly to the mitigation, if not the abolition, of the evils complained of, where the manufacturers themselves (the pollutors) are disposed to act fairly and liberally. But in many other instances, such, for example, as calcining of ironstone, it is next to impossible, without absolutely stopping the process, to cure the evil. No doubt, when the injury to crops became very serious the operation of calcining might be prohibited, excepting during the winter season or non-growing period of the year, when there would be no risk of damage to crops, whether cereal or otherwise. Indeed this remedy is practised in some districts where the ironstone is wrought in connection with coal, and is found in too thin a state, or in such small quantities that the damages to be paid for smoke pollution to crops would outrun the value of the calcined mineral. The stone is accordingly raised with the coal and "binged" on the hearth till the harvest is carried, when the fires are lit and the process completed.

Much, however, might be done by artificial means to aid nature in many instances when placed in situations so uncongenial to her well-being, as in the midst of towns or in their immediate vicinity. A very few adventitious appliances of no costly description would tend greatly to compensate to many town gardens and shrubberies for the conditions inseparable from their lot. These may be briefly summed up in the following recommendations:—(1) Give more free space around each individual tree; (2) Prune early, regularly, and judiciously; (3) Soak well during drought the entire garden; and (4) Top-dress with fresh soil over the roots and shrubberies occasionally. The first is rendered absolutely necessary by the confined circulation of air in town situations and vitiated conditions of the atmosphere, combined with the frequent absence of light in such localities ; and which is so essential an element in tree-growth. The second suggestion is proposed, with a view to the proper balance of head and due development of upright stem, and to prevent a flat-headed habit being acquired, to which trees in such sites are extremely prone—a habit which results in one or more of the stronger-growing side branches outrunning their neighbours and interfering with the upright form of the tree. Branch and stem pruning, and foreshortening the upper tiers of branches in the head, will be found most beneficial in trees so situated, when about from twelve to twenty feet in height; although gentle hand pruning should be commenced at a much earlier stage in their growth. The third recommendation we have made is a very important one to all trees which suffer from the smoke about towns, and although the moisture of our climate and frequency of rains mitigate to some extent the evils produced by the sooty deposits, nevertheless much may be done by judicious watering, especially to shrubs. A short supply of water at the roots, causing the premature shedding of the leaves before they have fulfilled their functions—a process which they require longer time to perform, owing to their vitiated and debilitated condition from the smoke and soot—is the principal cause of disease and stunted growth of town trees. Drains, both surface and underground, and hard paved streets, and walks impervious to water in the vicinity of tree roots,—as must be the case more or less in all large towns,—draw off the rainfall which in natural circumstances would go to supply the root moisture, and consequently in most situations of the nature referred to, only a very limited quantity ever reaches the rootlets and spongeoles at all. Thus summer watering and a thorough soaking, in dry, dusty weather, which could be quite easily accomplished in any town public gardens by the aid of the fire brigade engines, would be of immense value in restoring the proper equilibrium of moisture and health to the plants in such places, and an experiment which would repay itself in the enhanced amenity of the walks and gardens. The fourth and last suggestion made for the amelioration of tree and shrub culture in smoky localities, namely,—top-dressing the roots occasionally with fresh loam, is rendered necessary in such situations, by the loss which the soil sustains every season by the removal of the fallen leaves which in ordinary circumstances in the forest or park, would go to nourish and refresh as manure the surface of the ground, whence the decomposed ingredients are carried down by the winter's rainfall and action of frost into the soil, forming food for those young rootlets near the surface, which, ramifying amongst the otherwise exhausted earth acquire a stunted growth, and will produce impaired root action unless Nature's own and recuperative process, when removed by the force of circumstances, is replaced by artificial means.


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