Tweet

---

December, 1955

---

Interest in the amenities of our industrial areas has markedly increased in recent years, and the colliery spoil heap, which is such a prominent feature of the landscape, has naturally come in for its share of attention. It is no part of the aim of this article to argue the case for trees against other measures, such as levelling the heap and putting something useful on its site, but only to discuss the possibilities and difficulties of planting.

The first point, which cannot be over emphasised, is that the general environment is often of far greater importance to tree growth than any special attributes of the spoil heap. Before the latter are considered, attention must first be paid to climatic, atmospheric and biotic factors of the locality.

The general environment

Climate. Our coalfields sample a fair range of the British climate. Somerset and the Dean enjoy a better general climate than, say, Durham and the Scottish fields. Climatic changes of this order do not usually mean that quite different selections of trees must be made, but rather that, other things being equal, a wider choice of species capable of growing with vigour is available in the more favoured districts. In fact, it is not usually climate in the ordinary sense which limits the choice of trees for spoil heaps, but it is worth remembering that most of the broadleaved species and certain conifers (notably Corsican and Austrian pines) do experience some decline in general vigour as one goes north in Britain. Also there is the general trend of decreasing rainfall from west to east. This happens to be of chief concern to certain species which are not otherwise suited to waste heaps, but it may decrease the usefulness of at least one possible species (Japanese larch) in low rainfall areas.

Exposure is one of the most important (perhaps the most important) factor influencing tree growth in Britain. Though coalfields certainly differ regionally with respect to exposure, it is best discussed as it affects heaps of different size and shape, variation here being perhaps more important than between regions.

Atmospheric Pollution. Atmospheric pollution is the first general factor which seriously restricts the choice of species. Coalfields differ greatly one from another in this respect, rural coalfields such as that in Somerset are very free from pollution whereas the Black Country experiences quite extreme pollution. It is of course largely a matter of the type and scale of industry in the neighbourhood — heavy concentration of sulphur dioxide is the most damaging agency. Apart from industry, burning heaps themselves emit noxious fumes. Atmospheric pollution is selective in its effects on vegetation, in particular coniferous trees are much more susceptible than broadleaved trees and the former will only thrive in the least polluted areas. It is not possible to give a firm regional definition of pollution, since quite damaging concentrations can be developed locally in an otherwise reasonably pure region. But speaking generally the following coalfields can be considered reasonably free from atmospheric pollution and in them the use of conifers need not be automatically ruled out —

Somerset, Kent, Dean, West Cumberland, Flint, Dumfries-shire.

Intermediate areas in which there is some chance for conifers are —

South Wales, Northumberland and Durham, Leicestershire, Yorkshire Coalfield, Ayrshire, Fife.

The industrial regions such as the Black Country, North Staffordshire, Lancashire and Central Scottish fields are usually quite unsuited to conifers.

The Industrial Research Council maintains a continuous survey of atmospheric pollution and issues records. Undoubtedly these records are a valuable guide, but it is always safest to judge from tree growth in the immediate neighbourhood. If there is some evidence of atmospheric pollution and no direct negative evidence in the shape of healthy conifers it is wisest to avoid them altogether.

Biotic Factors. This is a useful omnibus term which covers such agencies as rabbits or small boys with penknives. It cannot be too strongly emphasised that the prime cause of failure in the majority of tree planting schemes has been man. With all due respect to the inhabitants of our coal mining areas, their virtues have not so far included any noticeable enthusiasm for trees. No tree planting scheme in mining neighbourhoods has the least chance of succeeding without appropriate attention being given to this factor — whether it be by fencing or by inspiring local enthusiasm is for the man on the spot to say. It is hardly worth devoting space to a dreary catalogue of the ways and means which have been successfully used to destroy trees.

A fortunate circumstance is that the rabbit has usually been poached to extinction in mining areas, and expensive rabbit fencing will usually be unnecessary. In most colliery districts fencing against stock will be required.

Characteristics of spoil heaps

Constitution. Whilst naturally there is some variation in the constitution of spoil heaps, the dominant materials are shale and mudstones interbedded with the coal, and it is safe to say that these argillaceous rocks give the spoil heap whatever special character it has. Sandstones appear to a lesser extent, and various forms of waste such as furnace waste, coal slack and washery waste may be found locally.

The shales and mudstones weather comparatively rapidly, the former to little flakes and the latter to small cuboid fragments, and the resulting soils are usually clay loams.

The proportion of slack coal has varied greatly with the method of working, its chief effect is to increase the likelihood of combustion, which is also related to the nature of the shales. Slack coal may also assist in the formation of open loamy soils. Ashes and clinker are rarely present in sufficient quantity to have much influence on the constituency of the heap, but washery waste is apparently becoming a more important material. It is tipped as a wet sludge of fine particles and is distinguishable on the grounds of its unpleasant physical characteristics rather than on its basic composition.

Combustibility is of course an important property of spoil heaps, firstly as a risk to access prior to planting (heaps even with long established woodland on them have been known to ignite), secondly because burned heaps appear to have somewhat different characteristics. In some coalfields half or more of the spoil heaps are burned, partially or completely. Typically, burning reduces the main constituents to a fine powder with many soft, flakey shale fragments and lumps of fired stone. Massive concretions may also occur.

Configuration.

The size and shape of spoil heaps are determined by the methods of tipping adopted. For the present purpose, they can be classified somewhat arbitrarily on the suggested following lines.

(1) Low/medium mounds. Including the Hill-and-Hollow formation of the older exposed coalfields and typical of South Staffordshire, they do not usually exceed about 50 feet above local ground level and are often much lower than this. They are often a series of low undulations with waterlogging in the bottom; or take the form of irregular plateaux with numerous small hummocks. Isolated low mounds are also common.

(2) High mounds and flat topped cones. These range in height from 50 to 200 feet or more, and are generally single mounds with a plateaux-like top, often with many minor hummocks on it.

(3) Cones. Modern mechanical tipping practices give rise to high cones often exceeding 200 feet in height. Many of these are still in use, and some of them are burning and will continue to do so for some time.

(4) Ridges. Tipping by tramway or aerial cable may arrange the spoil in long ridges; the crest is sharper when the latter process has been used.

Stability.

To support tree growth (or for that matter any vegetation) the coal heap must provide a reasonably stable surface, and an adequate supply of moisture and nutrients. Some degree of stability usually occurs within two years after the cessation of tipping, though after this there may be gully erosion and a good deal of surface movement of small particles. Severe gullying may occur on heaps of particularly fine particles such as washery waste; it is also encouraged where water can collect on flat tops and spill over. It is rarely a sufficiently important problem to call for large scale reshaping of heaps, and where locally serious can be combated by well-known erosion control methods. The surface movement of small particles is chiefly important in that it helps to prevent the establishment of natural vegetation by seeding; it is of much less importance to the establishment of planted trees.

Spoil heaps as soils.

Weathering of the typical spoil material takes place rapidly, but unless vegetative cover is present much weathered material is lost. The appearance of natural vegetation is no guide to the earliest date at which it is possible to plant trees; due partly to the surface shifting mentioned above and partly no doubt to the very exposed surface conditions (which must encourage periodic drying-out of the top few inches), the heaps are relatively slowly colonised. The chief constituents (shales and mudstones) appear to be adequately supplied with the main nutrients. At least we can say that no examples of failure of plantations have been observed which can be ascribed with certainty to poor fertility, and there are numerous examples of satisfactory growth of species which are known to be very demanding in this respect. The reaction of fresh spoil is usually basic or near basic, but on weathering it becomes more acid. There is some suggestion that this process proceeds more slowly under woody vegetation than under lesser vegetation or in bare conditions. This is to be expected, but great variations are to be found on individual sites and no clear trend can be established. The inference is that fertility will not improve from weathering under bare conditions; heaps do not become better sites for tree growth by being left for many years. This is to some extent supported by the evidence of planting in this country; there is certainly negative evidence that planting on quite fresh material has been quite successful. Also there are analogies with Continental and American experiences in the afforestation of various forms of mining waste where planting as soon as possible after stabilisation has frequently been found more successful than delayed planting. Part of the story is no doubt concerned with compaction; the open texture of the more recent spoils is favourable.

Some cases of what appears to be toxicity have been noticed. These seem to be connected with a high degree of acidity, perhaps brought about by the oxidation of sulphur compounds. But toxicity hardly seems a general hazard, and where it occurs it may be confined to patches of quite small extent.

Moisture. Spoil heaps are surprisingly moist, particularly those consisting largely of unburned shales and mudstones. They are by no means the arid sites one might well suppose them to he. Perhaps they may be likened to sand dunes, which seem to possess a perched water table. Whatever the mechanism, there is no evidence to suggest that there will be any serious limitation of tree growth from lack of moisture, but we must except the upper slopes of tall conical heaps since there has been no opportunity of observing tree growth in these conditions. It is reasonable to suppose that transpiration stress will be very high in such positions.

There is frequently a belt of very moist conditions round the base of spoil heaps, and in the old irregular hill-and-hollow formations there may be water-logging in the low places.

Special features of burned heaps

There is not enough evidence to give a clear account of how combustion alters the conditions, but the indications are that fertility is not reduced to any great extent. Certainly there are examples of successful planting on burned heaps, and demanding species have developed satisfactorily on them. Burned spoil typically lacks the coherence and colloidal properties of unburned material and the resulting soils tend to be gritty loams. The surface washes more readily, and occasionally large masses of fused material may appear at the surface. It appears that burned heaps are in general drier than unburned, certainly in the surface layers.

On burned heaps, birch is a common pioneer tree; whereas species such as hawthorn followed by oak and ash, which are associated with higher nutrient status, are more typical colonisers of the unburned spoils. This suggests drier, more acid, less fertile conditions on the burned heaps, and is a point to be taken into consideration when selecting species.

Exposure.

Since exposure is such an important factor to tree growth in Britain, any local eminence may increase it to the point where tree growth is seriously incommoded. It is not so much absolute as relative elevation which must be taken into account, and in addition the average "windiness" of the region. The sort of effect which may be expected is that in most inland districts the planting of low or flat-topped heaps will never be rendered impracticable due to exposure, but serious trouble must be expected on the upper slopes of tall conical heaps, more particularly in very exposed situations.

Exposure does not act very selectively, it will depress and malform all species of trees — but at the extreme only a few will be left capable of reasonable growth. It is unfortunate that the toughest happen to be conifers, which are most susceptible to smoke injury.

Even on quite exposed sites, there is considerable improvement in growing conditions to be expected as closure of canopy advances from the less to the more exposed situations. Poor stunted trees on the higher slopes of a spoil heap might be expected to improve as the plantation develops on the lower slopes.

(To be continued)

Drawings and Photographs accompanying the article