The Future of Tree Diseases

Trees are threatened with extinction. Those that survive the woodman's axe and the developer's bulldozer are struck down or menaced by an increasing number of epidemics. At the beginning of this century the American chestnut Castanea dentata which once made up 25 percent of the forests in the eastern half of the US was stricken by the fungus Endothia parasitica. It has now virtually disappeared from the landscape. The loss is incalculable.

Among other things its timber was highly prized for its beautiful grain and its resistance to dry rot. Thirty thousand million board feet of it are estimated to have been lost. The wood contained tannin that is used for making leather. The industry that extracted it is now bankrupt. The chestnut also provided a habitat for vast populations of squirrels and deer that fed on the nuts and great flocks of wild turkey. These have been decimated. What is more the fungus crossed the Atlantic where it is now wiping out European chestnut groves in Southern Italy. The loss was estimated 12 years ago at $1,000 million. [Carefoot and Sprott, 1967] But is money the right currency for expressing such a loss?

In 1930, forests containing 1,000 million American elms were struck with Dutch elm disease, which spread from the Atlantic along the St. Lawrence watershed around the Great Lakes, from Maine to Minnesota. Forty years later few survivors remained. Another 500 million or so elms scattered on farmland, along country roads and city streets were also annihilated.

The cost was estimated, twelve years ago, at $50,000 million, which included $12,000 million spent on cutting down the dead trees and burning them to kill the fungus; $3,000 million for replanting resistant trees; and $250 million a year to inject amenity trees with chemicals. [Carefoot and Sprott, 1967]

In the 1940s it was the turn of the oaks of which there were in America an estimated 12,000 million belonging to 35 different species. The disease was triggered off by a sap fungus which entered the tree through punctures in the bark mainly caused by an oak bark beetle. The disease is still spreading at the rate of about 50 miles a year. It is not as lethal as Dutch elm disease or chestnut blight, but is nevertheless decimating oak stands over a wide area.

The cost of this damage is enormous. The oak forests of America represented, among other things, over a billion feet of timber. At the low price of $30 per thousand this would mean $30 billion, and at today's prices possibly as much as $100 billion. But this is nothing compared to what would be the biological and ecological, not to mention the aesthetic costs of the destruction of America's oaks.

Only the Ash to remain?

I have mentioned the three tree epidemics that have attracted the most attention, but these are by no means the only ones that have broken out in recent years. For instance, a disease of the maple tree has been spreading southwards from Nova Scotia. A fungus associated with a tree scale insect is afflicting the hemlock, the red pine and the beech in New England and threatens to spread across much of America and possibly to Britain.

Another fungus, Ceratocystis ulmi, is killing off the plane trees in the southern part of France, while yet another, Coiyneum cardinale, is killing cypresses in south and central Italy. In California the Monterey cypress is in trouble. In South West Australia a particularly virulent disease is destroying vast tracts of eucalyptus forests. In New Zealand, willows are dying and in Jamaica and Florida the precious coconut palm, on which tens of millions of people depend for their livelihood, is being annihilated by Lethal Yellowing disease which is now spreading to the neighbouring islands.

In Britain the countryside is studded with dead and dying elms, our beech trees appear to be affected by some yet unidentified ailment which may or may not be related to the disease affecting the beech trees in New England, [Lonsdale, 1979] while sycamores in certain areas are afflicted with Sooty bark disease.

Elm, beech, sycamore, oak and ash make up the vast majority of the larger deciduous trees in this country. If the first of these is already being annihilated, and the second and third are afflicted - no-one knows with what consequences - the prospect is indeed grim. If oak wilt were to cross the Atlantic, this would leave us with only the ash - a truly terrifying prospect.

Playing down the Problem

The temptation for foresters and plant pathologists is to play down the current epidemics. They like to think that things are much as they have always been. This leads to the comforting thought that everything they learned at university is as valid today as it was then, and that there is no need to bring about any radical change to current attitudes or current forest practices.

Even Dr. Burdekin, the tree pathologist of the Forestry Commission, tries to convey this impression to the public. In an article in The Times, he quotes a letter that appeared on 11 June 1977 in the same paper, lamenting the disappearance of elms from the British countryside and showing that identical sentiments were expressed 40 years earlier - on November 11th 1930, also in a letter to The Times.

This is supposed to justify his conclusion that 'times do not really change'. The opposite is in fact the truth. Times have changed, and very dramatically at that, and the question we must ask ourselves today is whether our trees will survive these changes?

The technological approach to tree disease

It is the object of this article to try and answer this question. To do this we must undoubtedly review the major diseases affecting trees today. But I propose to inquire more deeply into the subject. Tree diseases are merely instances of diseases in general and must be bound by the same set of principles. So it is disease itself that we must consider.

Now there are two very conflicting approaches to the study of disease. The first is the technological approach. A disease is empirically associated with a parasite. The parasite is taken to be the 'cause' of the disease, which it is assumed, can only be cured by eliminating it.

The second approach is the ecological one which I shall consider later. The technological approach is very convenient since it provides the rationale for indulging in precisely what our society is organised and motivated to indulge in - economic enterprises, in this case, in the form of large-scale spraying programmes which contribute to GNP and provide research grants for scientists, development grants for technologists, profits for industrialists, dividends for shareholders and jobs for all. It also brings the rapid results which are required in a society that is so little concerned with medium to long term consequences.

Spray, spray and spray again

Systematic spraying is, of course, very irresponsible in that its effect on populations of insects, fungi and the various micro-organisms that inhabit the forest soil, cannot be predicted with accuracy. Even if it could, this would not necessarily help, as the exact ecological functions of the different populations in maintaining the fertility of the soil and contributing to the health of the trees and to the ecosystems as a whole is generally not known. What is known is that these populations will he significantly affected. [Kuhnelt 1976] Populations of some species will increase, others will decrease.

Since pesticides accumulate up the food chain, those at the top, i.e. predators, will be most adversely affected and it is these, it must be remembered, that in normal conditions, are responsible for controlling the population of target species. All sorts of micro-organisms live in the tree's roots, in symbiosis with the tree, contributing in all sorts of subtle ways to its long term health - these also can be affected.

Resistance, too, builds up very quickly among insects and micro-organisms to chemical poisons. Hundreds of insect species are already resistant to DDT and other pesticides. In any case insecticides can only really eliminate an epidemic if they can exterminate the pest population involved. But all they in fact do is thin it out, killing at most 80-90 percent of it.

As a result, the survivors, now in possession of an ample food supply, will tend to proliferate. What is more, the pesticides will give rise to a new population which, being descended from the survivors, must display some resistance to the chemicals used and is likely to be tougher and more difficult to eliminate.

More often than not, the plant pathologist faced with such a situation simply orders further spraying, which can only lead to a further increase in resistance to the chemical used, and of course to further biological and ecological deterioration. For these reasons attempts to control major plant pests by large scale spraying programmes have almost always failed.

What is more, except in the worst cases, the outbreak, if allowed to take its course, would have died a natural death. Natural controls would have eventually restored ecological stability, as shown in the following examples.

The Douglas Fir Tussock moth

Under 'normal' conditions this moth causes little or no noticeable defoliation. However on certain occasions, for reasons that are probably related to climate, the tussock moth population of a particular area can explode and cause a great deal of defoliation. Some 25 - 30 percent of the trees can die over a period of three years.

Most of them, however, die as an indirect consequence of defoliation, actually succumbing to bark beetles and other invaders. What is particularly important is that tussock moth outbreaks usually only last three years - after which the entire population tends to collapse as a result of an attack by their natural enemies, in particular a polyhedrosis virus.

For the last 25 years, the tendency has been to spray the affected forest. There is no evidence however that, this has had any effect. A study by the US forest services, for instance, concluded that "limited comparisons in California of two chemically treated areas with two untreated areas showed no significant differences in total tree mortality".

In both cases, the virus infection seemed to be the main cause of the decline of the moth population. Another report showed that 99.9 percent of the population of tussock moths in the Blue Mountain epidemic in 1972 had died by the end of the following year of natural causes. The polyhedrosis virus being mainly responsible. [Hermann, 1973]

The gypsy moth

The gypsy moth feeds on oak leaves, and when its population explodes it can cause considerable defoliation. However these population explosions cannot be sustained for long.

"Two years of defoliation are usually followed by a population crash with dispersal, disease and parasitization all contributing to a drastic reduction in gypsy moth numbers." [Hinckley 1972]

The affected trees tend to be leafless by the end of June but can refoliate during July and August. Some branches of older trees may die, but it is largely the pines which have little capacity for refoliation or deciduous trees in the forest under-storey that tend to succumb. As Hinckley points out,

"if this process continues long enough, a different forest emerges, one no less interesting and more inbalance with the gypsy moth. Lumber interests would have to put up with a temporary loss in production, but this would be made good if they had the patience to wait for the forest to recover. Unfortunately they do not have and insist on spraying."

Until 1961 DDT was used in massive spray programmes in New England and New York. Caterpillars were killed in May and a lot of defoliation was prevented. The eminent ecologist Kenneth Watt showed that these spray programmes had but little effect on the population dynamics of the gypsy moth. He considers that weather conditions are more important than anything else in determining the growth and the fall of gypsy moth populations. [Kenneth Watt, 1968]

More recently Sevin (Carbaryl) has been introduced to replace DDT which is now banned in the USA. It is non-persistent and breaks down on contact with water. But to be effective it must be applied at a very specific time: just after the majority of caterpillars have begun feeding, and before the leaf canopy has built up.

Because of micro-climatic difficulties, it is impossible to apply Sevin with this sort of precision over large areas, and as a result the pesticide merely tends to thin out the caterpillar populations, increase survival to pupation - because of the reduced competition for food - and hence prolong the outbreak. [Doane, 1968]

In addition this pesticide will also kill many insects, including parasitic flies and wasps, that are the natural predators on the moth. [Kamram, 1971] It will also kill bees which must thereby reduce pollination and food production.

The spruce budworm

The spruce budworm infests spruce forests in Canada, and the response has been to spray them, as in May 1978. In 1952 DDT was sprayed on 200,000 acres of forests in New Brunswick. The main effect was to increase the acreage affected and the need for further spraying operations. In 1963, about 25 percent of the land area of the province was affected - but by 1973, after 20 years spraying, the figure was closer to 90 percent. In 1976 9.5 million acres of forest had to be sprayed.

Eventually DDT was abandoned and an organophosphate called fenitrothion was used instead, but this has not been any more successful. As Elizabeth May writes,

"the killing of a large proportion of the budworm population (about 85 percent) was termed successful but left the survivors with an abundant food supply; starvation was no longer a limit to population growth. The lethal effect of both DDT and fenitrothion on the birds, small insects, spiders and wasps which prey on the budworms removed another check. The only remaining check on unlimited population growth was the annual dousing with chemicals from the air - the very thing which allowed the infestation to continue and spread."

The perpetual epidemic created in New Brunswick created a breeding ground for infestations in other areas, and forced both Quebec and Maine to start spraying too (Robert Paelhke 1978).

The biological and ecological damage done by all this spraying was of course immense. Among other things it caused an outbreak of a disease called Reye 's Syndrome - unknown before the 1950s - which causes liver and brain damage and is frequently fatal, primarily affecting rural children.

Recently, Prince Edward Island decided against spraying and instead set about replanting more varied, more resistant species, to replace the dying spruces. However, as Paellike points out,

"in all of the provinces where forestry was big business spraying was either carried out or fought hard for by the big forest companies. Their lack of concern with anything but maximising short term yields is truly disgraceful." [Paelhke, 1971]

In Nova Scotia where spraying had not occurred, the infestation came to an end by itself. When Nova Scotia's Forest Industries (NSFI) requested a permit to spray 100,000 acres of Cape Breton forests with fenitrothion, Nova Scotia's Deputy Minister of Lands and Forests turned down the request stating that

"bud worms have hit Cape Breton before but they died out on their own ... We feel that it is far better from a forestry point of view to suffer our losses now, rather than spray and prolong the inevitable, as New Brunswick has done. The forests of New Brunswick after 25 years of spraying certainly are not the envy of any one involved in proper forest management."

Other spraying failures: when will the experts learn?

There have also been attempts to use insecticides in the fight against Dutch elm disease in the US. Its use, to quote Frank Graham Jr., provided scientists with...

"one of the classic environmental horror stories. Enormous amounts of long-lasting insecticides were sprayed on American cities and towns. Robins, feeding on earthworms which earlier had fed on the sprayed leaves, died in untold numbers. Sewers carried DDT residues from city streets into rivers and lakes, where destruction became magnified. Yet, alter great financial and environmental cost, the cities lost their elms anyway, and the disease kept spreading into new areas."

Attempts to save trees affected by serious tree diseases by injecting them with chemicals have been a total failure. Efforts to inject trees afflicted with chestnut blight or Dutch elm disease have failed. First of all the cost is very much too high which means that the method can only be used for saving a few amenity trees around public buildings, for instance, or in private gardens.

In any case resistance soon builds up against the chemical. According to William B. Ennis of the Agricultural Research Centre, efforts to inject palm trees threatened with Lethal Yellowing disease can slow the progress of the disease but that is all, the tree cannot be cured and will eventually die.

Unfortunately, the experts never learn - largely of course, because they do not want to. In spite of the almost universal failure of spraying campaigns they remain the main weapon in the armoury used by plant pathologists. In this country, the Forestry Commission in 1978 mounted what was in its own words "its biggest ever aerial spraying operation". [Press release 1, 1978]. Twelve thousand acres of lodgepole pine plantation infested with the larvae of the pine beauty moth (Flammea panolis) were sprayed with fenitrothion - precisely the same pesticide use so unsuccessfully in New Brunwick and elsewhere.

Health and Stability: the ecological approach

We have seen that the technological approach to tree disease is an abortive one. If the parasite is the only cause of the disease then there is no solution to the problem because once the parasite is established we shall never succeed in eliminating it. Fortunately, the parasite is only the 'cause' of the disease in the very narrowest sense of the term. The problem is a very much more complex one, and cannot be understood in terms of the simple cause and effect relationships so beloved by our misguided empiricist philosophers.

To understand the real cause of the disease we must look much more closely into the question of disease in general, and this we cannot do until we can first explain what is 'health' - from which 'disease' is nothing more than some sort of deviation.

The term is normally applied to biological organisms only. A biological organism, however, is simply an instance of a natural system. If 'health' is a basic term, like such terms as 'control', 'stability', 'order' etc, then we should also be able to talk of the health of other natural systems such as ecosystems, social systems etc.

If a system is healthy, this can only mean that it is stable, i.e. that it functions properly. That state can only be established if one knows what is the system's goal. Since we know this to be the achievement of stability or continuity or homeorhesos, to use C. H. Waddington's term, [Waddington, 1957] then one can regard a system to be functioning properly to the extent that it achieves this goal. In other words health equals stability.

A natural system is hierarchically organised, i.e. made up of sub-systems and sub sub-systems. To maintain its structure, these must all fulfil their appointed functions and thereby cooperate towards the achievement of a common goal. Those that do not and have thereby ceased to be viable tend to be eliminated by natural selection. In this way 'noise' or 'randomness' or 'entropy' is reduced to a minimum, organisation or negative-entropy is maximised and the viability of a system, and hence its adaptiveness, stability or health, is maximised.

Disease in a stable society is, in fact, but a means of natural selection which explains why it eliminates mainly the weak and the sickly. If it does not eradicate the healthy, it is because it is not adaptive to do so, still less to wipe out whole populations of healthy organisms. It is the strategy of nature precisely to avoid such things. To achieve stability means precisely that: reducing discontinuities of this sort to a minimum.

Hence epidemics do not occur in stable ecosystems any more than do other major discontinuities such as large scale droughts, floods or massacres. Their occurrence is a sign that something has gone wrong, that the system has ceased to be stable, that a serious maladjustment has occurred.

Looking to the root of the problem

A serious disease is therefore a more complex phenomenon than is generally thought and cannot simply be attributed to the agency of the parasite or pathogen that appears to have triggered it off.

As Day writes,

"it is customary to speak loosely regarding diseases with which parasites are associated, assuming that the parasite is the sole factor with which one need be concerned, whereas other factors may be more necessary to the production of the diseased condition. One evil result of this is that fundamental causes tend to be overlooked and attention concentrated on the obvious factors in the problem, even though these may be of secondary importance." [Day, 1929]

The fundamental causes are often those that reduce the tree's resistance to parasite invasions. This reduction in their resistance must be regarded as an injury - unfortunately one that is not always normally visible. It is important that means of discerning them be devised before a combination of other injuries leads to tree losses. Keller has introduced the distinction between 'visible' and 'latent' or hidden injury. As Keller writes:

"the forester in industrialised countries should not wait until injuries are discernible (chronic or acute injury) and economic lossesdo occur. He should evaluate the situation well in advance and should recognize (and demonstrate) the existence of a potential danger to the forests as soon as the trees are under stress and long before they exhibit visible symptoms or even collapse." [Keller, 1976]

It is exactly the denial of the existence of 'hidden' injury and the use or occurrence of visible injury as sole criterion of a plant's reaction to air pollution which led to such erroneous conclusions as that "a concentration of 0.2 ppm SO2 is tolerated for several weeks even by young conifers". [Zahn, 1969] Such a conclusion of course would only be justified if a threshold existed below which pollution caused no biological damage - and such a threshold of course does not exist.

Keller suggests that the level of photosynthesis might be a useful indicator of hidden injury in the case of trees. Photosynthesis is a primary process of wood production. Respiration too depends on it because it utilizes substances produced by this process. Since photosynthesis reacts sensitively to any change of environmental factors, it is a good indicator of a plant's reaction to air pollutants.

In reality, of course, it is these factors - those that have caused the invisible injuries and have thereby reduced the tree's resistance - that are the real cause of the disease. For if they were not operative the parasite would be relatively harmless. Day comes to the same conclusion:

"If an indigenous parasite depends for successful parasitism on an already existing morbid condition in the host, it should be known what such a condition is and how it is brought about. For the means by which this condition is produced is the real cause and should be referred to as such." [Day, 1929]

One might add that in the case of the importation of an alien parasite, one can regard the importation as such as only a symptom of the disease it may give rise to. The real disease is clearly the state of mobility and world trade in wood products that must inevitably lead to such a situation.

For the biosphere to be stable it must be made up of ecosystems displaying great diversity. To maintain this diversity means maintaining them in relative isolation from each other, each in that area in which it has evolved as an adaptive response to a specific set of conditions. Once conditions no longer favour the existence of this isolation and once mobility is such that the components of these ecosystems are simply shuffled about like a deck of cards, biospheric complexity can only be disrupted and stability drastically reduced.

Experts: the art of sitting on the fence

Unfortunately not all plant pathologists are necessarily interested in working out what is the condition referred by Day. Their tendency is often to regard such an inquiry as sheer speculation unworthy of professional scientists.

Consider the following answer I received from a professional plant pathologist to an inquiry about the underlying causes of a specific tree epidemic in the area in which he lived.

"I share with you your concern with regard to the apparent rapid degradation of the vegetation component of ecosystems in many parts of the world ... At the philosophical level I think it is reasonable to hypothesise that many of our problems are a consequence of modern society's desire to impose uniformity on ecosystems which survived in the past because of their diversity. However, as I am a public servant engaged in scientific research, I cannot engage publically (sic) in philosophical discussion."

He is thus telling us that to consider the real factors giving rise to a forest epidemic is outside the brief of public servants and scientists. Their role, it is assumed is to keep their noses firmly glued to their test tubes, come out with learned descriptions of the actual mechanics of the disease, and then propose technological remedies for destroying the parasites that appear to be involved, whether such methods work or not.

What keeps trees healthy?

Indigenous versus exotic species

In order to determine what are the conditions leading to reduced resistance to disease we must first of all establish what are those that favour the maintenance of health. Since a tree species is a product of evolution which is a directive process tending towards the maximisation of stability, tree populations must display the greatest stability, and hence the highest degree of health, when growing in the environment to which they have been adapted by their evolution.

Again all the empirical evidence confirms this thesis. Thus Day points out that in general, "indigenous species are less affected by disease than exotics". [Day, 1949] He points out, too, that in Britain the species most seriously affected by butt rot are exotic conifers such as the Sitka and Norway spruce, the European and Japanese larch, Douglas fir and Thurja plicata. Indigenous trees such as beech, oak and Scots pine may be grown for much longer rotations than can these exotics and remain free from serious butt rot.

Significantly too, Florida's native palm species appear unaffected by Lethal Yellowing disease which exclusively affects exotic species. Pine blister rust in North America also appears to affect pines when they are planted outside their natural range. [Bingham, 1971] This means that to maximise resistance to disease, we should ideally be planting (whenever possible) our native trees - those that are designed by evolution to survive in the particular environmental conditions present on this island.

This would mean planting pendunculate oak on the heavy soils in the south east of England, the sessile oak in the lighter soils of the north and west and in some parts of the Scottish highlands. It would mean planting in all these areas alongside the oaks, such trees as elms, limes, poplars, ash, beech, hornbeam and birch. The under-storey of these large trees must also not be neglected. In general it should be composed of hazel, holly and thorn trees.

In ideal conditions the only conifer we should plant would be the Scots pine which once covered much of the Scottish highlands. Nor should we think simply in terms of species. A species was once looked upon as a group of uniform individuals but we now know that this is not so. There is very considerable diversity within a species.

Provenance: a new field of study

Indeed a whole academic field has developed to study this approach, referred to today as provenance research. [Langlet, 1962] It has revealed that in a widely distributed species, there are not only considerable differences in the strains to be found in different areas but differences too in the types of diseases to which the strains are subject.

Thus among species of trees that are widely distributed such as Scots pine, widely differing 'climatic races' occur. As Day points out, when a number of these 'climatic races' of a single species are planted together in one place, so that some are more and others less suited to their environment, the different races will differ not only as regards vigour of growth, but also susceptibility to disease.

"The pine needle cast fungi, Lophodermium pinastri, rarely acts in Britain as an important cause of disease on Scots pine but in some regions on the continent of Europe it frequently does so. In general the Scots pine is affected by a different sort of parasite in the north of Europe than further south. The same in general is true of all tree species." [Day 1950]

All this is obvious. The evolutionary process has adapted trees of a particular species and of a particular strain within the species to living in a particular environment. As soon as these are moved into a different environment to which they have not been adapted, maladjustments must occur. As Day puts it:

"any attempt in practice to treat a species as though (i) its environmental range was relatively unlimited, or (ii) it was within itself a constant constitutionally, so that strains native to significantly different environments might be planted without regard to their particular requirements, will lead to a greater or less extent to a breakdown of health." [Day, 1950]

This does not mean that exotics cannot be cultivated, but again as Day points out, the successful establishment of exotics in reality

"depends on the movement of the strains of species so that they remain within the limits of their environmental range both for living and non-living factors." [Day, 1950]

Hence it is by studying all the main features of their natural range and seeking to reproduce them in the area in which exotics are to be planted that their resistance can be maximised.

Since forestry in Britain is distinguished from that of most other countries by the extent of our reliance on fast growing exotic species, it is worth examining what are the main 'living and non-living' factors involved.

Climate

The most obvious one is climate, and one of the aspects of climate that most affects the growth and health of trees is the presence or absence of frost. Frost can weaken trees of certain species in such a way as seriously to reduce their resistance to specific diseases. Thus the fungus Dasyscypha willcommii, which causes the dieback and canker of the European larch, is normally of little importance. But if the larch are planted outside their normal range, particularly in areas where there is heavy frost, its susceptibility to this parasite increases very siginficantly and the disease associated with it can reach epidemic proportions.

The same is true of the Corsican pine which, when planted in areas with heavy frost, is subject to a type of dieback accompanied by canker from which, in normal conditions, it would not suffer. [Day, 1950]

Of course cold weather can have the opposite effect and can actually prevent the development of certain diseases. The cold weather of the Northern states of the USA and Canada, for instance, provides an effective barrier against the spread of oak wilt fungus. [Carefoot and Sprott, 1967]

Cloudy weather may be a factor predisposing pines to infection by the fungus Cronartium ribicola, in that it provides the conditions necessary for the successful production and distribution of the infecting spores. The rate of evaporation is also relevant. Thus the needle cast fungus (Meria laricis) of the European larch was transported with its host from their mountain habitat, where the rate of evaporation is usually high and the opportunity for spore development and infection slight, to Western Europe where the rate of evaporation is often low and opportunity for spore development and infection are very much higher. [Day, 1950]

Soil

Another essential factor is soil. Different tree species require very different soil characteristics - different mineral nutrient requirements for instance. The ash, the elm and the sycamore require soil with a high mineral content. Two needled pines and birches on the other hand are less exacting. The oak is intermediate in its demand for minerals. [Anderson, 1956]

To plant trees in soil which does not have the suitable mineral contents is to weaken them and thereby to increase their susceptibility to disease. At the same time this can lead to the degradation of the soil which can still further affect resistance to disease. The growing of conifers on sandy soils and sandy silts over a period of two centuries can acidify (podzolize) soils to the depth of 20 or 30 cms, (noirfelise). This often leads to a reduction in productivity from the second plantation onwards.

The difference in the mineral content of the leaves and detritus of different species is extremely important. Their cast-off leaves form a little layer which is slowly broken down into humus. The various organisms that decompose the litter are most active when it has a high content of basic mineral salts, also when the litter provides them with an alkaline, neutral or only slightly acid medium.

If the medium is too acid, as Anderson points out, they tend to be absent which causes the litter to accumulate. When this occurs the soil tends to lose its basic mineral elements and becomes increasingly acid. [Anderson, 1956]

Organic acids develop which move downwards through the soil removing its mineral salts, thus depriving it of many of its micro-organisms, causing it to lose its basic structure and to become podzolized. [Anderson, 1956] The number of tree species that can grow satisfactorily even in moderately podzolized soil is very limited. The most important is Scots pine.

This being so everything should be done to prevent podzolization. Trees should be grown that produce the appropriate litter, which mainly means deciduous trees. Unfortunately these do not satisfy today's requirements for fast growth and high economic yields, but in the long term it is undoubtedly by submitting to these ecological constraints that the health of the forest can be maintained and yields maximised.

Clearly a sound forestry policy in this country would not only aim at preventing further podzolization but would seek to reverse the process wherever possible. Vast areas of podzolized soil in this country should be systematically improved so that they can be returned to sound forestry. This means planting trees that can grow on podzolized soil and which will produce a litter that is attractive to soil micro-organisms.

Among such soil-improving trees are the rowan or mountain ash, the alder and the birch. Unfortunately they do not produce timber of any commercial value which explains why they are not planted on any scale and thereby why few efforts are made to reverse podzolization.

In general, the land available for forestry even when not podzolized is marginal land, mainly in hilly areas - the best land being reserved for agriculture and often too for urbanisation. In itself, marginal land is known to contribute to reduced health and hence resistance to disease.

For this reason alone it is important to prevent any further soil deterioration. Among other things, this means avoiding such practices as clear-felling which is still in general use by our own Forestry Commission, and scrub clearance, since both these practices expose the soil unnecessarily to the wind and rain, leading to soil erosion which, particularly on sloping ground, can be very serious.

If trees are to be planted on relatively unfavourable soil, then by way of compensation, all other conditions for tree health should be maximised. As Day writes,

"if forestry is, perhaps necessarily, to be confined in the main to the less fertile land, then the composition of woodland should be such that it is able to withstand the relatively great adversities in environment which determine this relatively low fertility. If these are too great, then no amount of care will prevent the forest from being burdened by a great weight of disease in the development of which both non-parasitic as well as parasitic factors may be expected to take part." [Day, 1949]

Planting trees on land that has been used for agriculture also favours disease, since its mineral content as well as its micro-fauna demand will have been modified by agriculture. Thus, according to Kujala, the parasitic fungus Fomes annosus is particularly destructive to Murray pine plantations in Finland when these have been established on former cultivated land. However, "In young stands of natural regeneration in the south the fungus never reached any importance." [Kujala, 1948]

Planting trees in land displaying an inappropriate moisture content can also favour the spread of disease. If the spruce, for instance, is planted on ground that is too dry or too compact to enable it to strike deep roots, it tends to be weakened and in a very dry year, it becomes susceptible to attack by parasites, in particular a ground mushroom that causes rot disease.

This also especially affects Picea plantations that have been established on previously cultivated soils. According to Noirfelise, planting pine plantations on peaty soil can in certain conditions increase their vulnerability to cryptogram attacks.

It has been noted how changes in the water table, as a result of mining, affect oak mortality in England. Today with increasing water abstraction for both agricultural, domestic use and industry, and falling water levels in many parts of the world, the health of trees is likely to be increasingly affected by this factor.

It is to be noted that the 1976 drought in Britain led to considerable acceleration in the spread of Sooty bark disease of sycamore and also of beech tree disease.

Dry soil also tends to reduce the resistance of various trees to the attacks by the parasite Armillaria mellea. On the other hand, the collar crack disease of cacao only seems to become epidemic when there is high atmospheric humidity and high moisture. According to Day, on well drained sites death appears to be only sporadic and losses insignificant. [Day, 1929]

Biotic Environment

The third factor is the trees' biotic environment. Foresters are rarely ecologists. They do not seem to realise that, in nature, trees are not arranged at random but in a specific pattern, that which most favours their health and survival as well as that of the forest ecosystem of which they are an integral part. If they are to maintain a stable relationship with their environment, i.e. if they are to be healthy, they must clearly be planted in a biotic environment which resembles as closely as possible that in which they evolved.

There is considerable empirical evidence to show that it is in such conditions that they are most resistant to disease. Keller shows that trees planted in the correct ecological conditions are more resistant to pollution than species "whose demands regarding habitat are only incompletely met, and which in consequence, exhibit diminished vitality."

Unfortunately scientific forestry does not always take these things into account. Its main object is to achieve maximum short-term yields. As Anderson points out, modern scientific forestry ignores

"the fundamental fact that the components of the forest are living organisms with their likes and dislikes and not so many match-sticks, automatically and mechanically increasing in girth, height and volume until they are large enough to meet the fleeting needs of man." [Anderson, 1956]

It is likewise ignored that

"forests are communities of innumerable living organisms and that they refuse to be bound entirely by rules laid down for them by man." [Anderson, 1956]

This means first of all that tree monoculture must be avoided. Specific tree species and strains must be planted as much as possible along with the other trees they are accustomed to living with. Ideally the under-storey they have co-evolved should also be reconstituted.

Nor should trees be unduly overcrowded. Overcrowding is also a factor increasing susceptibility to disease. It is known for instance that canker in larch trees is most destructive in overcrowded woods. In modern dwarf apple orchards in which dwarf trees are planted to a density of 1,000 an acre, canker can affect as much as 30 percent of the trees. In the old fashioned orchards with trees planted far apart canker deaths were very much lower. Nor should we plant rows of trees, as is done today, that are all of the same age, for susceptibility to specific diseases varies according to the age of the trees.

As Day points out, the larch in England is particularly sensitive at the age of 15 years to the fungus Armillaria mellea. Spruce, according to Nechleba, is most sensitive at the age of 25, (though Frombling finds it most susceptible in the first years after planting). A natural forest, by being made up of trees of different age groups, is very much less vulnerable to disease than a plantation of this sort.

There is another reason why trees of the same age and species should not be planted together. It is that their roots will be competing for the same space and hence for the same minerals and moisture. Trees of different species and of different ages, on the other hand, would be making the best use of the soil's fertility. [St. Barbe Baker, 1979]

Causes of tree disease

Importing parasites

It remains true that the most serious tree epidemics have been triggered off by changes brought about to the biotic environment of a species or strain of trees by the introduction of one or more alien parasites whose proliferation is not checked, as it would be in their natural environment, by climate, soil or biotic factors.

Thus the canker affecting the Monterey cypress (Cypressus macrocarpa) in 1928, was caused by a hitherto undescribed fungus later named Coryneum cardinale. After wiping out three quarters of the Monterey cypresses in California, it spread to France in 1940; the Argentine in about 1971; and then to Australla, England, Georgia, Italy, Northern Ireland and Spain. Wagener, considers that the main cause of its spread was the transport by man of contaminated plants, though the carriage of infected cones by birds was also partly responsible. [Wagener, 1948]

The gypsy moth Porthetria dispar, the great oak tree defoliator, was imported into the US in 1869 from France where, without adequate climatic and biotic controls, it assumed epidemic proportions. The chestnut blight in the US was caused by a parasite fungus, Endothia parasitica, that was imported from China at the turn of the century - and later spread to Italy.

Dutch elm disease in the USA was caused by the Dutch elm fungus, Ceratocystis ulmi, a native of Asia, which had already been introduced to Europe. It was probably brought to the US on a shipment of elm burl logs imported for the manufacture of veneer. Its introduction would probably not have proved fatal were it not that the European elm bark beetle Scolytus mulistratus was introduced on the same logs and was soon spreading the deadly fungus throughout the USA.

Significantly, there is a native American elm bark beetle, but it is comparatively harmless in America, the American elms having achieved a stable relationship with it. It is a new strain of the fungus that appears to have evolved in Canada, and that was taken back across the Atlantic to Britain, that is now annihilating English and Wych elms in the UK. [Gibbs and Frank Graham Jr.]

Sooty bark disease of sycamore is caused by a saprophyte fungus, Cryptostroma corticale, which was introduced into Britain in the 1940s on imported timber from North America where it is a parasite of the sugar maple. Scleroderris canker, which is wiping out red and Scotch pine in New York State, appears to be caused by a fungus of a particularly virulent strain which was probably introduced from Europe. [Press release 4, 1976] [Press release 5, 1976]

Felling affected trees: why governments won't act

How does one deal with this problem. One obvious thing to do is to fell the affected trees and burn them. If this is done early enough the disease can be nipped in the bud. This would probably have been effective incertain cases, in particular in the case of an alien parasite that has still only caused a local infestation. It would have been possible, as Hedger suggests in this issue of The Ecologist, in the case of Dutch elm disease. [Hedger, 1979]

But the elm was partly at least the victim of government priorities. Unfortunately little political capital can be made out of fighting forest diseases. The general public is simply not interested. Thus we find that the Department of the Environment dismisses the idea of felling all dead elms, stocking them and sawing them up. [Press release 2, 1977] The economic demand does not justify it. Assuming that a total of 500,000 cubic metres of elms had to be dealt with over a five year period, the felling, transport, handling and storage, together with the payment that would have had to be made to tree owners etc, would cost £15 million.

Since, according to their calculations only £10-12½ million would be recoverable, there would be a shortfall of £2½-5 million, and "there is no prospect of the government being willing to underwrite a loss of this kind". The attitude of government to important issues of this sort is incredibly depressing if one thinks of the massive sums of money wasted every year on projects that do little to foster the long term interests of the people in this country or of their natural environment.

This problem is not peculiar to Britain. The American government has proved to be about as short-sighted. According to Dr Mark McClure of the Connecticut Agricultural Experiment Station in New Haven, the current epidemic affecting red pine in eastern Connecticut is spreading to the native red pine forests of Massachusetts and practically nothing is being done to stop it - among other things, shipments of nursery stock to the yet unaffected areas is still unrestricted.

The reason is that money is not available. The government has different priorities and funds are only likely to become available once stands of major economic value are threatened; by then it is likely to be too late. [New York Times, 10 August 1977]

Keeping out the Parasites

Legislation can, of course, also be passed to prevent the import of timber which might harbour alien parasites. An attempt has been made to do this in this country in particular in order to prevent oak wilt being introduced into Britain. The importation of oak plants from North America is, in fact, now prohibited and imported wood of oak must have the bark removed to eliminate the bark beetles which could transmit the disease. The wood must also have a moisture content of less than 20 percent to eliminate the fungus involved in causing this disease.

However, it would be naive to suppose that all imports will be meticulously examined to see that they have no bark on them and that the moisture content of the logs is appropriate. As Burdekin himself admits, "such measures cannot guarantee exclusion of oak wilt or other comparable diseases." [Burdekin, 1978]

Pollution

Pollution also reduces the resistance of trees to attacks by parasites. High concentrations are not essential, a combination of different gasses in small quantities is often sufficient. Damage by smoke favours susceptibility to pests such as bark beetles, weevils and fir lice. [Schwerdtfeger, 1957] A pest attack by the small spruce leaf aphid was found to be more serious the closer the trees were to the source of the smoke. [Wentzel and Ohnesorge 1961]

Smoke also tends to reduce resistance to frost. [Wentzel 1956] Frost damage to spruce was higher in smoke polluted areas than elsewhere. The loss of needles to conifers during a frost was very much higher in areas of chronic pollution.

Coal dust has been found to reduce starch in the leaves of trees apparently by impairing the supply of light. According to Ersov, pollution by cement dust has reduced photosynthesis on lime and elm trees by 34 and 21 percent respectively. The reason seems to be that dust from cement, soot or other products blocks the pores and reduces light intensity, thereby interfering with photosynthesis. [Ersov, 1957]

Damage from Chemicals

Sulphur dioxide from blast furnaces, coke plants, fertilizers, soda cellulose and sulphuric acid plants, oil refineries and thermal power stations, is particularly damaging to trees. Again the reason is clear. SO2 paralyses the regulation of the stomata. Normally to protect themselves against the midday sun the stomata tend to close. When they are paralysed they remain open which means that transpiration proceeds unimpeded and the plant rapidly dries out, the more so since SO2 also interferes with moisture supplies. As the stomata remain open the leaves absorb more SO2.

Up to a point the plant may try to neutralize the effect of SO2 by absorbing more mineral salts but extensive damage cannot be prevented. Affected plants exhibit chlorosis of the assimilation organs with necrotic patches, and conifers lose some of their needles. Fruit bearing is interfered with, and if pollution persists seeds, may be deficient or completely absent. [Jahnel, 1955] Tests with rain acidified with sulphuric acid was also shown to have a considerable effect on increasing susceptibility to pests. [Keller]

Pollution by fluorine also has an adverse affect on trees. It occurs largely in the vicinity of aluminium foundries, fertilizer factories and glass smelting plants. Its main effect is to interfere with chlorophyll synthesis. As in the case of SO2 pollution, it leads to increased respiration, causes the leaves to dry out and gives rise to necrotic patches. [Keller]

Pollutants do not only affect the trees directly but also indirectly by changing the chemical composition of the soil on which they are deposited and the composition of the micro-organism populations. This is particularly true of sulphuric acid which seriously affects the soil in Scandinavian forests leading to reduced tree growth. [Jonsson, 1975] [Katz, 1949]

It goes without saying that all the various forms of damage done to trees by the different pollutants to which they are exposed, directly or indirectly, tend to reduce their viability and hence their health, thereby increasing their susceptibility to attacks by parasites.

How does one avoid pollution damage? Partly by planting trees away from sources of pollution and partly by refraining from introducing polluting industries into forested areas. But here we must run up against our society's set of basic priorities - a far greater importance is attached to the industrial process than to the maintenance of tree health.

Genetic Deterioration

A further factor in the incidence of tree disease is genetic deterioration. There are many reasons why it is occurring. The first is simply the tendency of loggers over the last centuries to remove the best trees of any particular stand, leaving behind only the second rate and less vigorous ones. This constitutes negative selection and must thereby lead to genetic deterioration. In many cases, the results are clearly discernible.

Thus, in Sweden, according to Lindquist, the

"good qualities of oak and the 'noble' qualities of birch have already been reduced to an alarming degree. Definite degeneration of important tree characteristics may be observed on the whole Norrland coast tract from Gayle to Haparanda." [Lindquist, 1948]

In this area, felling for the large sawmilling industry has been going on since the year 1600. Pine woods are now dominated in that area by "broad-crowned and intermediate types and the timber quality is generally poorer than in the interior parts of the country". According to the same author, the degeneration of pine trees on agricultural land in south and middle Sweden is even more striking. [Lindquist, 1948]

A second reason for genetic deterioration is to be found in current methods of seed collection for forest cultures - largely without consideration for the genetic characteristics of the parent tree or for the appropriateness of the strain for the area in which it is to be cultivated. According to Lindquist, this involves "very serious risk of getting seed with bad hereditary properties and of poor timber production in the future."

A third reason is the effect of pollution on the reproduction of trees. Sulphur dioxide pollution has a serious effect on plant reproduction. So much so that, according to Keller, there is a "negative linear correlation between sulphur dioxide concentration and pollen viability". [Keller, 1976] Thus he found that after 141 days of fumigating white fir Albies alba with sulphur dioxide in a concentration of no more than 0.1 parts per million (ppm), germination was reduced by almost a third, while if he used a concentration of 0.2 ppm, it was actually reduced by half.

Such data showed

"a significant detrimental influence on reproduction of an important tree species. It indicates particularly a loss of genes, i.e. an impoverishment of genetical resources with consequences which are presently still unknown," (Keller 1976).

Significantly, his evidence fits in with observations made by other researchers such as Wentzel [Wentzel, 1963] and Mamajew and Shkarlet (1972) on the effect of sulphur dioxide on beech and pine.

A fourth cause of genetic deterioration is the mutagenic effect of various pollutants and combinations of pollutants and, in particular, of the various pesticides with which forests continue to be sprayed. I have not found any material on this subject, but the mutagenicity of many of these chemicals has now been well established largely with experiments on bacteria and mammalian cells cultivated in vitro.

Since chemicals which are mutagenic to one form of life also tend to be so to others, the genetic material being similar in all cases, one must expect trees to be equally affected - which must eventually lead to still further genetic deterioration.

Wounding Trees

A further point to consider is the physical damage caused to trees by people, vehicles and other mechanical devices. This can also lead to disease. Let us see how. A tree is normally capable of resisting invasions by parasites of which it has some phylogenetic experience. It has a number of lines of defence against invaders. Firstly, it forms chemical barriers in the wood behind the wound to prevent infection. These barriers are usually effective and the wound heals. In some cases, however, micro-organisms may penetrate these barriers.

All is still not lost because the tree has another line of defence. The injured cambium produces a zone of special cells that seals off the affected area or 'compartmentalises' it. The compartmentalised zone may decay but the decay is contained and does not spread into the new wood that forms around the affected compartment. If a tree is wounded too often however the tree is gradually weakened, so much so that it may succumb to invasions it would previously have successfully resisted. [Shigo, Larsen 1975]

Many serious tree diseases in built-up areas appear to have occurred largely because the trees in the area have been wounded by man's activities and their resistance thereby reduced. This appears to be true of the fungus that causes oak decline (not to be confused with oak wilt) and which mainly affects trees in Texas and on the coast of the Gulf of Mexico. This disease appears mainly to affect trees that have been wounded and hence debilitated by man's activities.

A Combination of Factors

The various factors leading to ecological maladjustments, and hence the reduced resistance of trees to disease, rarely occur independently. Very often trees are affected by various combinations of these factors which have, at best, an additive effect, at worst a synergic one - causing greater damage than any one factor could by itself.

Consider the case of defoliation of oak trees by the oak roller moth. In 1923/4 this led to the death of many oak trees. It was found that the dead trees were covered with honey fungus which extended right up the trunks. It was also found that most of the dead trees had previously been weakened by other factors.

Thus mixed woods from which conifers had been removed during the war suffered excessively, as did those in which the removal of conifer nurses had been too long delayed. It also appeared that alteration of the water table by mining and pumping operations could have had a predisposing effect. It may well be that all these factors were involved.

Consider another example; a disease that broke out in Slavonia in June 1926 and led to the death of pedunculate oaks over a wide area. The trees appeared to have been killed by the fungus Armillaria mellea, but according to Robinson this fungus would not have had such a destructive effect if the trees had not been weakened successively by defoliation by various insects (Liparis dispar, Liparis chrysorhea and Melacosoma neustria) and if they had not then also been affected by mildew. [Robinson, 1927]

The disease affecting beech in England described by Lonsdale in The Ecologist has been attributed by some to the combined efforts of the beech scale insect and a fungus, Nectria coccnea. [Lonsdale, 1979]

Other plant pathologists have regarded environmental factors as the main cause. The German plant pathologist Zycha considers that drought and frost are the main factors involved. Others, in particular Brown, consider that the scale insect alone causes the disease. Lonsdale considers the thesis that several if not all these factors may be involved. Before a fungus can kill the beech, a lot of conditions may have to be satisfied and this thesis seems to be consistent with the material we have considered in this article.

Treating diseased trees: biological control

This of course, makes tree diseases particularly difficult to treat. A large number of maladjustments must be corrected and the methods used to correct one may actually increase the extent of others. Ideally, one would allow evolution to do the work for us, for the changes brought about by evolution assure the adaptation of systems to their total environment, not just to a single aspect of it. In this way, precisely those changes would slowly be brought about in the different components of the ecosystem affected that would assure their perfect readjustment to the new conditions.

Needless to say we cannot afford to wait that long; the adaptive process must be speeded up. This as we have seen can be done by planting soil improving trees when poor soil can be incriminated. Planting different species from the most appropriate strains to replace a single monoculture must also increase stability. When it comes to dealing with a maladjustment caused by a very destructive imported parasite more adventurous remedies are clearly required, even though this means taking certain risks. One such strategy is biological control.

Biological Control

If an imported parasite can be particularly damaging, this is because it is no longer subjected to the climatic, topographical and biotic controls that normally keep it in check in its natural habitat. To introduce the appropriate weather or topographical conditions is obviously impossible once the trees have already been planted and affected by the parasite.

What can be done however is to introduce one or more of the missing biotic components of the parasite's natural environment i.e. the tree's natural predators. Many successes are supposed to have been achieved in this way. It has even proved effective to introduce (from other areas) predatory insects that are not the pest's natural enemies. A pine forest in Colombia appears to have been saved in this way from destruction by leaf-eating moth larvae.

The defoliating moth larvae (Oxydia trychiata), previously of little consequence in that country, had suddenly become very destructive to plantations of exotic pines for the pulp and paper industry. Small wasps (Telenomus alsophilae) from North America that prey on canker worms which defoliate such broadleaved trees as oak and maple were released into the plantation in the winter of 1975 with very satisfactory results. [Science News, 1977]

Biological control is of course fraught with danger. The imported predator may well cause more damage than the target species it is supposed to control. Also it still involves accommodating undesirable trends, i.e. the planting of trees in a biotically unsuitable environment - in the case of commercial conifer plantations in an area in which they were previously unknown and in which they encountered parasites of which they had had no phylogenetic experience.

Biological control is nevertheless a far sounder strategy than is chemical control since, in the best conditions, it can make a positive contribution towards recreating a less unbalanced forest ecosystem, while chemical control can only have the opposite effect.

Finding and breeding new Strains

If biological control does not provide a means of controlling the population of an imported parasite, the only constructive course of action that can yield fairly rapid results is to find or breed strains of a particular species that are resistant to the disease.

It has been found for instance that the Chinese chestnut is resistant to the blight that has affected the American and European chestnuts. The Malayan dwarf coconut palm appears to be resistant to Lethal Yellowing disease as is a newly found variety, the Maypan hybrid. The Chinese elm also appears to be resistant to Dutch elm disease, and is being planted in the USA. A strain of the red oak has been developed that appears to be resistant to oak wilt. [Press release 3, 1977]

The problem, of course, with breeding resistant strains is that trees can be affected by a lot of potential parasites. The poplar for instance, as Lanier points out, is susceptible to an astonishingly high number of plant and animal parasites and these are particularly likely to attack certain clones of the cultivated hybrids. [Larnier, 1974]

If the hybrid, created as an adaptation to an introduced parasite, is too radical a diversion from the native strain, it may simply cease to be adaptive to all the other constituents of its natural environment; this, in the long run can simply increase the tree's susceptibility to all sorts of other possible diseases.

As Bingham writes,

"If we can only have the forebearance not to rush into stop-gap breeding work that might seriously alter the genetic structure of both host and pathogen, we will be ahead in the long run." [Bingham et al, 1971]

Burdekin also considers it

"important that great care and rigour be taken in these breeding progress to ensure that other factors such as tree shape and vigour, ability to withstand climatic influences and resistance to other diseases are all incorporated in the rigorous testing procedures." [Burdekin, 1978]

But this may not always be possible.

Another problem is that breeding hybrids is a slow process. Once a resistant tree has been found or bred and planted in large quantities, a long time has elapsed, still longer before it reaches maturity. That is why short cuts must be used. Vegetative techniques, including tissue cutting and rooting, must be resorted to. To reproduce the trees sexually would take too long.

Yet it is by doing so that a population displaying the genetic variety necessary for resistance to diseases would be created. When clones are all identical, they display minimal genetic variety and minimal resistance to disease. These techniques in any case are only applicable to commercial forests and amenity trees. When tree populations are annihilated in natural forests they cannot be replaced by such methods. The logistics of the problems involved would be too great as would the cost, especially in view of today's priorities.

Looking towards the future

We have established what are the real causes of tree diseases. Theyall involve changes that have tended to reduce the stable relationship established by evolution between a tree population and its climatic, topographic and biotic environment, thereby giving rise to various forms of phylogenetic maladjustments. [Boyden,1973] These changes tend towards reversing the evolutionary process, since it is precisely the latter's goal to increase the stability of these relationships correspondingly reducing the incidence and seriousness of such maladjustments.

The ecological approach aims at correcting these maladjustments by restoring the stability of the relationships between tree populations and their environment, thus restoring their real health, and hence that of the ecosystems of which they are part. To implement such a programme in our modern society is difficult for a number of reasons.

To begin with, there are no technological means of carrying it out, and in a society that is specifically organised to provide technological solutions to the exclusion of all others, this is very serious. Indeed ecologically orientated policies would not provide research grants for scientists, profits for entrepreneurs, taxes for our government or jobs for job-seekers (at least at the current rate of pay) to the extent that technological solutions would. Nor would they make the same contribution to our Gross National Product (GNP).

Also there are no instant solutions to ecological degradation, no magical cures for it, although unfortunately we have been hoodwinked by many scientists into believing that such things exist and are generally available as the solution to our worsening problems.

In addition, the ecological approach involves accepting a certain level of tree losses. They may decrease as overall forest health improves, but losses there must always be. Parasites will never be entirely exterminated. For this reason alone the ecological approach is unlikely to be accepted by those who have been misguidedly taught that science can actually eliminate pests.

Indeed one might go so far as to say that so long as our society remains on its present course, committed as it is to continued economic growth, it is unlikely that any ecologically orientated policies can be implemented. The reasons are clear.

The planting of fast-growing exotics in climatic, soil and biotic environments to which they have not been adapted by their evolution is necessary to achieve the high rate of tree growth and the short-term profits required to maintain the economic viability of our forestry enterprises.

The planting of very high density monocultures, in which trees are all of the same age, and the adoption of such unsound practices as scrub clearance and clear-felling are usually necessary to render forestry sufficiently economic and, in the short-term, to assure its survival in ever less propitious conditions.

Further growth of the forestry sector would probably require the adoption of still more aberrant practices. Indeed Peter Wood of the Commonwealth Forestry Institute stated at a recent conference that we were still at the hunter-gatherer stage in forestry. If we farmed trees as we farm other agricultural crops, we could, according to him, increase yields by up to 800 percent. [Elkington, 1978]

Today's Epidemics: only the Beginning?

If misguided people of this sort are allowed to influence developments - and political and economic pressures must tend in that direction - then the destruction we are seeing today may well be but the precursor of even more terrible tree epidemics that yet lie in store.

The abstraction of stili greater amounts of water to satisfy the burgeoning industrial, domestic and agricultural demand in a growing economy must lead to further falls of the water table in remaining forested areas with the associated reduction intree vitality.

The current loss of forested land to agriculture to make up for the hundred thousand acres or so of agricultural land eaten up every year by urbanisation will continue to increase as more motorways, air ports, factories and housing estates are built to provide the physical super-structure of a growing economy. [Coleman, 1977] This must have the effect of pushing forestry to ever more marginal areas - which as we have seen must also further reduce tree health.

The generation of increasing levels of pollution by smoke, coal, cement dust, sulphur dioxide, fluorine and the other by-products of the industrial process will also be inevitable. The cost of systematically reducing these levels in an expanding economy is prohibitive. On the contrary, as economic problems worsen, the amount of money available for pollution control, which is still low down in our government's list of priorities, is likely to decrease rather than increase.

The same is true of pesticides. The total amount put to agricultural and non agricultural use in Britain of even the most poisonous varieties, such as the chlorinated hydrocarbons and organophosphates, has increased rather than decreased and in a growing economy will continue to do so.

As for mobility, which we have seen to be an important cause of tree diseases, its growth is an essential feature of economic progress. Every year more and more people are herded across the oceans for business and pleasure. Trade in wood and wood products is also likely to go on increasing. We in this country import every year £2,000 million worth of wood, and the gap between local production and total needs continues to increase.

Given the growing ineffectiveness of controls in a chaotic and disintegrating society in which everyone's sense of responsibility is being rapidly eroded, one can only predict the introduction into this country of more rather than less dangerous parasites from abroad.

In the context of a growing industrial economy the future of trees in this country is indeed grim. If we continue in this direction, a treeless Britain, to use the title of Elkington's article in New Scientist, is not only a possibility but a definite probability. [Elkington, 1978]

This is particularly so if we consider that trees are among the most vulnerable of organisms. They cannot move to get out of the way of their enemies nor migrate to avoid bad weather. They are highly complex and only have a new generation every 30-3,000 years as opposed to every two weeks or so in the case of many insects. This means that they are very slow to adapt to changing conditions. Indeed, it may be that an environment capable of supporting healthy populations of such sophisticated forms of life as trees may have to display a degree of complexity and stability that could not be achieved on a much more heavily industrialised planet.

In this country, the situation is particularly acute; because of the density of our population, the already high level of industrialisation and our very low tree diversity, only a few alien parasites, operating in the appropriate conditions, could annihilate the major part of our remaining trees.

Only a reversal of current trends towards increased industrialisation would assure their survival. Fortunately, such a reversal can be predicted with absolute certainty. [Goldsmith, 1971] Indeed, it is already beginning to happen - not of course, as the result of conscious public policy, but simply because world conditions are becoming ever less favourable to the functioning of the industrial system. It is for this reason, and this reason alone, that I entertain some hope for the future of our trees.

References