Why measure forest disease impact?
Kinds of disease impact
Natural versus plantation forests
Geographic relationships
Attention focused on disease impact
More pathologists needed
Reports presented at the Symposium, as well as the discussions that followed, clearly showed that tree diseases are properly regarded as universal problems having a serious impact on forest productivity. Specific reports were presented from North America (3 countries reporting), Latin America (9), Europe (16), the Near East (16), the Far East (8), Africa (24), and Australia (3). The U.S.S.R. is the most important of the major timber-producing regions of the world for which no summary statement was presented. For this vast area, it can only be presumed that the disease picture is, in the southern part at least, not too dissimilar to that on those same and closely related forest tree species which occur also in neighboring Czechoslovakia, Poland and Yugoslavia.
While there is universal agreement that forest tree diseases do have significant impact on forest productivity, and hence also on forest management in general, there is tremendous lack of agreement as to just how significant this impact really is. There is great variation as to where quantitative measurement of this impact has been attempted and how effective these efforts have been. Consider the ubiquitous fungus Fomes annosus, for example. From India it is recorded only that annosus root rot is moderately damaging to Cedrus deodara; in Belgium, it is considered that 100,000 hectares, or 17 percent of the total forested area, are endangered by this fungus; in Norway, the annual loss in spruce to this fungus is estimated to be 200,000 cubic meters, valued at 10 million kroner ($1.4 million), and it is conservatively estimated that the disease takes a quarter million cubic meters annual toll in the United States. Perhaps it could be concluded from these assessments that the impact is more serious in Norway than in either Belgium or India, but it would be almost impossible to judge the degree of difference. Any attempt critically to compare the seriousness of this disease between the United States and Norway would be but an obvious exercise in futility. Any true appraisal is further complicated by the fact that Fomes annosus sometimes causes outright mortality, sometimes slows down tree growth, and at other times acts as a heart (butt) rot leading to actual loss of, or degrading of, merchantable volume.
The above was but one of many examples that could have been cited of the complexities involved in attempting an appraisal of the impact of forest tree diseases. Appraisals are needed, however, not just to satisfy the curiosity of forest pathologists but, as set forth in the presentation for North America:
1. They make possible more accurate net inventories of timber assets and all that is thereby implied to forest management.2. They show just how serious forest tree diseases really are, and hence also where disease prevention or control could be most helpful.
3. They point up where additional research is needed in order to develop the essential preventive or control measures.
The importance of such appraisals was emphasized by J. C. Westoby, Deputy Director of the Forestry Division of FAO who stressed the need, in his opening remarks, to quantify the losses or the negative influences caused by forest tree diseases. Moreover, that the second General Recommendation, drafted at the conclusion of the Symposium, was to the effect that more general and better quantitative evaluation should be made of the direct and indirect losses from forest diseases [and insects] as a basis for determining the correct orientation of research programs and appropriate expenditures for forest pest research, surveys, and control.
Only the United States and Canada have made a concerted attempt fully to evaluate their losses to forest tree diseases. A few European countries have tried to make quantitative measurements of loss to specific diseases, as mentioned earlier in the case of Fomes annosus damage to spruce in Norway. Even in the United States, however, where estimates have been most complete and inclusive, the 140 million cubic meters of total annual loss is of debatable accuracy.
Annual losses of 30 million cubic meters for Canada are obviously most conservative because only outright mortality and heart rot losses were considered. In short, even the best of attempts fall far short of a precise measure of forest disease impact. Most countries of the world have only some awareness that diseases are doing some obvious damage to their forests.
As might be expected, concern about forest disease impact seems to be generated only when forestry itself attains a certain degree of intensity. In short, only when man starts to grow his forests rather than merely to reap nature's bounty does he concern himself too seriously with the "negative" influences. Thus, while the records are still incomplete and of debatable accuracy, European countries long ago began to take disease losses into account. In other countries, where forestry is not yet intensively practiced, it can only be said that "no diseases of importance have been noted or reported" Even in the United States, an appraisal of disease impact did not seem too important until it became advisable to determine how well the old-growth forest would meet timber requirements until it was converted completely to a system of managed forests. As plantation forestry increases, so will the awareness of the deleterious effects of diseases.
Just as increased emphasis on plantation forestry gives rise to increased concern over tree diseases, so does an abundance of virgin or old-growth forest seem to generate an attitude of apathy toward disease losses. The feeling may well be, as was so aptly expressed by I.A.S. Gibson in summarizing the situation in Africa, that as nature has provided both crop and disease, the cost of the latter need not be counted. Measurement of these losses in old-growth stands in the United States and Canada has shown, however, that this could be a most dangerous attitude. As an actual fact, the single big loss factor proved to be the presence of excessive heart rot in these old stands. Imagine the predicament in which North American forestry would soon find itself if these losses were neglected in planning the orderly conversion from natural to managed forests!
The adverse effects of forest tree diseases may be either obvious or subtle. Unfortunately the obvious effects, death for example, are not necessarily the most important. In their presentation for North America, Davidson and Buchanan listed eight ways in which they could visualize disease as adversely influencing a tree or a forest. These are listed here without any order of relative importance:
1. Mortality - trees killed by any disease.2. Destruction of wood already formed - meaning essentially the losses from heartrots that destroy or disintegrate wood. The decays of living trees are hence a type of loss distinct from all others.
3. Reduction in growth increment - decreased height and/or diameter growth which adds up to volume loss.
4. Delayed regeneration - years lost in getting a new crop started.
5. Deficiencies in stocking - too few stems, uneven spacing, or holes in the stand. Site not fully occupied and, hence, not up to full production potential.
6. Degeneration of species composition - disease of preferred species permitting volunteer inferior or weed species to take over the area.
7. Deterioration of site - soil erosion, compaction, leaching, etc.
8. Reduction in wood quality - incipient decay, stain, cracks, excessive knots, pitch pockets, and other responses to disease. No change in volume of production but loss in value of products.
Most of these eight types of impact are not too difficult mentally to visualize. Except for mortality, however, they are difficult ocularly to observe in the field, and are extremely difficult to measure or otherwise evaluate in quantitative terms. Also, it is obvious that any given disease may induce more than one type of loss. Reduction in growth increment eventually followed by mortality is an especially common sequence of events. Where more than one type of impact occurs, either concurrently or sequentially, their combined effects are additive.
While this list may seem all inclusive, discussions at the Symposium revealed still another type of disease impact that is assuming increasing importance in proportion to the increase in plantation forestry. This impact, extremely difficult to assess and even more difficult to quantify, might best be described as "enforced species substitution." This is related to, but not the same as, degeneration of species composition as defined above. Reference is made here to those innumerable situations where the presence of disease makes it impossible to grow the preferred species. Site, markets, and all other circumstances may be ideal for the growth (perhaps following introduction as an exotic) of a premium species, but disease impact forces substitution of a second choice, and all too often a poor second choice at best. The thousands of hectares in Europe and other parts of the world not planted to Pinus strobus because of the presence of Cronartium ribicola are the best possible example of this kind of disease impact. How should or how can it be measured?
Other examples of this type of disease impact discussed at the Symposium that warrant inclusion in any such listing are the enforced abandonment of Pinus nigra as a plantation species in Denmark because of Soleroderris lagerbergii, enforced abandonment of Cupressus macrocarpa as a plantation species because of Monochaetia canker in many areas of East Africa, enforced substitution of the slower-growing but resistant Pinus patula for Pinus radiata because of the damage from the foliage pathogen Dothistroma pini in parts of East Africa, and the inability to grow Pinus radiata in parts of South Africa because of the depredations of Diplodia pinea.
In contrast to this situation, there are all too many examples of failures resulting directly from the introduction of an exotic to be planted into a totally unsuited site. Here, improper moisture, temperature, soil fertility or pH, or some other edaphic or environmental factor - not disease - doomed the venture to failure.
If generalizations are at all possible or feasible, it might be concluded that virgin or old-growth forests are subject to one group of diseases and that the managed forest, more especially the plantation forest, is damaged by another and somewhat different group of diseases. Whether a plantation is composed of an indigenous or an exotic species seems mainly to influence the possible intensity of disease impact rather than the type of disease. This is not particularly surprising, for the mere establishment of a plantation renders even the most common native species an exotic to the extent that it is growing in an unnatural environment.
In the old-growth or natural forests, the greatest losses are incurred from diseases which do not greatly affect the continued survival of the tree. Heartrots, stains, and other diseases affecting the physiologically inactive heartwood become increasingly important, as was so clearly shown by the loss estimates for Canada and the United States. This is not at all startling, if one pauses to analyze the situation, since the virgin forest is composed only of residual individual trees that have survived the killing diseases of youth and attained sufficient age to form heartwood susceptible to decay.
With the advent of the managed forest, especially with the establishment of plantations, killing diseases take on a role of primary importance. These diseases may have caused the same losses many years ago in the virgin forest, but they were either unobserved or unevaluated. Root diseases seemingly tend to assume a certain localized importance. These are generally caused by pathogens already known to be established in the area and generally characterized by having a worldwide or at least continental range with a wide array of susceptible hosts. The introduction of exotics to establish a forest plantation is often followed by diseases affecting the living tissues of the host exclusively; that is, killing diseases. These, by contrast, are typically caused by pathogens of limited host range and geographic distribution. Under these conditions, the introduced disease reaches the acme of development. Most plantations have either not been established long enough, or are managed on a rotation so short that heartrot losses have not, and generally cannot, become critically damaging factors.
To draw or even attempt to draw comparisons between the disease situation in the forests of any two continents or other major land masses does not appear realistic or even helpful. The region referred to as the Far East, for example, may have certain political, social, and even economic similarities, but the climate is extremely diverse, ranging from subfrigid to tropical. Hence, there is a corresponding range and diversity of ecological niches, forest types, and, most important of all, in forest pathogens and their impacts. The ecological and pathological diversity within this one region may be as great as that between regions or even continents.
North America displays an equally wide variability in latitude, altitude, and climate, with a corresponding variability in forest types and forest pathogens, again the differences within being as great as any that may be found between continents. Surely Latin America is a unit only by virtue of a generally common language since the vegetative types of that tremendous land mass range from the treeless plains to the dense tropical jungles. The disease problems peculiar to coniferous plantation forestry in South Africa are extremely different from those of the tropical rain forests of Liberia. Only Australia and New Zealand, having evolved in relative isolation, tend to have an individualistic flora that may influence also the pathological picture. Certainly accidental introductions of forest pathogens should be easier to control. Here too, the equilibrium between indigenous hosts and pathogens achieved by centuries of natural selection and isolation from outside influence has in relative terms only recently been upset by man through utilization, modification of environment, and introduction of exotics.
Europe, as a geographic entity, may be a more useful term of delineation in reference to forestry and forest tree diseases than other areas combined for reporting purposes at the Symposium. Climatic variability is somewhat less extreme, native tree species are not too numerous, and, most important, forestry has long been practiced there with the result that tree diseases and their impact have been the subject of consideration over a longer period of years than elsewhere in the world. However, even in relatively compact and homogeneous Europe, the forests are divided into small units, which are managed differently according to tree species, site and owner. It can also be concluded that even these small units still differ in the disease problems which may afflict them.
It becomes obvious that only general comparisons can be drawn and only general lessons learned from the disease problems in different parts of the world. This means generalizations based on one situation in one area may not hold in a different region. Most forest tree diseases of serious impact are caused by fungi, and since fungi are living plants they respond to temperature, moisture, food source (suscept), and other environmental factors just as is the case with host plants. Hence, there may be expected to occur in South Africa, for example, ecological niches almost identical in suitability for the development of a given forest tree pathogen to other ecological niches, say somewhere in North America. It is within these identifiable but restricted areas of similarity that the experiences gained with tree diseases in one geographic unit of the world may most profitably be applied to another.
It is not especially important to know whether or not forest tree diseases are taking a bigger toll in the Far East than in Africa, or causing more damage in North America than in Europe. It is important to know, however, that forest tree diseases are causing losses in forests in all parts of the world, and that reductions in the amount of this damage by disease control may well be one of the most readily attainable means of increased forest productivity.
The FAO/IUFRO Symposium has certainly focused worldwide attention on forest tree diseases and the impact they are having on the forests. Reports and discussions there revealed the range and diversity - and similarities - of forest tree diseases throughout the world and gave cause to wonder just how serious they really are if the whole truth could be learned. Many nations will doubtless make a more concerted attempt to find out - and then to take more drastic and effective remedial action.
To go back once again to the stated needs for measuring the impact of forest tree diseases, namely:
(a) procurement of more accurate net forest inventories;
(b) guidance to forest disease control programs;
(c) direction to forest disease research efforts,
determinations of this impact must be made for any given region, country or management unit before these benefits can be realized. Probably the best qualified person to be evaluating forest disease impact is the forest pathologist, provided he has the necessary co-operation from specialists in silviculture management and economics. The same forest pathologist may also be conducting research to develop controls for these diseases except for the fact that he cannot gain the necessary financial support. One important reason for this is that these losses are not understood by administrators, which means backing for pathological research may be lacking.
The understatement of the year is that no country in the world is sufficiently staffed with adequately trained forest pathologists. Indeed, many countries do not have any forest pathologists at all and the only semblance of knowledge as to forest disease impact available results from purely botanical (mycological) surveys and an occasional chance observation of abnormality on the part of a passing agricultural pathologist. Even the countries best staffed with trained forest pathologists have been able to measure only part of the total impact from forest tree diseases.
Knowledge of the prevalence, distribution, and impact of forest tree disease in one part of the world serves to alert other parts of the world possessed of closely similar environment and possible tree hosts as to potential hazards. Thus forewarned, they can be forearmed to prevent disease introductions and have advance information as to effective control measures should efforts to exclude them break down. Perhaps it has been the heterogeneous character typical of most forests of the world that has prevented disease losses from being even more destructive. Man with his plantation forestry is changing all that, and drastically so in many instances, by the introduction of exotic species into strange and often ill-suited environments. Therefore, killing diseases in particular may be expected to increase and cause significant losses of forest productivity with adverse effects on forest management plans throughout the world.
One fortunate ameliorating circumstance may indirectly lessen future forest disease losses. Just as increasingly intensive forest management practice, that is, pure plantations of the best tree species selected on a world wide basis, may increase the potential for disease losses so will the possible improved utilization practices permit economic salvage of at least some of the dead trees. Even today, this happy circumstance is already clearly evident in many parts of Europe if only because it is economically practicable to utilize fuel wood made from partially decayed wood unsuitable for lumber or still higher value products.
Finally, the impact of forest tree diseases throughout the world is caused primarily by hundreds, if not thousands, of different fungi. Some are restricted to one continent, one host, one precisely defined environment. Others have somehow gained essentially worldwide distribution, may attack a wide range of hosts, and cause diverse types of damage under a multitude of environments. Obviously, diseases of the first class lend themselves admirably to control through exclusion, while diseases of the second class will always be, in all probability, problems to be contended with. Armillaria mellea, Fomes annosus, Endothia parasitica, Diplodia pinea, Cronartium ribicola, and Lophodermium pinastri, for example, are specific Latin epithets defining important forest pathogens. They are also, unfortunately, almost household word, among even the most poorly trained forest pathologists, be they in North America, Latin America, Europe, Africa, or elsewhere.
The Symposium has served to emphasize the extreme importance of quantitative evaluation of forest tree disease impact in order that management plans, disease control programs, and disease research efforts might be properly conceived and directed. The leading forest pathologists, all too often the only forest pathologist, from about 40 countries around the world were unanimous in their expressed needs for more precise disease loss figures. Forest pathologists recognize and appreciate this basic need but, unfortunately, all too rarely are forest pathologists also forest land managers occupying positions of decision-making authority and responsibility. The purpose of this chapter will have been served if even a few forest land managers, who did not attend the Symposium, read this report and appreciate the need for improved forest disease loss data. These administrators will then be better prepared to support and encourage the pathologists in their efforts toward development of controls for forest tree diseases - efforts which, in the final analysis, are directed toward improved forest and forest land management practices and increased productivity of our forest resources.
