FAO Forestry Department
In the preparation of this study. FAO is indebted to Professor D. Poore and Mr C. Fries, the consultants who researched and wrote the study: to Messrs Gutierrez de la Lama (Spain), W. de P. Lima (Brazil) and C. Malvos (France), who supplied much useful specialized advice: and to the director, librarian and staff of the Centre technique forestier tropical (France).
Readers can obtain the full-length paper by writing to FAO at the address given.
This article is extracted from the newly published FAO Forestry Paper 59, The ecological effects of eucalyptus. Since, on the one hand, eucalypts have the potential to confer large benefits on forestry programmes in developing countries, but, on the other, there is mounting criticism of their potentially harmful ecological effects, FAO decided to accept the offer of the Swedish International Development Authority (SIDA) to carry out this study. Its aim was to analyse as dispassionately and objectively as possible the available information on the ecological effects of eucalypts. Because of the importance of the subject and the critical need to disseminate information to foresters, land-use planners and decision-makers in general, Unasylva is publishing these excerpts.
EUCALYPTUS: A CONTROVERSIAL TREE native to Australia, now in 80 countries
· World forests are currently being cut at many times the rate at which they are being replaced. In tropical countries, on average, only 1 ha is planted for every 10 ha of natural forests cleared. There are high and increasing demands for wood for industrial use and fuel needs, especially in developing countries of the tropics with their growing populations. To cope with this situation, the choice is often made to plant fast-growing, nonnative tree species that have a multiplicity of uses. One such exotic group comprises the more than 600 species of the genus Eucalyptus, whose popularity as plantation species is attributable to their high adaptability, fast growth rate and wide range of uses: from sawn-wood and processed wood products to fuelwood of high calorific value to a variety of environmental and ornamental uses. Their popularity may be judged by the fact that more than 80 countries have shown an interest in eucalypts, planting more than 4 million ha worldwide, outside their natural range of Australia. Southeast Asia and the Pacific.
Yet along with this popularity, a growing body of opinion claims that eucalypts cause a variety of short-to long-term ills, impoverishing the environment in terms of the soils, water availability and wildlife - even where plantations have been planted on waste lands devoid of tree cover. Some countries have even banned the planting of eucalypts.
The questions that will confront those who have to decide whether to plant Eucalyptus are the following:
· Does the planting of Eucalyptus in certain circumstances carry costs, in the broadest sense of the word, that would not be associated with the planting of some other species?
· If there are such costs, will these be offset by the advantages (in rapid growth or adaptability to adverse conditions) that would be gained by planting Eucalyptus?
· Is there enough evidence, on the basis of this study and what is known of the social implications of planting eucalypts, to reach valid decisions? If there is not, what kind of information or research programmes are necessary to correct the deficiency?
· How important is the problem in general rather than local terms, and what investment in research does it therefore warrant?
More than 80 countries have shown an interest in eucalypts, planting more than 4 million ha worldwide, outside their natural range of Australia, Southeast Asia and the Pacific.
One of the major difficulties in undertaking this study has been to produce any valid generalizations on a subject covering so many different species and local circumstances. Another is the artificiality of the distinction between costs associated with "ecological" effects and costs that may be considered more strictly "social".
The second of these difficulties can be overcome only by a more critical study than has been possible here of the social attitudes toward the growing of eucalypts and of the social costs and benefits associated with it. It is strongly recommended that such a study be made to complement the present work; it is likely that it will be undertaken soon by FAO.
The first difficulty is not so easy to solve. If sufficiently critical experiments were to be set up comparing eucalypts with alternative plantation trees and with natural vegetation in all the possible sites and circumstances where the problem is likely to arise, the labour and cost would be out of all proportion to the importance of the problem. On the other hand, limited unreplicated trials are not even valid for local use.
The most useful research for the purposes of this review has been of two kinds: fundamental research concerned with elucidating the general processes of the water cycle and catchment hydrology, of nutrient cycling and of the physics of shelter-belts; and that which has dealt in a comprehensive and critical manner with a whole system (notably the total catchment studies). But even the latter can only be extrapolated to comparable conditions elsewhere, and it is unreasonable to suppose that such studies can be carried out for all the numerous circumstances in which a comparison between eucalypts and other species is needed.
TENDING EUCALYPTS IN BURKINA FASO do the benefits outweigh the costs?
Natural eucalypt forest appears to control the leaching and run-off of nutrients as well as or even perhaps slightly better than other natural forests.
The most profitable way forward would seem to be to use the systems already developed for catchment studies and for the nutrient cycle, refining them where necessary to apply as precisely as possible to the case of eucalypts, and to identify the critical measurements that would be required in each particular case to assess the likely effects of any interference with the system (in this instance, planting eucalypts). Devising these relatively simple measurements would give early warning of any serious consequences and would enable prescriptions to be made in advance (for example, for fertilizer treatment). It is recommended that FAO encourage research along these lines.
The following is a brief summary of the main findings of this study.
Critical evidence is lacking on two important questions: Do eucalypts use more water or have a greater effect on the water regime than other species of trees? and Are eucalypts more efficient in their use of water (producing more wood per unit of water used) than other species? Perhaps there is no general answer to either of these questions.
Water Catchments under forest have a lower water yield than those under scrub or grassland, but they may regulate flow better, depending upon the nature of the ground cover. There is evidence from the humid tropics, however, that young, rapidly growing eucalypt plantations consume more water and regulate flow less well than natural forests.
There are indications from Australian work that eucalypt plantations may reduce the yield of catchments more than pine, but the evidence is not conclusive.
Eucalypts are often planted where there have been no trees before. Under these circumstances, the water yield of catchments is reduced and water-tables are drawn down. The effect is greatest when trees are young and growing rapidly. Other tree genera would probably produce comparable effects.
The strong surface roots of some - but not all - eucalypts mean that they compete vigorously with ground vegetation and with neighbouring crops in situations where water is in short supply.
Erosion Eucalypts are not good trees for erosion control under dry conditions, as ground vegetation is suppressed by root competition.. This effect is accentuated by collection or burning of the litter, but reduced by terracing. Their performance as shelter-belts is similar to that of other trees of the same size and shape.
Nutrients Natural eucalypt forest appears to control the leaching and run-off of nutrients as well as or even perhaps slightly better than other natural forests.
The effects of uncropped; eucalypts on soil depend upon the state of the soil in which they are planted; they are beneficial in degraded sites, but probably not so when replacing indigenous forest. There is evidence from Malawi that eucalypt litter decomposes less well than the litter of indigenous Brachystegia because termite activity is reduced.
Where eucalypts are planted in bare sites, there is an accumulation and incorporation of organic matter. There is no evidence of podsolization or irreversible deterioration of soil.
Eucalypts planted on nitrogen-rich peat have been shown to take up large quantities of nitrogen. They could thus be used for reducing eutrophication.
The cropping of eucalypts on short rotation, especially if the whole biomass is taken, leads to rapid depletion of the reserve of nutrients in the soil. This is a direct consequence of their rapid growth; it would apply in much the same way to any other highly productive crop, and is also closely associated with length of rotation. There is some evidence that the removal of nutrients in comparable crops of pine is greater. In each instance, calculations of nutrient cost should be made and fertilizer treatment decided accordingly.
Competition The effects eucalyptus has on ground vegetation depend very much upon climate. Most have to do with competition for water; the effects of reduced light are probably less than those caused by some other broad-leaved trees or pines, since eucalypt foliage casts only light shade. Ground vegetation is less affected in wet conditions than in dry, when it may be greatly reduced, leaving the soil bare and prone to erosion. There is evidence that some eucalypt species produce toxins that inhibit the growth of some annual herbs.
The numbers and diversity of mammals, birds and insects are less in exotic eucalypt plantations than in natural forest. The usual relation seems to apply: forests produce more diversity than plantations of indigenous species, which in turn produce more diversity than plantations of exotics. This effect can be reduced but not eliminated by proper management to produce suitable habitats.
Displacement Eucalypt plantations largely displace the ecosystems that were there before. The relative importance, both ecological and social, of these original ecosystems should be carefully balanced against the advantages to be gained from the new plantations.
After reviewing the evidence very thoroughly, it must be stressed that there can be no universal answer, either favourable or unfavourable, to the question of planting eucalypts. Nor should there be any universal answer: each case should be examined on its individual merits. It is difficult to see how further general research, however detailed, can alter this conclusion.
Eucalypts, especially on a large scale, should be planted only after a careful and intelligent assessment of the social and economic consequences, coupled with an attempt to balance advantages against disadvantages. This can probably best be done by a sympathetic examination of the ecological circumstances and needs of local people, assisted by an understanding of the results of the fundamental research on water, nutrients, etc. referred to in this article.
Short-term ad hoc research on particular sites may be of some help in making local decisions, but the results of such research must not be extrapolated to different circumstances nor must unwarranted generalizations be made from it.