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The right trees in the right place

Hans M. Heybroek

HANS M. HEYBROEK IS with the Dorschkamp Research Station for Forestry and Landscape Planning, Wageningen, the Netherlands


Whenever plantation forests are to be established, important questions must be answered: which species and which provenances should be chosen for the available sites, and what are the needs, potentials and possibilities for genetic improvement? Good results require careful matching of species and provenance with the site.

These questions are not isolated from the other aspects of forest management. Plantation forestry requires adjustment between the planting stock and site conditions, soil preparation, spacing, silvicultural treatment, protection, rotation age, exploitation techniques and end uses. This adjustment is not irrevocable but a continuous process of trial and error. While all aspects are interdependent, the genetic quality of the planting stock is a key in the adjustment process. It is clear, for example, that a different use may necessitate a different genetic make-up of planting stock, which, in turn, may reflect on the protection needed, and vice versa. Increased frost resistance may allow the use of a species in new areas, where it may require a different treatment. Similarly, improved, faster growing stock may need different nursery techniques, reduced (or increased) protection, wider spacing and a shorter rotation cycle. Certain species may become interesting possibilities for other sites as well, even in places where forestry previously could not compete with other forms of land use. The foresters' art is in finding the best balance between all these variables in order to obtain an optimal production. New, better provenances or genetically improved stock may change that balance and have far-reaching effects.

Why choose hardwoods? When comparing hardwoods and conifers as groups, no general statement can be made as to when and where one or the other should be preferred. Site by site, and case by case, a choice has to be made between the many candidate species, irrespective of their being hardwoods or conifers. Nevertheless, hardwoods have some especially advantageous traits. Many species have the ability to sprout from stumps, thus allowing coppicing or pollarding. As this makes expensive replanting operations superfluous, it allows for short or very short harvest rotations of wood of small dimensions on a continuous basis, including bark and leaves for fodder. Regeneration from the old stumps, in addition, may be safer than replanting operations on sites that are prone to erosion or landslides. In conifers, this ability is rare, Sequoia sempervirens being an interesting exception. The ability to fix nitrogen with root nodules is confined to some hardwood genera. Hardwoods often blend better with agriculture: they are widely used for shelterbelts and village plantings as well as for shelter, shade, leaf fodder, utensils and fruits.

Choice of species. A wealth of information on the ecological properties of tree species is embodied in the mini-monographs presented at the FAO Consultation on Fast-Growing Trees, in the specialized handbooks on eucalyptus and poplars, and in the traditional silvicultural handbooks. Some of this information can be given in an organized form such as lists of species that can withstand certain ecological extremes, such as:

· Species resistant to periodical flooding: Fraxinus spp., Salix spp., certain Eucalyptus spp., several Populus spp., Liquidambar styraciflua, et al.

· Species resistant to (some) salt: Acacia spp., Robinia pseudoacacia, Ulmus pumila, Populus alba, certain Eucalyptus spp., et al.

· Species for dry soils in cold-winter climates: Robinia pseudoacacia, Ulmus pumila.

Such information looks impressive but, on close inspection, is full of gaps, being mostly qualitative instead of quantitative. When looking for the best species for some new site, for example, only suggestions about which species should be tried are offered. The establishment of species trials is thus imperative.

The design of species trials depends entirely on the information that is sought. Arboretum-like comparisons and line plantations can supply only some qualitative information on health and on general adaptation to the site. If quantitative data on yield per area are required, trees must be planted in replicated plots. Larger plots allow a better estimate of the potential yield but their size must be kept to a minimum for practicality. Repeating a portion of the trials over two or three consecutive years might be an alternative. In several such trials, considerable "year-effects" have been noted: a species or provenance that grew poorly after being planted in a certain year developed much better when planted in another year. This might be caused, for example, by accidental cultural or weather conditions that lead to a slow start of growth after planting but which may have a very long aftereffect.

Choice of provenances. The choice of species can never be fully separated from the choice of provenances. Most tree species are highly variable genetically. Genetic homogeneity, as exhibited by Pinus resinosa and Pinus omorika, seems to be a rare exception, certainly in the temperate hardwoods. Unfortunately, knowledge is limited about the amount of variation, and about how variation is distributed over the range of a species or between and within provenances. Even if a single provenance looks homogeneous, another provenance of the same species may well be quite different in appearance and growth. Even species with a limited natural range can be very heterogeneous genetically, as exemplified by Alnus cordata (Cianci, 1980). In many species, this variation has not yet been studied and this seriously complicates species trials. Evidently it is impossible to judge the suitability of a given species on the basis of one sample. The species cannot be tested, only certain provenances of the species. One may be disappointing' while another may be satisfactory. A sensible species trial, therefore, has to include several promising provenances per species. This leads to a large and unavoidable increase in the size of the trial.

The full scope of geographic variability of a species has to be tested separately in special trials, sited in various locations under different ecological conditions, using large numbers of provenances. These should be systematically collected inside and sometimes outside the species' natural range. Interactions between provenances and sites should be analysed. This requires an international programme and close international cooperation. The results of such studies may very well lead to the conclusion that earlier species trials did not include the correct provenances, so that possible rejection of a certain species was not justified, and that it must now be retested using the better provenance(s).

Genetic improvement. If a research and development programme on plantation trees does not include genetic improvement, the programme is likely to be unbalanced. Adaptation of the genotype of a plantation species to the local conditions and to its envisaged uses is essential. Genetic improvement can be combined with the establishment of local and dependable sources of seed. The genetic improvement of a species may have to begin with the identification of the best provenances for local use. This can be important. There is one example of a full-improvement programme in an introduced species, successfully carried out, including plus-tree selection, establishment of a seed orchard and determination of the genetic gain accomplished. It was only discovered later that a different provenance from the natural area of the species easily out-produced even the improved material. In the case of a species that was introduced long ago, the improvement of the local "old" material should be carried out concurrently with a more systematic testing of "new" provenances from the natural area. Experience has shown that "old" material can be valuable, apparently because natural selection over the years has already removed some less well-adapted genotypes.

During the Consultation, improvement was variously recommended and generally considered feasible for many important properties such as disease resistance, insect resistance, faster growth, better timber quality, better fibre characteristics, higher wood density, better drought resistance, salt resistance, frost resistance, better rooting of cuttings, better coppicing ability, and even for higher nitrogen fixation, as in Alnus. Similarly, the different qualities needed for use in agro-forestry can be improved. In some cases, rapid multiplication of genetically improved material can be obtained by vegetative propagation, which allows large-scale use of rare and superior genotypes or species hybrids. The "genetic gain", or the degree of genetic improvement obtained, can be very high indeed and can lead to a revolution in the culture of a species. While research into the facilitation of vegetative propagation was, therefore, urgently recommended by the Consultation, the risks involved, particularly the narrowing of the genetic base, were also noted.

A TREE NURSERY IN SUDAN Which species? hich provenances? Which sites?

Diversification. There are several regions in the world where just one species, provenance or clone has become the dominant or single plantation material. In such cases, apparently, the species do well: they are easily grown and planted; foresters have learned how to handle them; and customers are satisfied with them. Other species may or may not have been tested at all, or tests may have been cursory.

Even if everyone concerned is content with such a situation, it is not a healthy one. The forester, at least, should be very uneasy about it. Where it exists, research should be directed to finding and developing other species and clones that serve some of the same purposes. Wherever possible, complete dependence on a single plantation species should be avoided. Such a situation is inflexible and highly vulnerable, while the world is dynamic and unpredictable.

First, the market may change. Today, only poplar 1-214 for a local pallet factory that prefers stems of 35-cm diameter may be grown. In 10 or 20 years, when the trees now planted will be mature, needs may be different, and new customers may have replaced the old ones. Diversification is, in fact, a safeguard. The insecurity and unpredictability of future markets are strong reasons to diversify to meet the rather uncertain needs of the future. Such needs should be a stimulus to try out, if only on a small scale, many different species, provenances and clones, in order to find some that are promising on at least some of the available sites, even at a lower total production. Market insecurity should be seen by the silviculturist as both an incentive and an opportunity to plant different species on different sites, each where it is thought to thrive best. It is a challenge to the forester's professional skill.

Casuarina eguisetifolia BEING USED TO STABILIZE SAND DUNES IN VIET NAM any new plantation is a gamble

A second consideration is that any new plantation is a gamble, but especially on a "new" site or in a new region. The decision to plant trees often has to be taken with imperfect knowledge and before the chosen species has proved its health, value and desirability over a full rotation. There is the risk that a tree may prove disappointing at a later age for some unpredictable reason.

Biological considerations lead to exactly the same conclusions. The ever-increasing intercontinental traffic in goods threatens to continue to bring in new species of parasites and insects that can be devastating to trees. Even if a particular parasite is present already, the importation of a new strain or variant can cause an unpredictable worsening of the situation: this happened recently with the introduction of a new strain of Dutch elm disease in Europe (Gibbs, 1978). It is important to stress the unpredictable nature of this process. Some newly introduced parasites may fail to become established or else fail to cause any significant damage under the new conditions. However, the parasite may thrive in its new country and cause damage far beyond that in its country of origin, even on exactly the same host species. For example, the European Melampsora-rust, which was introduced in Australia and New Zealand in the last decade, can kill clonal poplars there which it damages only slightly in Europe. Virtual extermination can also occur if a parasite finds a host species on the new continent without resistance to it; that happened when the chestnut light fungus was introduced in North America.

Many such calamities can be predicted for the future. Nature's potential for destruction is far from exhausted. It is impossible to foresee, however, where and how it will strike and on which species. The forester has to prepare, as well as possible, for such contingencies. Plant quarantine, useful as it is, cannot keep out all the plant enemies all the time. Diversification is the main line of defence which the forester can build. It does not avert the risks, but it spreads them.

While inter-species diversity is very effective - a eucalypt, for example, is not likely to suffer from the same new parasite that damages pine - the maintenance of genetic variation within a species is also very important. Most tree species are genetically highly diverse, and quite often certain representatives will prove resistant to a parasite that severely damages other representatives of the same species. Therefore, the greatest vulnerability occurs where a single clone is used to the exclusion of others over a large area. For maximum effect, genetic diversity should be employed in a structured way at different levels. If we take poplar as an example and imagine a flexible and relatively safe situation, genetic diversity at the stand level could be accomplished if seedling varieties could be produced. These, unfortunately, are not available. The use of multi-clonal mixtures has a similar effect and is advocated by many (Schreiner, 1972; Libby, 1980). Monoclonal stands, however, have some silvicultural advantages and seem acceptable in poplar culture to some extent (Kleinschmidt, 1979), provided the situation at higher levels is safe.

At the region level, there should be considerable diversity with 10-20 good clones of diverse genetic background being marketed and used regularly, the commonest occupying no more than, say, one third of the poplar forest area. At the country level, the experiment station should maintain tests with perhaps 100-200 experimental clones on different sites. These should serve as a reserve from which released clones that appear to fail and have to be withdrawn can be replaced (Koster, 1980). At continent and world level, gene conservation should be used to ensure that potentially valuable gene material is preserved and kept available for breeders.

International cooperation. Forestry research traditionally fosters international cooperation. It is no accident that the International Union of Forestry Research Organizations (IUFRO) is one of the oldest international scientific organizations in the world. The reason is that such cooperation pays, especially in forestry. In choosing the best species or provenance for a certain country, an international approach seems to be the best answer.

It is evident that each country should try the species or provenance that has proved successful in a neighbouring country. An exchange of plant material and an agreement to set up parallel experiments with identical materials are the logical first steps in assisting this process. Tests in different countries should use the same standard provenances. Such testing gives multiple information. Weather conditions that occur only once every 10 or 20 years in one country may be common in another one. Thus, the experiences of the latter may allow the former to shorten the testing period. Certain diseases or pests may tend to be more severe elsewhere; pooled experience can advise either caution or confidence about the probable utility of certain plant materials under given conditions.

Naturally, nothing can replace local testing. However, interpretations of the results of local testing can be made earlier, and projections about the usability of certain materials can be firmer, if data from trials in neighbouring countries are known. The more the trials in different countries are set up along parallel lines, the stronger and more detailed the conclusions arising from the comparisons. Sometimes the opportunity occurs to fit or to expand cooperation into an international scheme to study the geographic variation of an entire species. Several such studies are under way under the aegis of IUFRO; many more await initiation. International cooperation has clear advantages. The gain in time and information is apparent. Because international trial series can be a sound investment in future production, they are good prospects for international funding.


CIANCIO, O. 1980 Mini-monograph on Alnus cordata L. FAO Consultation on Fast-Growing Trees. FGB-79-6/ 4.

GIBBS, J.N. 1978 Intercontinental epidemiology of Dutch elm disease. Ann. Rev. Phytopathol., 16: 287-308.

KLEINSCHMIDT, J. 1979 Limitations for restrictions of the genetic variation. Silv. Genet., 28(2-3): 61-67.

KOSTER, R. 1980 Personal communication.

LIBBY, W.J. 1980 What is a safe number of clones per plantation? (In manuscript)

SCHREINER, I. 1972 Mass production of improved forest tree planting stock through synthetic varieties. In Biology of rust resistance in forest trees, p. 571-590. USDA, Misc. Publ. 1221.

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