Hans M. Heybroek
HANS M. HEYBROEK, Forest Geneticist, is with the Dorschkamp Research Institute for Forestry and Landscape Planning, Wageningen, Netherlands.
The first task of a forest breeder is to identify or produce superior genotypes of forest trees, be it through selection, provenance tests, introductions, test crosses or hybridization. In most cases, this does not lead immediately to products ready for use by the foresters. Before improved genotypes can be used on a large scale, it is necessary that problems be solved about their multiplication and about the required degree of genetic diversity in the planting material. Though these questions are not in all cases reckoned to be the immediate task of the forest tree breeder, he can never neglect them as they may have serious implications for his breeding strategy.
Seed orchards are one of the main tools of the forest tree breeder. Through them, he hopes to provide the foresters with quantities of seed of an improved genotype for large-scale use in afforestation and reforestation.
PROGENY TESTING IN SCANDINAVIA a basic step in tree breeding
Seed orchards require a large investment in land and labour, in planning, establishing and maintenance. The expectation that they really will produce the desired quantities and qualities of seed is at the basis of these investments and often even at the basis of the major decision of starting a breeding programme.
Many questions about seed orchards have now been answered in an excellent handbook, the outcome of the activities of the Working Party on Seed Orchards of the IUFRO (Faulkner, 1975). It should be stressed that many seed orchards are beginning to fulfil their aims. Others, however, present difficulties. This has recently been summed up as follows.
It would appear that the yield of seed orchards is not living up to expectations. The most common (problem) was that many families are failing to produce an adequate number of male or female flowers. The actual seed yield is then produced by relatively few individuals, thus defeating the orchard design for a broad genetic base. Self-fertilization may become a dominant feature of such orchards. Even when flowering appears sufficient, there still occurs an extremely high abortion rate. Even when the cones mature, the ratio of full to potential seeds per cone may be extremely low. We need to re-evaluate our concepts and approaches if we are to realize the maximum seed yield and quality of "tree improvement effort" (Krugman, 1976; for a further discussion see Sweet and Krugman, 1977).
In the humid tropics, introduced pines but also native species may offer big and unexpected problems for propagation by seed through irregular flowering, poor pollination, flower abortion and low seed set. More research on these aspects is needed (Sheikh Ibrahim, 1977; Tufuor, 1977).
A fundamental problem of the seed orchard may further lie in the fact that the seed orchard aims at a high seed production whereas the seed produced should give rise to trees with a fast vegetative growth. These two goals may conflict where there is an inherent antagonism in a tree between seed production and vegetative growth. Such an antagonism is very well-known in apple trees and described for several forest trees (Heybroek and Visser, 1976; Teich, 1975; Heybroek, 1974). It may even be that the earlier and heavier flowering populations represent less desirable biotypes (Delaunay, 1977; Heybroek, 1974).
For that reason, genetic means of improving the seed production of seed orchards, for example by selecting heavily seed-producing clones or by selecting those clones that start flowering in a seed orchard relatively soon, should be considered to be not without risk. From this point of view, also the fact that in some seed orchards the bulk of the seed is produced by a few precocious and heavy-seeding clones should be viewed with suspicion. The resulting seedlings might similarly be early and heavy flowering and have an early culmination of growth.
This means that more attention should be given to non-genetic and even to quite artificial means of increasing the production of a seed orchard. For instance, there is promise in locating the orchard in an area with a climate that is conducive to flowering and seed production. This may be effective (Sweet, 1975; Werner, 1975; Gansel, 1973) but it often requires much international cooperation. Unfortunately, our knowledge on the ecophysiology of flowering is often insufficient for choosing locations with optimal conditions. Placing the entire seed orchard in a greenhouse (Koski, 1975) or treating the constituent trees every second or third year in a greenhouse (R.C.B. Johnstone, pers. comm.) may provide a solution in some cases. The influence of soil types, pruning, girdling, irrigating and fertilizing on seed production has been reviewed by Sweet (1975), Werner (1975) and Schmidtling (1975).
Genetic diversity is the only defence we have against the many unknown dangers which plague trees throughout their long lives.
Manageable methods to control flowering in the seed orchard may ultimately result from an important recent scientific breakthrough showing the big role of certain gibberellic acids in the flower induction of the Pinaceae (Pharis et al., 1976; Ross and Pharis, 1976).
In looking for parallels in the apple orchard (which undoubtedly was the model when the term "seed orchard" was coined), it is striking to see how much modern fruit growing depends on special rootstocks to regulate fruit production. It is hard to believe that what applies to apple trees would be entirely unapplicable to all forest trees. More likely, the lead that apple culture has over us is based on ages of practical experience and on much research. Research aimed at finding precocious and dwarfing rootstocks for important forest tree species like Norway spruce seems feasible and highly desirable. There are indications of the existence of such types (Anon., 1975; Dyson, 1975). The dominant role which has been played by a single research station in the development of the various apple rootstocks may show that a concentrated research effort, perhaps in international cooperation, is needed to develop a range of rootstock types fit for the establishment of fast- and high-yielding seed orchards.
Once the seed orchard produces sufficient female flowers, the seed production may be limited by insufficient pollination. Evidence accumulates that supplemental artificial pollination can increase production (Denison and Franklin, 1975; Hadders, 1977) as well as reduce the high proportion of selfing that occurs in some seed orchards (Furukoshi, 1977), but there remains a need to develop good methods of mass pollination. Depending entirely on artificial pollination, Sweet and Krugman (1977) have designed a novel type of seed orchard for Pinus radiata. In these orchards, male flowering is prevented by clipping the cone-bearing clones at certain intervals, so that they form hedges. This also keeps female flowers and cones at a workable height.
Until recently large-scale vegetative propagation of forest trees has been limited to some species of Populus and Salix and to some clones of Cryptomeria japonica and Ulmus. As these are relatively easy to root, breeding can proceed from crossing, via selection and clonal tests, to large-scale clonal planting (except for Cryptomeria, see Toda, 1974).
The majority of forest tree species does not allow cheap large-scale vegetative propagation. Though most trees can be propagated by grafting, this is too expensive for use in forestry. Grafting incompatibility sometimes adds another difficulty.
Modern research and technology, however, have developed feasible methods for mass propagation by cuttings of a few other important forest trees: Norway spruce, radiate pine and in Africa Triplochiton scleroxylon, Eucalyptus and others (Longman et. al., 1977; Chaperon and Quillet, 1977). This has offered a multitude of new interesting possibilities and problems (Kleinschmit, 1977). Fundamental to these developments is the recognition of the fact that all trees seem to have a juvenile stage during which they are easier to root from cuttings than later in the adult stage. In the species mentioned, cuttings of juvenile plants root easily enough for mass propagation.
This fact has many consequences. In the absence of methods to rejuvenate a tree, clones must be made from young trees before these have proved their superiority. This stresses the value of family selection and necessitates the use of early tests. It requires methods to maintain the juvenile stage in some part of the basic material of the clones forever or at least for a much longer period than in the undisturbed seedling. It poses questions about any other differences that may exist between the juvenile and the adult stage, for example, to what extent fast growth of the ortet is found in the cuttings for which root development is a different process, and on the essence of the process of ageing (Zimmerman, 1976). It makes us wonder how much gene-environment interaction there is at the individual tree level in a population, and it stresses the need for genetic variation in planting stock (Kleinschmit, 1977). It may entail problems of certification. Kleinschmit (1977) has shown how a higher initial cost of the cuttings can be offset by a higher genetic gain or by the opportunity to plant at a wider spacing.
It offers, on the other hand, prospects for a much higher genetic gain than is possible with sexual propagation (Kleinschmit, 1977) and it circumvents all problems with seed orchards. It may also allow us to use rare provenance or species hybrids or other exceptional genotypes (Schreiner, 1966). If handled with care, it can increase rather than diminish genetic variation, and it can be built into a very flexible improvement programme. It may prepare us for the problems and potentials that will come with future methods to produce trees large-scale by tissue culture.
Trees and forests are more to man than a source of wood. They are an environment and their perceptible qualities influence man's emotional and spiritual well-being.
Breeding can easily lead to situations where the naturally occurring populations of a species with its wide genetic variation are replaced by much more uniform, "improved" materials over wider and wider areas. Similarly, a promising introduced species may be planted to cover whole provinces. While each extension of the area with such new materials may be expected to lead to a proportional increase in productivity, this may not come true. At more than one level and in several respects, genetic variation is a positive value and therefore it should be considered a breeding goal. Its neglect entails disadvantages as well as considerable risks. Genetic diversity generally is part of nature's strategy for the survival of populations.
Genetic diversity in the planting stock is an obvious and necessary precaution whenever a chance exists that the next generation of forest will be founded by natural seeding. To prevent inbreeding depression in the first generation and to minimize it in following generations, genetic diversity in the planting stock - that is, absence of relationship between components - is required. The amount of genetic diversity required can be computed; it is comparable to the amount of plus trees needed in a collection to be used for second-generation breeding.
Genetic diversity in the planting stock may prove to be very useful to the breeder later because it enables him to select and use outstanding individuals in the resulting stands, maybe also for some additional property that was not considered in the earlier breeding programme. In this way, genetic variation in the planting stock may contribute to gene conservation.
Four other reasons for cherishing genetic variation will be discussed in more detail, with an emphasis on the "why", on the mechanisms that can make variation more desirable than uniformity. The popular theory on diversity-stability relationships, rightly criticized by Goodman (1975), will not be invoked.
Forests and other plantings of trees are more to man than a source of the commodity wood. They also form an environment to man, and as such their perceptible or visual quality can have a significant influence on his emotional and spiritual well-being. Especially in densely populated areas, this is one of the main functions of trees. This situation offers special problems and opportunities to the forest tree breeder (Koster, 1974; Townsend, 1977).
It should be realized that for amenity or landscape use plantings consisting of one clone may have a visual effect that is entirely different from the effect of a planting of seedlings of the same species (Heybroek, 1976). Clonal plantings are especially suited for situations where the need for a formal or monumental effect and for a strict preconceived pattern requires the geometric repetition of trees of exactly the same shape in rows or blocks, or where one wants to emphasize the "rational" aspect of a certain landscape. By their uniformity, even-aged trees of clonal stock emphasize their occurrence as a group rather than as individuals. Where a relaxed, natural atmosphere is required, however, seedlings are at an advantage. Contrary to clones they stress the tree to tree variation, demonstrating the ever-present individual differences in shape, growth rate, pattern branches and leaves, colour, density, flowering, leafing etc., which form a constant source of interest and job for the onlooker. The human eye loves and searches for variation and detail. It immediately notes damaged or missing trees in clonal plantings; in seedling plantings, each tree differs from its neighbours and gaps are less conspicuous. Clonal plantings, especially if even-aged, can bring an element of monotony to the landscape. The replacement of seedling trees by uniform clonal stock may result in a definite loss in the visual quality of a landscape.
Obviously, the amount of variability present differs between populations: there may be populations with too much and others with too little variation for certain purposes.
Variation at the species level is equally important. An extreme diversity of species is rarely the ideal, however. Ideally, a landscape should have a recognizable pattern or a theme on the one hand and plenty of variation and detail on the other hand: "variety within order." The preponderance of a certain species in an area can thus be right if it fits the pattern-aspect (compare Rapoport and Hawkes, 1970).
It seems astonishing that so few firm data exist on the differences in effect between genetic variation and uniformity in trees on production per unit of area. Model experiments with agricultural crops seem to lead to the conclusion that a mixture (of pure lines or species) produces, in general, at an intermediate level, that is, less than the better component and more than the poorer component when these are grown pure and under the same conditions.
There are indications, however, that the mixture can produce more than any of its components over a long series of years in case the weather during that period fluctuates sufficiently to make now this rather than that component the better producer. This can be explained by the phenomenon of compensation: if one of the components suffers a set-back (by forest, drought, parasites, etc.) early in the development of the mixture, another component may be able to use the vacant space. Thus the mixture will yield more than the arithmetic mean yield of the components.
What holds for weather may hold for site: on a highly heterogeneous site, a mixture may do better than any of its single components; each component will tend to dominate on the site it can utilize best. Depending on its composition, the mixture can thus be more flexible and better buffered against variations in time and space, in weather and site quality in its environment.
Especially in forest trees, which must live many seasons on sites that can be much more variable than those of agricultural crops, and often on sites that are not too well known, good mixtures of genotypes may have an advantage by being better buffered.
In this view, gene-environment, or GE-interaction in the materials is a prerequisite for superiority in the mixture: only if the different genotypes react differently to different environments has a mixture a chance to be superior. GE-interaction at the individual tree level is what counts here and this can be measured with the help of clones (Matheson, 1977). It is largely independent of GE-interaction at the population level, which is the level usually studied (Goddard, 1977).
With regard to production, mixtures become even more interesting if and where overcompensation can be shown, that is, where the mixture produces more than any of its single components. In such cases, apparently, other mechanisms than GE-interaction are involved. A case of overcompensation has been reported for poplar by Tauer (1975). De Wit (1977) suggests such overcompensation may occur especially on very poor sites, not on the better sites.
The problem has similarities in silviculture where the pros and cons of mixed versus single-species forests are being discussed; in both fields, experimentation to determine the magnitude of the advantage is difficult
Genetic diversity is the only defense we have against unknown dangers and risks, which play such a big role in forestry, due to the long lifespan of a tree. It certainly is not a perfect defense, it just is the only one we have. Rather than being a real defense, genetic diversity is a way to spread the risks.
These risks are of many kinds. A risk familiar to most tree breeders lies in the fact that we cannot test the new - and hopefully better - materials we produce over a full rotation and under all conditions. We often release new materials at a stage when they are "promising," which implies that their use contains the risk of disappointment at a future stage. Later in life, or when planted outside the testing area, they may show unexpected physiological weaknesses, susceptibilities to parasites, etc. (Toda, 1974). Similarly, the once-in-fifty-years' weather falls outside the normal testing period and thus constitutes an unknown risk. Because of these unknowns a breeder will often try to release different materials to spread the risks.
Basically the same holds for a much more grave and complicated problem, the danger of "new" diseases and insects. The examples of the introduction of chestnut blight and Dutch elm disease in North America and Europe, of Marssonina brunnea in Europe, of Rhabdocline pseudotsugae in Europe, of two poplar rust species in Australia and New Zealand and of Phytophthora cinnamomi in Western Australia are well-known. Others can be listed, and many more fungi seem just to be waiting for a lift to a new continent to make history in a similar way. For the purpose of this discussion, a genetically based increase in the pathogenicity of a parasite (Snow et al., 1976; Gibbs et al., 1975) falls in the same category of "new" diseases.
REFORESTING LEBANON WITH CEDARS part of a national "green plan"
With regard to genetic variation within the species, the impacts of the mentioned epidemics show clear differences. It seems important to recall that the full natural variation of the species did not save the American chestnut from a virtually complete eradication by the chestnut blight. Similarly, in large parts of the species area, the American elm as a tree seems headed for the same fate due to Dutch elm disease. Both are native, not introduced, tree species. This shows that natural variation is not a magic cure for all problems.
In other cases, a different picture emerges. While the first impact of the importation of Cronartium ribicola on the western white pine Pinus monticola was disastrous, the species apparently has sufficient genes for resistance in its gene pool to come back as a more or less resistant species in few generations, a process that can be accelerated by the breeder (Hoff et al., 1976). Poplar culture in Italy was hit hard after the advent of Marssonina brunnea because of the preponderance of the clone I 214 that just happened to be very susceptible; in the Netherlands on the other hand, more clones were cultured widely, of which only one proved highly susceptible. Additional genetic variation that was accumulated in an active breeding programme allowed the speedy release of two highly resistant clones, so that the new disease resulted mainly in a shift in the assortment of clones used. In the case of Rhabdocline pseudotsugae in Western Europe, only a fraction of the varied host population was attacked severely; after the disappearance of that part of the population, the disease became of minor importance.
These examples may suffice to show that the impact of a "new" disease on a species can vary widely. How big a part of the species will turn out to be susceptible under the local conditions and to what degree is mostly unpredictable. It is clear that the presence of genetic variation in the host species can play a major role in preventing catastrophic results and saving the culture of a species even after initial losses.
Yet because these dangers are really unknown and unpredictable, it is impossible to say how much genetic variation should to reduce risks to some acceptable level. One can only strive to include as much genetic variation in the planting stock as possible, and decide in each case how much one is prepared to sacrifice for it in the way of cost or productivity.
Genetic variation at the species level is a very obvious and generally reliable line of defense against the dangers concerned. It is risky to rely on a single species over large areas; diversification seems advisable.
The multiline in small grains as described by Browning and Frey (1969) is a highly refined and sophisticated cultivar, consisting of a mixture of pure lines being practically identical as far as most properties are concerned but differing in resistance to the different races of the rust. Each line has a resistance to a certain set of races, none is resistant to all. Lines with different resistances are mixed more or less in proportion to the share each rust race had in the spore cloud last season. The resulting multiline cultivar is not free from rust, but it never suffers real damage by it. Equally important, it aims at stabilizing the rust population in its composition over the years, preventing the development of new races. It is a safe way to employ "vertical" or specific resistance.
This goes to show that this sort of multiline is a defense against an exactly known and defined parasite. Genetic variation is carefully dosed against that single enemy; for the rest, the cultivar is homogeneous. It is an inspiring example because it shows how genetic variation can be used; we foresters do not yet have the knowledge or the plant materials needed to copy this example in forest trees.
For some host-parasite combinations, trees have the disadvantage of a large size, limiting the usefulness of mixtures. The efficacy of the multiline in the small grains in resisting a leaf rust is partly based on the fact that each single host plant is long, narrow and relatively small. When a certain plant gets infected with a compatible race of the rust, a high proportion of the spores produced will be intercepted by the leaves of neighbouring plants which are not susceptible to this race. Conversely, from those neighbours, that plant will mostly receive rust spores of other races to which it is not susceptible. So a high proportion of the rust spores produced in the field will land on leaves where they cannot cause infections and thus will get lost. This mutual protection slows down the build-up of an epidemic; so the crop will be ripe before it can get severely rusted.
Such a reduction in the rate of multiplication of the parasite might be less pronounced in many tree diseases, simply because of the size of the host. In the large crown of a poplar, for example, a rust will find all neighbouring leaves equally susceptible and the build-up of the rust population will therefore scarcely be slowed down if the neighbouring poplar tree happens to be resistant to that rust. For many diseases, a single tree is large enough to allow a complete build-up of a population of the parasite; some diseases can take years for that process, as the tree does not move.
A useful delay in disease extension in a stand is not to be excluded for all host-parasite combinations and conditions. In a case where a disease spreads slowly from a focus, as may happen with a root rot that needs root contacts for its spread, it can be expected that a certain percentage of resistant trees (either of the same or of a different species) in the stand might effectively reduce the spread of the disease. If that same disease, however, can enter the same stand at many points, little will be left of such an effect of the resistant trees.
It follows that the extent of a possible retarding effect of a mixture of genotypes on the build-up of an epidemic will differ from case to case, and that only under very specific conditions a useful retarding effect may be anticipated. Thus the hope that mixing a more or less random collection of genotypes will improve the health of the individual trees seems mostly unwarranted.
It is sometimes suggested that genetic uniformity per se can increase the susceptibility of a stand to diseases. The above considerations make clear that this is hardly the case. Genetic diversity will rarely decrease the impact of a disease on the individual genotype, but it can decrease the risk by spreading it.
Extensive planting of a single clone over large areas produces the extreme and horrifying situation of minimal genetic variation. Obviously, the use of clones can easily lead to reductions in the genetic variation, with all the negative effects and dangers mentioned. These are not inherent to all vegetative propagation, however. A wise production and use of clones can lead to genetic variation equal or even superior to that of seedling populations.
First of all it seems desirable to produce and release as many good clones as the nursery trade can reasonably handle, and keep more in stock and under test to substitute for those that need to be withdrawn. The clones should be of widely different genetic backgrounds. Here, vegetative propagation can make a big contribution to an increase of generic variation in the planting stock. It allows the propagation of rare recombinants, hybrids and other outlying genotypes which cannot easily be maintained in seedling populations. Thus it is not difficult to maintain within a restricted number of clones a range of genetic diversity that is much wider than in a large seedling population.
The question remains, how clonal material should be used: in pure stands or in mixtures at the single tree level. Both seem acceptable under certain conditions, both have specific pros and cons. Mixtures can utilize possible effects of GE-interaction, compensation and mutual protection and in some cases be more attractive for the eye. Unexpected damages can differ as to effects: slight damages, affecting a low percentage of the components will hardly be noticed in a mixture, especially if they occur in a young stage of the stand. They would however have a full impact on pure stands of that component. On the other hand, damage to a larger proportion of the components would leave all mixed stands defective; while if components were planted pure, some stands would be entirely undamaged whereas the others could be salvaged and replanted with better clones. Pure clonal stands may be easier on the silviculturist, the nurseryman, the designated authority for control of planting stock and on the industrial user; but if the clones are marketed pure and the balancing is left to the user, there is a danger that one or two clones will be favoured so that in the end they will constitute the majority of the plantings. In order to prevent this, each single released clone should have a high or specific usability. It can be advantageous to have both seedling populations and clones of a species on the market.
Mixtures are at a clear advantage in cases with many "unknowns" about the components, the sites, or their interactions. The same applies if the site varies considerably. If mixtures of many clones are desired - Kleinschmit (1977) prefers mixtures of over 100 clones - it may be better to release not the clones but the mixtures, and have their composition determined and guarded by a research station, a designated authority or some other organization with a long range responsibility.
From the foregoing it should be clear that genetic diversity in the planting stock provides no guarantee against diseases and damages. In many cases, however, it can limit their impact, first of all by spreading the risk, further by the mechanism of compensation, sometimes perhaps by reducing the rate of spread of a parasite.
For this and other reasons, genetic variation is an important goal for the breeder. It is his responsibility to maintain or develop genetic variation in his materials as he is supposed to be the person with the long range overview. If needed, he has to convince the nurseryman and the user of the wood of its advantage. Even though how much variation is needed cannot be defined, he should try to increase rather than decrease genetic variation.
FARMERS IN CENTRAL AMERICA MAINTAINING A TREE NURSERY combining forestry and agriculture
A highly effective way to diversify is to use different species. This is so obvious that it should not be over-looked. Countries that depend heavily on one or two species should try to introduce additional suitable species. Countries that have a few major and many secondary species should resist the temptation to concentrate all their breeding efforts on the major species which would only accentuate their dominance. Instead, the secondary species should be improved too, so that a range of good propagating materials will be available to the planter.
References are at the back of the magazine following the last article.
