By C. SYRACH-LARSEN, Director Forest Tree Breeding Station, Hørsholm, Denmark
It may seem remarkable that the silviculturist should lag so far behind the farmer or the gardener in the use of improved plants. There are perfectly good reasons for this.
Genetic improvement is based on the fact that no plant within the same species is identical with any other and that, by using the best individuals, the desired better types can be raised. Similarly, it is possible to make crosses between species and types in such a way as to get a combination of the qualities desired.
By sexual reproduction from seed sorts can be produced, as in the farmer's cereals, root crops, and grasses, or in many of the gardener's flowers and vegetables. These are mostly plants which furnish a sexual generation every year or every other year. By using generations which follow each other rapidly it has been possible, by constant selection and by artificial interpollination, to propagate fixed strains.
Clones, which are used in the propagation of fruit trees, roses, and similar growth, are a quite different form of "sort." They are widely distributed by vegetative reproduction. Clones can be applied by grafting, budding, cuttings, or layering. From plants with choice fruits or fine flowers, which may have arisen more or less by chance, shoots (grafting) or individual buds (budding) are taken and transferred to wild seedlings (stocks). Every one of these sorts is thus an individual in itself which, if grafted or budded, will grow on countless alien roots or give rise to complete or independent plants through cutting or layering.
The method chosen by the farmer or horticulturist in individual cases of improvement depends upon the characteristics of the plants and on the practical possibilities. It is particularly important that attention be paid to the rate at which generations succeed one another; to flowering conditions, such as the time of appearance of male and female flowers; to insect pollination or wind pollination; to the amount of seed yielded by the individual plant and, hence, the necessity to observe the capacity of the plant for vegetative reproduction; and to many other points.
In forestry, any attempt to obtain "fixed strains" of forest trees by artificial pollination runs up against the difficulty of tree height. It is not easy to pollinate by hand, high up in the crowns of trees. Furthermore, the trees are often scattered over wide areas and cannot be transferred to convenient experimental areas. In addition, it may take ten, twenty, or even forty years for a tree to set its own seed.
It is therefore not surprising that the idea of artificial pollination was abandoned and that no one had any faith in the regeneration of forests by grafting or by cuttings. Broadly speaking, therefore, intensive silviculture places most reliance on improvement of the forest's growing stock by the introduction of exotic tree species and by selection of the best indigenous trees under conditions of free, uncontrolled pollination. Very important progress was made, however, when it was recognized that forms of forest trees do exist which can be determined by heredity, and when first steps were taken towards intentional improvement, even though that progress did not go beyond selection on the basis of free, uncontrolled pollination.
The first and main step was to look for the best possible seed for reforestation. Accordingly, the demand for authoritative information on the provenance of seeds grew steadily, and various systems of control were set up in several countries. International cooperation in this field is of the greatest importance, so that information can be exchanged and, if possible, some progress be made toward establishing uniform rules of procedure or, at least, toward an exchange of views on the inquiries submitted and on the methods of dealing with them.
In this connection it would seem wise to pay attention not only to the climatic races and types which occur naturally and which show a particularly good form from a forestry point of view, but also to those types which presumably have been developed under the influence of man. Particular mention should be made of the oaks of the Dutch avenues, the European larch (Larix decidua), and the Douglas fir (Pseudotsuga taxifolia) of western America growing in Scotland, not to speak of many other examples both in the Old World and in the New. Reference should also be made to the widespread propagation of Hevea brasiliensis, which furnishes by far the greater portion of the world's crude rubber, almost all of which is derived from plantations located outside the natural growth habitat of the species; the extensive plantations of this tree are based on extremely small quantities of original seeds.
Such a segregation of species may have considerable advantages, but it may also involve the risk of excluding valuable chances of variation. This is, however, of no account in comparison with the great advances made in agriculture and horticulture, especially during the past fifty to one hundred years. The bumper crops of today, in fields and in gardens, exceed by many times the crops of a few generations ago. The modern improvement of maize through hybridization - the first theoretical tests of which were made about 1917 - is estimated to have yielded in the year 1947 alone, a profit of $750,000 to $1 million.
We have seen with envy how some other users of the soil have been able to progress through the use of plant improvement techniques. In forestry, unfortunately, too much emphasis has been placed on the difficulties involved, and these techniques have been neglected. The difficulties are great, if the same procedures are tried in forestry as in agriculture and horticulture. Forestry should go its own way and then, perhaps, things will assume a different aspect.
The most important methods of procedure in forest plant improvement work are:
(a) Investigations to determine hereditary differences within species: races determined by climatic factors and special form types, which have grown both naturally and in cultivation - that is, the problem of provenance in its widest sense - with planned experiments based on free, uncontrolled pollination.
(b) Artificial pollination on the basis of previous studies of the flowering of forest trees.(c) Hybridization, especially the utilization of hybrid vigor, or heterosis.
(d) Cytological work to identify trees with chromosome numbers differing from the normal, and the reproduction of such trees.
(e) Vegetative propagation, which is of the greatest importance as a technical method of forest improvement, by the establishment of seed-source plantations and tree shows or displays, which may also be of special importance for the utilization of a tree of any given character (clone).
Pinus strobus: a. - Old tree, Uggeløse Skov, Denmark. b. - Seedlings propagated by controlled pollination carried out in 1944 in the crown of the old tree. c. - Grafts from the old tree, grafted in 1948; the grafts flowered in 1950. (b and c were photographed 30 June 1950.)
First and foremost, the best possible types of economically important trees should be preserved for the future. Each country should register its most valuable stands, preferably in virgin forests, or any: particularly outstanding individuals elite or champion trees. This task is closely related to maintaining national parks and preserving natural sites. The elite trees should be used in planting work, either by use of their seed or preferably by vegetative propagation (grafting, budding, or by cuttings) so as to achieve the preservation of these presumably valuable genotypes, which will at the same time become available for further use. This step would in any case offer a certain safeguard against deterioration.
In Denmark this work is carried on by the State Arboretum. Because of its forest-botanical nature it is a task which comes naturally within the scope of the arboreta in various countries, whether they are attached to forestry colleges, research stations, or other institutions. It is time to get away from the old collections with all their monstrosities. It used to be the custom to collect such items as weeping ash, snake spruce, and globosa- or pygmaea-shaped trees, instead of concentrating mainly on elite individuals from the forest.
Distance is an important problem. Often the trees which should be collected are scattered over a wide area, so that it seems impracticable from the start to make use of them. Much would already be accomplished if selected champions were taken into the arboreta. The best way to do this would be to get seed from the good stands and through vegetative propagation of fine individual trees. A jeep with a Swedish steel ladder can travel far afield and can be used for collecting material from quite high trees. If it is still difficult to reach very high into the trees, small bits of branches can be shot down.
For transportation over great distances, air mail and air express are available. Within three days I succeeded in getting quite an amount of grafting material into Denmark from California. On Wednesday the grafts were dispatched from San Francisco, and on Saturday the grafting began in Hørsholm. We have been sending pollen of Pinus from Denmark to Placerville, California, and, since the spring of 1946, we have annually received pollen from Canada, both for experimentation and for commercial production of Populus hybrids. We have received pollen of Pinus from Australia and have made quite a number of other exchanges of the same kind. In the course of a few years this material, the exchange of which is so essential to us, will certainly become a very common cargo.
Even more than distance, perhaps, time acts as a deterrent. However, there is a whole series of important forest trees which produce considerable quantities of seed at comparatively young ages, even if they are grown as ordinary seedlings. As examples from among the important trees in European forestry Betula, Larix and Pinus may be mentioned. Trees aged from ten to fifteen years may still bear considerable quantities of seed, provided that individual plants are spaced widely enough apart so that the crowns can develop properly.
Grafting offers certain advantages. It appears that grafts are capable of producing seeds at still earlier ages, even if they are planted in the forest in the same manner as other trees. Wounding, strangulation, or root-pruning can also be used to hasten seed-bearing. It has been clearly proved that grafts, treated by special methods - grown in pots or subjected in some other way to special treatment - can be used as small, early-fruiting plants for experimental purposes. It is a great advantage in a greenhouse to work with seed-bearing potted plants. If it can be ascertained that the grafts have exactly the same genetic characters as the elites from which they are taken, this material can afford great opportunities for future research.
Identification of the genotype in forest trees is also a difficult problem. Considerable headway can be made through proper silvicultural treatment, but it is nonetheless a great advantage if one can work with trees which possess an inheritable tendency to furnish the strongest growth, the straightest stems, the: high-fist resistance against diseases and insect attacks, and: have other valuable silvicultural qualities. Forest trees are dwellers in communities where, like other living organisms, they have to adapt themselves to the existing conditions. These conditions may differ considerably in the course of time, all the more because trees live so long end may be subjected to many different external influences.
As an aid to identifying the genotype of trees in Denmark we have initiated "tree shows." These consist of plantations of groups of various sizes of plants propagated vegetatively, so that each group, usually 30 plants, represents its own particular tree - its clone. For this purpose the most reliable results are obtained by using cuttings, but grafts also permit reliable observations. In the arboretum at Hørsholm we have found these methods very helpful in identifying the genotype of the clones of Fagus, Fraxinus. Betula, Larix, Picea, and Pseudotsuga. 1
1 C. Syrach-Larsen: Estimation of the Genotype in Forest Trees, 1947.
These tree shows are a means of demonstrating the existence of various genotypes. They also present a good establishment for continued observations and tests. Moreover, they can be laid out in such a way that they permit the testing of individual genotypes under different environmental conditions, and the work can be developed in other directions.
Douglas fir 1
Original tree, Wind River, State of Wyoming U.S.A. (photographed 17 April 1946)
Graft (V.1061) on old stock in Denmark, 29 April 1946. (b was photographed 25 September 1946 and c, 26 June 1950.) When comparing Douglas fir 1 with Douglas fir 2, it becomes evident how extremely different are the genotypes of the two trees.
There is space for only a hint, but tree displays may also be very valuable for phenological observations and as laboratories for studies in plant physiology and plant pathology. Through such studies the great individual differences in plants become quite apparent; these differences are the basis of improvement. It is surprising how much knowledge can be derived from this kind of work, and how rapid is its progress. Another matter of interest is that, by making grafts - or, better, cuttings - we can try to verify the importance of provenance. It might be possible to set up "living meteorological stations," so to speak, or to initiate the use of "type trees." It is often said that plant improvement work should follow the plant from "the cradle to the grave" in order to be sure of finding the right and most valuable plant, but I believe that in forestry we should follow the individual tree from "the grave to the cradle," as we are doing-in our use of tree-displays.
Douglas fir 2
Original tree, Wind River, State of Wyoming, U.S.A. (photographed 17 April 1946)
Graft (V.1062) on old stock in Denmark, 29 April 1946. (b was photographed 25 September 1946 and c, 26 June 1950.)
The farmer sows his seed and watches the plants develop until time for the harvest. He can easily do this, for the whole process only takes one or two years. But doing the same would be a tedious pursuit for the forester. He must use a short cut, such as the tree display. The shapely, full-grown tree in the forest stands at the edge of the grave, but it has made clear to us that it could become the ideal tree. If we take grafts from it and establish them in a tree-display, we then have an opportunity to study the congenital characters which have made it such a valuable tree. In the course of a few years we can find the reason for its superiority - whether exceptional vigor, great hardiness, or a special resistance to diseases. This is what I mean by saying, "we have followed it from the grave to the cradle." It is, however, a striking example to illustrate that we must follow paths other than those usually taken.
With regard to seed-production, "I strongly urge taking up vegetative propagation and, in conjunction with experiments on artificial pollination, the establishment of seed plantations for the supply of seeds for practical use." ² It is an old quotation but it states an objective which we should strive to attain.
² C. Syrach-Larsen: Forest Tree Breeding, 1934, page 112.
There may have to be certain modifications, as for instance that in certain tree species the greater part of the planting will be done with seedlings taken from controlled pollinations on grafts from champion trees. There may also be other useful combinations, but certainly there should be special seed-source plantations: or - as they might better be called - seed-source gardens. It is quite certain that an intensive forestry such as we have in Denmark should employ seed-source gardens. I believe, however, that they can also be recommended for use under more extensive conditions. For example, in the United States of America and Canada, they would be the natural link in advance of the more general tree-farms grown there.
In Denmark the work has been so organized that most of the experimental work on forest-tree improvement is concentrated at the State Arboretum, whereas the establishment and control of seed-source gardens is the main task of the State Forests Plant-breeding Station.
Above all, seed-source gardens, which are formed by vegetative propagation from elite trees, will be our trump card. We should not cast envious eyes on the favored position of plant improvers who work with herbaceous plants, but rather look at the matter the other way round. Paradoxically, it may be claimed in respect of short-lived plants that we must confine ourselves to types which seed consistently or breed true in order to preserve them, whereas with forest trees we can preserve valuable hereditary tendencies in the same individuals, perhaps for centuries. Therefore let us not lament the slow succession of generations, but rather rejoice at the long life of individuals.
In seed-source gardens we have, in addition, special opportunities for making use of hybrid vigor (heterosis). We can not only readily produce seed of any desired hybrid but we can also produce it from parent trees selected for that purpose.
In this connection it may be mentioned that F. I. Righter of the Institute of Forest Genetics at Placerville, California, recommends the utilization of hybrids by employing seed from F1 generations without controlled pollination. Placerville works almost exclusively with Pinus. They have quite a number of hybrids of Pinus, of which P. attenuata x P. radiata is the oldest (sown in 1929), and their experience is doubtless based on it. I have obtained with Larix decidua x L. leptolepis astonishingly good progeny in controlled F2- and back-crosses, and I also recollect seeing in Scotland beautiful plantations derived from free pollination of both F1 and F2 at the same crossing.
Fraxinus excelsior:
Graftings from the same tree with seeds in greenhouse, 22 August 1939.
Seedlings of the seeds shown in Fig. b, germinated 1941 (photographed 1942).
I should like to direct attention to the "polycross-method," used with success in recent years in agricultural plant improvement. ³ The method is particularly recommended for plants which are cross-pollinators, which can be divided into clones and thus be preserved for a number of years, as is the case with forest trees.
³ H. N. Frandsen: Imperial Agricultural Bureaux Joint Publication No. 3, 1940, H. K. Hayes: American Naturalist, Vol. LXXX, 1946; and H. M. Tysdal and Bliss H. Crandall: Journal of the American Society of Agronomy, Vol. 40, 1948.
By employing the "polycross-method" plants can be sought with high "combining ability." If, from the forestry point of view, trees with sufficiently high combining ability could be found, it might pay to introduce them into stands, even where natural regeneration is being used.
Seeking always for the best, we must, working with the greatest precision, come to grips with the heart of the problem so we can make later use of the discoveries in practice. It is up to each of us to tackle the problems as we see them, but at the same time we must co-operate with others the best we can.
This is pioneering work: Studies of growth stimulants, which may make possible a more general use of cuttings, and painstaking cytological researches are of equal importance with theoretical meditations or the use of well-tried horticultural methods.
We must strive for vigorous growth - perhaps not exactly gigantic stature - we must to a great extent hunt for resistance to disease, and it must be our special aim to increase the volume production per unit of area. In these ways forest tree breeding may make its contribution to an expanded afforestation.
When the pioneers reached the North American prairie it was covered by a "sea of grass," which at that time they considered themselves lucky to get gratis, but the farmer has long since found that it is to his advantage to turn under the original grass and cultivate new and more desirable kinds of grasses, even though it costs money and labor. The same must hold for the forester, if he is to succeed in meeting the demand for wood by the paper and chemical industries, in which the consumption becomes steadily greater, as well as for many purposes which make precise demands on the structure and technical qualities of timber. There will be a steady demand for special biological qualities in trees to provide shelterbelts for farm crops against wind and blown sand, and in those species and types of trees and bushes which are needed to assist in regulating rivers and in protection against erosion. It may well become the task of the forest tree improver to find material for planting on vast areas where man's activities have had a specially disturbing effect. Special planting materials may be needed where industrial areas poison the atmosphere, or where the gigantic mountains of sand from the South African diamond mines blow right into people's houses, or in the struggle to free the Suez Canal from the incessant blowing of sand from the adjoining desert.
To succeed to any appreciable extent in forest tree improvement, we must soon approach the stage where it becomes obvious how much more profitable it will be to create rapid-growing pure plantations on favorably situated areas rather than going on the hunt for countless different species in virgin forests, or waiting on the regeneration of the forests of the Par North. Those regions of the world favored by abundant light, heat, and moisture will hold out highest possibilities.
We must be keenly alive to ah the possibilities that may arise, from whatever quarter they may come. But, whatever happens, we must take up the task with optimism. If we dwell rather on long-lived individuals than on the slow succession of generations, the future of the work immediately becomes brighter - possibilities are opened up for attacking tree improvement in an entirely new way, and the silviculturist may yet overtake the agriculturist and the horticulturist.
By agreement with the President of the International Union of Forest Research Organizations, FAO is prepared to publish in Unasylva short notifications and requests concerning research and research materials sent in by Research Institutes and individual research workers.
The following item has been received from Carl Muhle Larsen, Union Allumettière S. A., Institut de Populiculture, 230 rue Buizemont, Grammont, Belgium:
"For our breeding work within the genus Populus, we are looking for specimen material from P. nigra. Both the true species and varieties are of interest; but the branches must be from healthy stock, and preferably from trees with good form and growth. For direct utilization the first year the small branches, 30 to 50 cm. in length, must bear flower buds"."We should be glad to hear from research institutes of possibilities for obtaining such material, and for its consignment to us. Carriage expenses and other costs will be paid by our Institute."
