BY JEAN POURTET, Inspector-General of Waters and Forests, Nancy, France
(Current and Future Work Programs)
Poplars (genus Populus) together with willows (genus Salix) constitute the family Salicaceae. The genus Populus comprises five divisions or subgenera: Leuce (aspens and white poplars), found throughout the Northern Hemisphere; Tacamahaca (balsam trees), in Asia and North America; Aegirus (black poplars), in the temperate zone of the Northern Hemisphere; Leucoides, in southeastern North America and in the Far East: and, finally, Turanga, in the Mediterranean basin.
The poplars of the first three subgenera play an important role in world economy; about 5 million cubic meters of white poplar and balsam wood are produced in a wide circumboreal zone outside the U.S.S.R., 3 million cubic meters of black poplar wood are grown in artificial stands in Western Europe and about 2 million cubic meters of varied types in the Mediterranean basin.
Little has been recorded concerning the aspen and even less concerning the balsam poplar, because these trees are important only in natural stands and have been cultivated only exceptionally. The black poplar, however, has an interesting history.
The first poplars introduced from America and the East into France were intended only for ornamental purposes and as park trees; through grafting it became possible to propagate, rapidly and indefinitely, each of the types introduced, even single specimens.
Because of this, poplars trees soon became widely known and appreciated, whereas normally a long period of time, frequently a century, elapses between the introduction of a tree species by seed and its large-scale use in afforestation The rapid growth of these trees made possible early crossings of new species with the native Populus nigra. The first American poplars were introduced into Europe at the end of the eighteenth century. Henri-Louis Duhamel de Monceau in 1754, mentioned the results obtained in his poplar plantations, giving details concerning the ecology and technological qualities of the poplar and its timber
Hybrid poplars were planted at the same time, certain specimens of which are still alive; for example the P. euramericana f. serotina planted in 1752 in the Botanical Gardens of Nancy. This has attained a girth of 6.30 meters at 1.30 meters above ground.
Some of these hybrids were reimported into America by emigrants. Thus, at the beginning of the nineteenth century, François-André Michaux a naturalist and explorer in the eastern United States of America, was uncertain as to the classification of the black poplars found in the New York region. In his remarkable treatise on trees of North America 1 reference is made to the "Swiss or Virginia poplar", a tree widely cultivated in Europe which, although it was not encountered, "is probably native to some part of America."
1 Histoire des arbres forestiers de l'Amérique septentrionale. Paris, 1810-13, 4 vols.
During the nineteenth century, intelligent empirical experimentation led to the selection of a few valuable hybrids. These became an important feature of the French countryside and a valuable source of production. Gradually, these hybrids spread to other Western European countries.
A comprehensive study of poplars undertaken at the beginning of the twentieth century by the French botanist, Louis Albert Dode, failed to achieve practical results, because of its oversystematic treatment. Later, the work of such men as Edmond Henry, Candsdale, Gijsbertus Houtzagers, Regnier, and Piccarolo, contributed toward a reorientation of theoretical studies along practical lines.
Since 1947, most of the widely dispersed research has been co-ordinated within the International Poplar Commission under the chairmanship of Prof. Philibert Guinier.
The International Commission, for practical reasons rather than for the sake of pure science, undertook as its primary task the establishment of identification and nomenclature of poplars. Great confusion had prevailed in this field since the eighteenth century, when François-André Michaux referred to the same tree by two different names, "Swiss poplar" and "Virginia poplar." The facile propagation of poplars and their commercial value had led to the use of an infinite number of names for the same tree species. The complex situation arising from the empirical outlook of certain poplar growers, who were indifferent to nomenclature, would have rendered impracticable all international or regional exchange of poplar seed or grafts. It would also have led to disappointments.
The International Commission labored for two years before completing the nomenclature of black poplars. Three major botanical groups were established, namely:
(a) Populus nigra L.: Old world black poplars;
(b) Populus deltoides Marsh: Black poplars of eastern North America;
(c) Populus euramericana (Dode) Guinier: Hybrids derived from crossings of the two above species.
Each of these groups is divided into several subgroups containing various selections usually designated by local common names.
(a) Populus nigra L:P. nigra f. italica: the fastigiate male, grown for ornamental purposes throughout the world, and for industrial purposes around the Mediterranean.
P. nigraf.: the "Vert de Garonne" (France)
f.: the "Hamoui" of Syria and others in the Mediterranean area: very erect female trees yielding timber which is valued for carpentry.
(b) Populus deltoïdes Marsh:P. deltoïdes f. carolinensis (female) and angulata (male):southern types with excellent timber, but requiring a great deal of care and difficult to graft.
P. deltoïdes f. missouriensis: of erect stance; the male was introduced into the Netherlands and is still cultivated there.
P. deltoïdes f. monilifera (female) and f. virginiana (male): types resistant to cold and of rapid growth; the female widely grown in France.
(c) x Populus euramericana (Dode) Guinier:
These vigorous, rapidly growing hybrids are found throughout Western Europe and have been distributed commercially throughout the world. There are numerous clones, the main groups of which are listed below:
x P. euramericana f. serotina: male, with late leaves, cold-resistant, excellent timber.
P. gelrica: selectively grown in the Netherlands, apparently closely related to the preceding type.
xx P. euramericana f. regenerata: female with deciduous catkins; excellent timber, but susceptible, under certain circumstances, to oozing canker (Fr. chancre suintant), a serious disease.
x P. euramericana f. robusta: male, with very early leaves, remarkably straight and erect trunk. It is of comparatively recent origin, and has spread over the entire world, although it requires great care and its timber uses have not yet been explored.
x P. euramericana f. I-214, 155, etc.: excellent hybrids selected in Italy by Prof, Piccarolo and well adapted to the favorable conditions of the Po valley.
In the future, any new types evolved or appearing spontaneously will be designated by a number preceded by the initial letter of the name of the country where they originate. They can thus be added systematically to one of the groups already mentioned.
The International Commission also specified, f or practical purposes, the characteristics to be noted in the determination of species, namely: the date at which the leaves appear, the sex of the tree, erectness of the trunk, the characteristics of the bark and of the branches, the type of fruit borne, etc. On a recent trip to the United States of America, a group of European experts had occasion to clarify several obscure points.
No difficulty was encountered in establishing the nomenclature of balsam poplars, in which clones of species and varieties need not be distinguished by a binary system of nomenclature.
White poplars are not very common, except aspens: Populus tremuloïdes Michx. and grandidentata Michx. in America; P. tremula L. in Europe in two forms: one, with large, glabrous leaves, found in the boreal zone and in mountainous areas; the second, with small villous leaves in the early stages of growth (var. villosa).
Southern white poplars are characterized by a persistent coating of down on the underside of the leaves (tomentose). In botanical gardens and arboreta, the following types from the Mediterranean region are found: P. nivea Willd, Hickeliana Dode, subintegerrina Lange, etc. In the forests of Europe, the orphan hybrid, the result of a cross between the aspen and P. nivea: P. megaleuce Dode (Ypreau or "blanc de Hollande") and its close relative, P. canescens (Grisard), occur.
However, less is known of the comparative qualities of these types, their ecology, and methods of propagation.
As previously mentioned, nurserymen and planters, until the twentieth century, were content merely to propagate the types of trees growing spontaneously, or hybrids which they found most valuable because of their rapid growth or their resistance to certain natural enemies. Many types selected in France during the past hundred years are still being multiplied and planted throughout Western Europe, competing with the best of the more recent types evolved.
It was considered necessary, however, to increase the range of types offered for selection by planters and, therefore, to attempt systematically to evolve new types; this was undertaken after the first World War by the Institute for Poplar Culture at Casale Monferrato in Italy.
Artificial hybridization was used, and seed was also gathered from the best female trees; the latter method, which does not afford any means of positively ascertaining the male parent, has the advantage, however, of yielding a great number of hybrids, from which further selections may be made.
Results have been excellent: numerous new types have been evolved, which are sufficiently resistant to the more serious diseases, are of rapid growth, and have good technological properties.
Similar experiments are now under way-in France and in other European countries, Good results cannot be expected of this method everywhere, because some trees are sensitive to winter or autumn frosts and also because the soils on which-they are planted are generally poorer than the soil in the Po valley.
In Germany, research is basically concerned with the selection of species suited to each Gyps of soil and of trees which can stand rather dry climates; poplars are not very common except in the Rhine valley, where they are widely used as forest tress. The same applies to Switzerland, where the most valuable trees-were selected without concern as to their position in the systematic classification.
The Swedes have evolved new aspens of very rapid growth, either by the process of heterosis (P. tremula x tremuloïdes) or by increasing the chromosome number (Tremble triploïde): these do not seem suitable for lower latitudes, probably because the days are too short.
Finally, in the United States of America, until recently, research was aimed exclusively at the evolution and selection of hybrid crosses between black and balsam poplars.
Each country, therefore should undertake and pursue its own research for the evolution of new types best suited for its requirements. It is clear that it will not be possible to improve on past results in cultivation but we may expect to improve the technological properties and the disease-resistance of the trees.
The creation of a "Populetum" or collection of poplars is current practice in many countries. This will make possible the comparison of old and new types; in France, 70 different types have already been planted side by side in the Loire valley near Blois.
In practice, black poplars and balsams are propagated almost exclusively by grafting; the technique is simple and only a few types require special precautions to ensure success, such as the irrigation of nursery beds. No effect upon growth has been noted from the use of plant hormones.
Grafts must be taken from young trees and, if possible, from the tree tops. In this way, i.e., by using grafts from elite trees, the mother-trunks can be converted into pollards which furnish excellent grafts. Approximately every ten years, new pollards are required. Austria, Czechoslovakia, Germany, and the Netherlands have contributed to perfecting the grafting technique.
It is more difficult to graft white poplars: aspens are complete failures; white poplars proper, and particularly the southern types, are more successful. However, the use of such propagation methods is rarely feasible financially.
The Danes and the Swedes have perfected a method of greenhouse propagation by grafting budding, green suckers; to be both useful and practicable, this technique still needs to be perfected and simplified. Work along these lines has been undertaken at the French Forestry Research Station.
In Canada, aspens have been crossed with other members of the same group in an attempt to facilitate grafting.
The rules for the cultivation of poplars have been evolved gradually and mostly empirically. In the majority of countries where this tree is grown, plantations are still far too dense. Experiments conducted over several years on experimental plots in Belgium, France, and Italy have proved that only poplar stands with a maximum of 200 stems per hectare are economically profitable, yielding the best assortment of products. Research in these countries has demonstrated the relative ineffectiveness of thinnings, Observations made by a recent international mission to the United States of America indicated that the same rules apply to hybrid balsam trees; it is a fact that dense plantations have consistently failed instead of producing the magnificent yields expected by the theorists.
Aside from such thick poplar stands, there has been successful cultivation of poplars in rows, while, in some regions, poplars have been introduced into regular forest stands.
Information concerning suitable types of soils for poplars is generally good. The art of pruning is also far advanced. Research has shown that poplar trees should attain a minimum girth of 1.20 meters ² if they are to be commercially profitable. Trees of sufficient size are also preferred for pulp.
² This applies to Western Europe; in the Near East, on the contrary, small diameter poplar stands are considered most desirable.
These are both numerous and varied, ranging from mammals which eat the bark (rabbits, squirrels, and other small rodents in Western Europe; deer in North America) to the unknown bacteria which cause canker.
Most parasites exhibit marked selectivity; growers, therefore, require a knowledge of the position in the systematic classification of the trees they intend to grow and of their pests: for instance, the mistletoe of Western Europe (Viscum album L.) does not attack Populus nigra L., but does live on all the Euramerican hybrids. The unknown causal agent of oozing canker selects its victims even more carefully: among the Euramerican hybrids, it damages only the regenerata type and is only dangerous in the northern part of the area where this type grows.
The two above-mentioned poplar parasites attack healthy vigorously growing trees. Others, i.e., insects and fungi, destroy leaves, while still other insects bore holes and galleries in the wood.
On the other hand, many enemies of poplars only attack the trees when their resistance is low: parasites of sickly trees and deficiency diseases are prevalent in too densely planted stands or among trees planted under unfavorable ecological conditions. These are the most frequent cause of damage to poplar stands, but they can be avoided through proper silvicultural practice.
The International Commission recommended that its member countries make a survey of the enemies of poplar trees. This will bring together widely dispersed documentation on the subject.
Poplar is used for many varied purposes because it is light, easily worked, and has good mechanical properties.
Sawn timber. This is the oldest use for poplar timber and the one for which it is best suited. We may disregard carpentry which is chiefly important in the Mediterranean basin, because of the special properties of Populus nigra (forma italica in the West) other types in the East, are frequently used as small round wood, as in Turkey.
In box manufacturing, where lightness is of prime importance, knotty lumber and logs of too small a diameter are frequently used. The manufacture of crates for furniture is an important industry in France: fluctuations on the furniture market greatly affect the price of poplar wood in the Paris area. In Belgium and the Netherlands, much poplar wood is used for the manufacture of wooden shoes (sabots) Wood of the white poplar is used in carpentry and for ship construction.
Veneer. Poplar wood is excellent for veneer-making without requiring prior treatment, when it is of good quality; in fact, it is the percentage of veneer wood in a log which determines its price. Boards quality are used for packing boxes. Good splinters are used for match-making. Select wood of sufficient length is used for plywood manufacture.
These main uses of poplar wood, as sawn timber and for veneer, have stimulated research in many countries, notably in Switzerland. This research is aimed at lowering the percentage of waste by the elimination of certain defects, and at determining the influence of the clone, of environmental ecological conditions, and of proper care upon wood quality.
It has been fully demonstrated already that certain types have better technological properties than others (for example the clone P. deltoïdes f. carolinensis cultivated in southwestern France) but, in most cases, conditions of growth and management, particularly pruning, have a much greater effect on the technological properties and on yields of select wood than the intrinsic qualities of the type cultivated. This explains the surprising divergence of opinion found among even the best informed growers.
Cellulose. Aspens and balsam poplars have long been highly valued as sources of paper pulp, but the use of black poplar for this purpose is comparatively recent. Italy is foremost in this industry.
French technicians have lagged behind in this respect. They are engaged in determining the best proportions for combination with other longer pulp fibers.
The manufacture of fiberboard is equally important in America and in the Scandinavian countries, but is still in an incipient stage in Belgium. The use of poplar wood for pulp and fiberboard manufacture is less profitable than for sawn timber and veneer. However, these industries afford an outlet for small size wood and logs of unsatisfactory shape.
To date, good results have been achieved, thanks to the multiple efforts recently co-ordinated by the International Poplar Commission. They are summarized below.
Black poplars (Section Aigerus). The identification and classification of known types (either of wild or of hybrid origin) is on the point of being completed. As regards their nomenclature, a classification system has been established, into which new species may be fitted as they are identified. Furthermore, there now appears to be available a sufficient range of types to make possible excellent results from plantations on good, new, rich soil in the temperate zone of Western Europe. These types are sufficiently varied to supply the manifold requirements of our present-day economy, taking into account the special ecological conditions at the French Experiment Station and the danger of disease infestation.
While proper cultivation methods and rules have been well established, they still need to be applied more generally.
Balsam poplars (Section Tacamahaca). Further experimentation with this species and hybrids is required. Good results are expected in either the frigid or temperate zones.
White poplars (Section Lance). Only the Swedes have passed the experimental stage in their research and have put into practice some of the principles evolved. Elsewhere, natural stands are being felled and at times ill-advised plantations are being made due to insufficient basic knowledge.
The conclusions reached in this report point the way to future research. The following points require attention. First, the need for rich, alluvial soils of excellent quality, rarely found in extended tracts, if the poplars currently cultivated are to prosper. Second, insufficient present knowledge regarding the cultivation and possible industrial uses of the white poplars group, particularly the southern species. Third, the backwardness of research throughout the temperate hot zones, more particularly in the Mediterranean basin.
Research workers and growers should, therefore, direct their efforts toward:
1. Research on types of poplars which can be grown on comparatively dry soils, of average or below-average richness. This problem hardly concerns the northwestern European countries where the humidity of the atmosphere compensates for the deficiency of soil moisture. However, it is of capital importance in Mediterranean countries.
Black poplars of Eurasia (Populus nigra L.) are more resistant to drought than the southern types (P. deltoïdes); consequently, a study of the former may provide a solution.
In southeastern France black poplars of relatively satisfactory growth and shape are cultivated locally. By choosing the best of these, and selecting seed obtainable from them, and possibly, even by crossing them with Euramerican hybrids, it should be possible in a short time to grow trees producing marketable timber on soils where the growing of poplars has previously not succeeded. It is likely that a survey of the entire area where black poplars (P. nigra) grow would lead to the discovery of valuable clones.
In southwestern France and in the eastern Mediterranean basin, female clones of black poplar (P. nigra) of very erect stance, but not as slender as the Italian poplar, are grown: they are the "Hamoui" of Syria, and the "Vert de Garonne", etc., widely used locally. Their improvement might prove worthwhile, particularly since in Mediterranean countries poplar wood is used for carpentry, which requires the special technological properties of black poplar. The leaves of the black poplar are also used as feedstuff for livestock.
2. The scientific study of southern white poplars. These types are little known or used, except in the Near East where a cultivated form, known as "Roumi" a fastigiate form of Populus nivea Willd, is used for veneer-making.
While the studies of botanists have made possible a preliminary classification, probably incomplete and perhaps inexact, as further examination may show, our knowledge of the ecology, silviculture and utilization of this group is altogether insufficient.(a) Ecology. These trees are resistant to high summer temperatures, and can live on comparatively dry and even salty soils. However, the variations in behaviour of the various types and their limits of endurance in the face of adverse factors such as drought, stagnant waters, and extremes of temperature need to be defined. It is no less essential to know under what minima conditions they still produce marketable crops. This leads to silvicultural considerations.
(b) Method of cultivation. We have practically no knowledge as to the best method of growing these trees. It is reasonably certain, however, that, like all poplars of the Leuce group, these trees will grow in denser stands than black poplars; on the other hand, the economic limitations of such adaptability have not yet been determined.
Vegetative propagation is much easier in the case of aspens, however, the details of this technique still have to be perfected.
3. Scientific research on poplar wood. All research should be constantly correlated with the technological research being done in the laboratories and in factories using lumber from stipulated small stands.
Only those species which have proven completely satisfactory as regards their suitability for carpentry work, their sawing qualities, and paper- and veneer-making qualities, or any one of these, should continue to be propagated.
4. The future progress of poplar culture in the hot temperate zone. This will depend directly upon the solutions to the preceding questions and on research on the Turanga group. However, there are a few good alluvial soils in well-irrigated districts in the warmest parts of the Mediterranean area, which can be planted with black poplars with anticipation of good yields. It may also be predicted that the southern species P. deltoïdes Marsh and some hybrids now bred or selected at the Italian Institute of Poplar Culture at Cassale Monferrato will prove to be successful.