RICCARDO MORANDINI
RICCARDO MORANDINI is on the staff of the Silvicultural Experiment Station, Florence, Italy. Other members of the drafting team wore J. M. Fielding (Australia), R. Faulkner (United Kingdom), and O. Fugalli (FAO).
The genetic improvement of exotic trees is based on the same general principles that are valid for all forest trees and has the same object, namely, increased production, both qualitative and quantitative, and the development of all those characters, such as plasticity, tolerance and resistance, which enable the maximum benefit to be obtained from planting trees.
However, the practical achievement of this improvement comes up against difficulties linked with the exotic origin of the trees. For example, knowledge of the biology, ecology and genetic variability of the species in its natural home is often very incomplete and hard to obtain in underdeveloped regions. The supply of reproductive material as seed, grafts or cuttings of known origin is also difficult to organize.
Research on the improvement of exotic trees should therefore be done both in the country of origin (where the research should include the study of the genetic variability, provenances, selection of seed stands and plus trees), and in the country of planting (acclimatization tests, experiments with provenances, hybridization and the formation of seed orchards).
The current trends in wood consumption suggest the primary concentration of effort on those trees whose present and future economic value is well known.
The tremendous increase in the demand for wood, due to economic and industrial development, has brought about a revolution in forestry which traditionally has been linked with century-long rotations and a largely extensive type of production. Foresters have been forced to adapt themselves to the new industrial rhythm and to concentrate their efforts on transferring this rhythm to forest production. For this they must adopt new techniques and especially new means of production, in other words, new tree species. In certain regions the selection and improvement of the indigenous forest tree species has provided the means of production, but very frequently the local species did not have the required characters and it became necessary to introduce trees from other regions. This development has given us two new terms, namely "introduced" or "exotic" species.
It is not necessary to provide examples of the introduction of trees; during the last few years the exchange of trees between one country and another has become almost as frequent and widespread as the exchange of agricultural plants.
It must first be emphasized that the adjectives "exotic" or "introduced" apply to a species grown outside its natural habitat. This definition leads one to consider as exotics, for phyto-geographic reasons, some species that are "politically" indigenous. This applies to European larch (Larix decidua) when taken from the Alps to the German plains; to the Corsican pine (Pinus nigra var. calabrica) planted in Metropolitan France; and to the western white pine (Pinus monticola) when it is transferred to the eastern United States.
Naturalists were the first to introduce new tree species, mainly from the eighteenth century onward, largely to establish botanical collections or to enrich parks and gardens. The nineteenth century, however, saw the commencement of the introduction of trees to fulfill the needs of forestry. To-day four basic reasons for introducing species are recognizable:
1. to enrich the local flora;
2. to obtain resistance to disease and other unfavorable environmental factors;
3. to exploit superior growth rate;
4. to obtain wood quality.
Enrichment of the local flora
The present composition of the forest flora of a region is the result of natural evolution in relation to climatic variations throughout geological time (and especially the glaciations) and of the direct and indirect action of man.
Consequently, the forest flora often has a very different composition in regions which have similar ecological conditions. This applies especially to conifers which are the basis of the modern forest economy the distribution of which is extremely irregular. In tropical and subtropical zones, for instance, broadleaved trees are largely dominant, save for a few countries such as Mexico where there are also conifers. In other cases the local forest flora consists only of shrubby trees and bushes, even where the ecological conditions would permit the growth of large-size trees.
Thus the introduction of exotic trees can supplement the local flora and provide foresters with a new means of production.
Resistance to disease and other unfavorable environmental factors
The need for resistance to disease and other unfavorable site factors is often the chief incentive for introducing new species. The resistance of the Japanese larch (Larix leptolepis) to the agent of larch canker Dasyscypha (Trichocyphella) willkommii which affects the European larch (Larix decidua); the use of the Japanese and Chinese species of Castanea which are resistant to the ink disease of sweet chestnut and to Endothia parasitica; the plantations of Picea sitchensis which resists strong winds in the northern European lowlands; the use of certain species of Acacia and Tamarix in saline soils; all of these prove the importance of widespread testing and use of exotic species.
Superior growth rate and production
The need for greater rates of growth normally provides the reason for preferring exotic to indigenous species. Two factors must be considered. First, the capacity of the species to grow to a large size; and secondly, rapidity of growth which makes it possible to introduce the rhythm of industrial output to wood production. Some species can produce a given volume of wood in half the time required by others. Moreover, if volume increment is high from the early years, the thinning yields enable investment depreciation terms to be considerably improved and plantation programs to be drawn up which are financially feasible.
Wood quality
Another reason behind the introduction of exotic trees is the production of wood having a different quality from that of local woods. Formerly it was the production of high quality wood which attracted attention. The plantations of Tectona grandis in Africa and America, the spread of the various southern pines outside the southern United States, tests of the American ash (Fraxinus americana) and walnut (Juglans nigra) in Europe are well-known examples of this kind. Nowadays it is rather the great demand for coniferous wood and pulp wood that determines the type of plantations. The plantations of Pinus radiata in South Africa and New Zealand, and the plantations of Populus species in Europe strongly reflect these tendencies.
To sum up, all four reasons mentioned here as justifying the introduction of new tree species may be collated under a single aim - that of finding the means of producing in the shortest possible time on a site the greatest quantity of wood of the desired quality for industries based on wood.
It is therefore necessary to make economic decisions based on a knowledge of the present and future local conditions of the wood market, of trends in its use, and of the prospects and possibilities of developing industries which the availability of wood might affect. The validity of these statements is confirmed by the paper industry in New Zealand based on plantations of Pinus radiata, and by the program to plant Eucalyptus and establish a pulp industry in Morocco.
The geneticist is therefore called upon to draw up his program of work in close co-operation with economists and to keep within the overall plan for the development of his country.
It should be remembered that there is not complete agreement about the importance of exotic trees in the forest economy. A swing of opinion against exotics has arisen in the past because of disappointments from certain introductions, too optimistic an evaluation of the possibilities of some species and, chiefly, too early a passage from the experimental phase to large-scale plantations (Edwards, 1963).
There are two principal kinds of problems in developing a program to introduce exotic trees. The first concerns basic questions as regards choice of tree species, and a very effective treatment of these was recently made by Champion and Brasnett (1960) on the initiative of FAO. The greatest problem is how to obtain sufficiently detailed data on the various species, which are often little known even in their country of origin.
Preliminary knowledge of the ecology of the species in its natural range is clearly indispensable for any comparative study between the region of origin and the region in which the species is to be used. On the other hand, tests carried out on the sites in question provide the only means of assessing the productive capacity and plasticity of the species in the new environment; and these features are also linked to the variability of the species.
The introduction or the improvement of exotic species requires, therefore, basic research, and questions of genetics must occupy a very extensive place.1 Part of this research deals with the species in its natural range and should thus be done in the country of origin, the most important questions being the extent of genetic variability, ecological studies, and the selection of seed stands and plus trees. Introduction tests (in arboreta and test plantations) and tests of resistance and plasticity should clearly be done in the receiving country.
1 For this reason, mention is made in this chapter of several subjects which form the major part of the discussion in other chapters of this final report, for example, chapters 4, 10 and 11, to which the reader is referred for details of the various concepts discussed by Professor Morandini.
TABLE 6. - FOREST TREE SEED AND PLANTS MOVING IN INTERNATIONAL TRADE
|
Species |
Region of origin |
Region of use |
|
|
Latin name |
Common name |
||
|
Larix decidua |
Europe larch |
Europe |
Europe and eastern North America |
|
L. leptolepis |
Japanese larch |
Japan |
Europe and eastern North America |
|
L. x eurolepis |
Hybrid larch |
Europe |
Europe and eastern North America |
|
Picea abies |
Norway spruce |
Europe |
Europe and North America |
|
P. sitchensis |
Sitka spruce |
Western North America |
Europe, especially northern countries |
|
Pseudotsuga taxifolia |
Douglas fir |
Western North America |
Most temperate regions of the world |
|
Pinus contorta |
Lodgepole pine |
Western North America |
Northern Europe |
|
P. nigra |
European black pine |
Southern Europe and Mediterranean region |
Europe, eastern North, America, Australia, New Zealand |
|
P. sylvestris |
Scots pine |
Europe |
Europe and eastern North America |
|
Acacia mollissima |
Black wattle |
Southern Australia |
India, South Africa and other subtropical areas |
|
A. melanoxylon |
Australia blackwood |
Southern Australia |
India, South Africa and other subtropical areas |
|
Cupressus lusitanica |
Mexican cypress |
Mexico and Central America |
Australia, Jamaica, southern, central and eastern Africa. |
|
Eucalyptus globulus |
Blue gum |
Tasmania |
Africa, South America, subtropical regions of Asia and the Mediterranean region |
|
E. grandis |
Mountain gray gum |
Eastern Australia |
Africa, South America, sub-tropical regions of Asia and the Mediterranean region |
|
E. saligna |
Sydney blue gum |
Eastern Australia |
Africa, South America, sub-tropical regions of Asia and the Mediterranean region |
|
Pinus elliotii |
Slash pine |
Southern United States |
Australia, New Zealand, central and southern Africa |
|
P. patula |
Patula pine |
Mexico |
Australia, central and southern Africa and other subtropical regions |
|
P. pinaster |
Maritime pine |
Portugal and southwest France |
Portugal, France, Spain, South Africa, Australia and New Zealand |
|
P. radiata |
Monterey pine |
California |
New Zealand, Chile, Spain, Australia, southern Africa |
|
P. taeda |
Loblolly pine |
Southern United States |
Subtropical Africa and Asia |
|
Swietenia macrophylla |
Honduras mahogany |
Tropical America |
Ceylon, Fiji, Malaya, Trinidad, India |
|
Tectona grandis |
Teak |
India and Burma |
Throughout the tropics |
Secondly, there is a whole series of questions concerned with tree improvement, such as hybridization, the induction of mutations, investigations on early diagnosis in phytotrons, and so on, which, according to the individual case, may be done in the country of origin or in the host country or in both. In this connection, mention must once again be made of the importance of the exchange of information and international co-operation. The international provenance tests on Pinus sylvestris (1907, 1938), Picea abies (1938) and Larix decidua; (1944); the work of Section 22 of IUFRO which deals with the study of forest plants; the results of investigations made in close co-operation by research workers from different countries, all show that teamwork should be extended on a worldwide scale.
Studies on genetic variability
A close study of a species in its natural habitat can give a picture of its silvicultural and economic potential. The size to which a tree can grow, its rate of growth, and its ecological and edaphic requirements are all fundamentally the same in the country of origin and elsewhere.
Greatest attention is paid to the variability of the species which often - although not always - depends upon the extent of the area of growth. A species with a very limited range will probably not show great variability. Nevertheless, it should be remembered that the three populations which make up the natural range of Pinus radiata do show considerable differences; and other examples of this phenomenon are provided by Larix leptolepis in Japan and Picea omorica in Yugoslavia.
Studies on geographic races
There is always a certain amount of variation among tree species with a very extensive natural range. This variability is sometimes very marked, to the point of justifying the recognition of geographic or ecological races. The geographic variation of Pinus sylvestris is one of the best-known examples, and this has been elucidated by a great number of comparative experiments in Europe and North America.
Pinus ponderosa shows just as great variation and the different values of its botanical varieties and numerous races have been demonstrated by the contradictory results obtained from Europe and from the provenance tests now in progress. Zobel (1961) has stressed the variability of Pinus taeda and P. elliotii in the southern United States and the even greater variation of P. flexilis and P. montezumae in Mexico. In this connection the biochemical research of Mirov (1962) is of great interest. In Mexico the importance and extent of this variation suggests that certain groups of species are still in the full process of evolution. Under these conditions the geneticist is able to find precious material for his work.
Certain species, particularly European and North American conifers, have already been studied in detail. A great deal has yet to be learned but, by and large, there is already sufficient basic data. In those countries where scientific forestry has just made its appearance the geneticist has an enormous field of work. What is known, for instance, of the Swietenias or the Araucarias or the Acacias, or other tropical species of great forest value?
For each genus, and for each species which is worth introducing, there must first be a study of the conditions in its natural range and one must distinguish, according to circumstances, the morphological, ecological and edaphic differences and examine the technical properties of the wood on the different sites.
It is not necessary here to go into details of the problem because it was fully discussed in Chapter 4, but it is enough to emphasize its importance.
Selection of seed stands and plus trees
Because of the variation shown by the great majority of tree species, the provision of seed of known origin is crucial. It is known that this problem is often difficult to resolve even in those countries with a long forestry tradition where the seed crop and its distribution have long been subjected to more or less efficient control. Germany and Sweden have always been to the fore in this respect, and several countries are following their example. In scientific work, co-operation between research institutes often results in the supply of material of well-defined origin, but sometimes, even in forest research and often in forest practice, it is not possible to get such material, especially of species which are still little known and from certain countries.
Species of the genus Eucalyptus provide an important example. Because of the increasing use of Eucalyptus the demand for Australian seed has increased very rapidly, both for reforestation and for research. It was above all the supply of seed for research, often of the rarer species or special provenances, which complicated the work. The goodwill and co-operative spirit shown by the Australian Government have led to the formation of a section of the Forestry and Timber Bureau at Canberra specially charged with the supply of seed to foreign countries.
This is a happy solution but one which unfortunately so far finds no counterpart in other countries. In fact, reliable guarantee of seed origin is no easy matter. Seed stands, and in the later phases of breeding, plus trees must be selected, often in regions which are badly served with roads and transport. Moreover, it is necessary to organize the collection of the material in the forest and keep a check on its processing right up to delivery. As will be seen in Chapters 10 and 11, this problem does not solely concern exotic trees, but it is particularly accentuated in certain of these species.
In some cases the countries interested have arranged ad hoc expeditions to the country of origin. South Africa, New Zealand, Rhodesia and the United States (Loock, 1950; Hinds and Larsen, 1961; Zobel, 1961) sent experts to Mexico to collect seeds of Pinus species. This interest, stressed by the Seminar and Study Tour of Latin-American Conifers organized by FAO in 1960, has led the Mexican Government to organize, with the assistance of FAO, a center for the selection of seed stands of these pines, and for the collection and distribution of the seeds.
Similar seed collecting expeditions will probably be necessary to obtain seeds of other species, particularly those growing in remote areas and in countries where trained seed collectors are not available. Where collections of research material can be adequately supervised by local staff, all or part of the costs may have to be defrayed by the country or countries making the request. It is also desirable that demands for seeds that are difficult to obtain should be well thought out.
The question of seed origin is such a basic one that the authorities of every country should make it their responsibility. European foresters have not yet obtained any real guarantee, for instance, regarding the bulk importation of seed of Pseudotsuga taxifolia from America.
It is desirable that international co-operation, which has been greatly facilitated by the bodies which organized the World Consultation in Sweden, should lead to a solution of these problems. Members of (FAO), and IUFRO can play an important role by providing information and facilities for international seed collection and exchange. In this respect, the following are worthy of urgent attention:
1. production and circulation of a list of research workers who are willing and able to collect seed on behalf of other countries;2. production of a practical code of procedure for standardizing the selection of seed trees and seed stands and site descriptions for both research and practical purposes; the importance of obtaining local meteorological data should be emphasized (see Figure 15).
Tolerance and plasticity
Although a comparative study of climatic and other ecological conditions of the region of origin and the region of introduction permits general forecasts of the chances of success, only plantation tests make possible valid conclusions about the economic value of the species, and this is the basis for any large investment.
It is unusual to obtain very detailed information about the region of origin and certain phenomena escape precise assessment. The plasticity of a species is very important because some show very marked requirements and cannot adapt themselves even to conditions which are apparently very near those which obtain in their natural range. By contrast there are other species which survive and grow vigorously in apparently very different conditions. A striking example of such differences in plasticity is given by the genus Eucalypus which includes several hundred species covering a more or less widespread range. Some of the species will spread only in their own particular habitat or under conditions which are strictly similar while others, even those with a very restricted range, are able to adapt themselves to very different ecological conditions.
In this context Pinus radiata, may again be mentioned. This species was taken from a limited area of origin and transported with great success to very different regions where the climatic variations cover an extremely wide range. Again, some species which are of no particular interest in their country of origin because of their slow and limited growth, have shown exceptional vigor as exotics.
The choice of species to introduce is more difficult when the introducing country has extreme ecological conditions. Resistance to certain factors, or adaptability to difficult habitats, are then often of greater importance than growth rate or wood characters.
Resistance to drought and cold are frequently sought after, particularly in the introduction of exotic trees to the Mediterranean region where the unreliability of the climate increases environmental difficulties. Otherwise the limiting factor in the region is that of exceptional soil conditions, particularly salinity.
Resistance to stated factors and plasticity are sometimes found throughout a species, especially in one with a very limited natural area such as Pinus radiata, But more frequently these characters are linked to geographical and ecological races. Attempts at introduction should take these two possibilities into account.
Tests of races and provenances
The existence of differentiated races within a species complicates the problem. If the preliminary study enables attention to be concentrated on a more limited sector of the range and makes possible the elimination of races from other sectors, it is still necessary to consider the introduction of as wide a range of provenances as possible, and to establish tests to compare these provenances in different environments. Apart from provenances taken directly from the region of origin, the possibility should be considered of seeking material from other countries where the species has already been introduced with success. Quite often such provenances, already subjected to natural selection under the conditions of the intermediary country, have a plasticity which is far superior to that of material taken directly from the country of origin. An example of this is provided by Eucalyptus camaldulensis.
In some countries at the present day it is rather unusual to find that a new species is being introduced for the very first time. The presence of plantations, even small plots or strips or of trees in gardens and arboreta, is a great advantage. These provide the basis for useful preliminary studies and may represent a precious source of material.
On the basis of these plantations or individual trees which could provide a natural spread of the introduced species (which is sometimes to be found in the second or third generation as seminatural stands), it is possible to begin the work of selection while awaiting the first results of new introductions or of provenance tests which require a longer period of time and hardly ever less than ten years.
If the early introductions have reached a considerable age, they will have been subjected to climatic extremes and their state of health and growth will make it possible to draw conclusions and use them in the selection program, particularly with regard to resistance to cold.
Hybridization
The main aim of hybridization is to bring together in a new individual the favorable characters of the parent trees. However, the effect of increased vigor, which can occur in F1 hybrids, often assumes greater importance. The cross may be between an exotic parent tree and a tree of an indigenous species, or between two exotic parent trees. In the first case, there is a general tendency to improve certain characters of the indigenous species, such as wood quality, stem form, and especially resistance to disease. Thus the hybridization programs in the United States and in Italy with the indigenous chestnuts (Castanea dentata and C. saliva) and the Japanese and Chinese chestnuts (Castanea crenata and C. mollissima) aim at making the indigenous trees resistant to ink-disease and to canker. Hybridization between Abies alba and A. veitchii in Germany aim at improving the resistance of the indigenous silver fir to frost. Crosses between European and American poplars (such as Populus nigra x deltoides) on the other hand, stress the factor of growth rate.
Hybridization programs using exotic species are in progress in several countries. In the United States the Institute of Forest Genetics in California places great importance on this means of improvement. In Korea, members of the Institute of Forest Genetics at Suwon, Kyunggido, are working to increase volume production.
The origin of hybrids is often fortuitous; this is the case with species whose natural ranges are separate. When individuals of each species have been brought together by chance in plantations they have given rise to hybrids which have demonstrated superior characters and have then been reproduced artificially. In Eucalyptus, mention can be made of the hybrids E. x algeriensis, (E. camaldulensis x rudis x tereticornis), E. alba x saligna and E. saligna x grandis, which have already achieved a well-defined place in the culture of Eucalypts. But the attention of geneticists today is directed toward the natural or artificial hybridization of many Pinus species. Many examples could be quoted here, but the most interesting include P. strobus x monticola and P. sylvestris x densiflora in America, P. taeda x elliottii in Korea, and P. attenuata x radiata in America and New Zealand.
Seed supply and seed orchards
The supply of seed mainly limits the introduction of exotic trees, especially in forming large-scale plantations. This problem has already been mentioned in this chapter, and attention is again drawn to the paragraph referring to the importation of seed from the country of origin.
Careful inspection for pests and diseases of all imported seed, cuttings and scions should be made by the importing country; adequate quarantine facilities should be available for handling cuttings and scions. It must be remembered that not all exotic species are of value, and precautions should be taken to insure that undesirable freely-seeding species do not become serious weed problems.
Old plantations are sometimes good seed sources. All early introductions should be examined and, if worthwhile, should be classified with reference to seed collection. The risks of obtaining undesirable inbred plants from seed collected from small groups and isolated trees must be carefully considered. For this reason, collections from such sources are not recommended. On the whole, the general criteria for the classification of seed stands will be found valid and the same holds for the choice of plus trees (see especially Chapters 10 and 11 of this final report and also Appendix A).
There is now a marked tendency to concentrate seed production in seed orchards. This practice eventually insures the supply of seed of known origin, thus avoiding the hazards involved in importation, particularly with regard to provenance. It will suffice to note the problems associated with seeds of Pseudotsuga taxifolia, Eucalyptus and Pinus radiata to appreciate the importance of this method. The basic material for the formation of these seed orchards is carefully selected on representative sites or the material is introduced and subjected to progeny tests (see Chapters 10 and 11).
The production of hybrid seed poses special problems. Several countries already have their mass-production programs, either by controlled pollination or by natural crossing, an example being the mixed plantations of Eucalyptus viminalis and E. camaldulensis for the production of hybrid seed in Morocco.
The ecological properties of each species, its production possibilities and the economic role it can play in the receiving countries are all factors influencing the prospects for the improvement of the species, the research lines to follow and the methods of work.
It is not possible to examine here the details of the improvement of every species. Five species and genera of great current interest have therefore been chosen, on which large-scale improvement work is either in progress or projected. These species are Douglas fir, the poplars, the eucalypts, Monterey pine, and the Mexican pines.
Douglas fir (Pseudotsuga taxifolia Britt) ²
² See the reports: Le Douglas en Europe by R. Schober (FAO-FORGEN 63-4/5), and The use of exotic trees in increasing production by M. V. Edwards (FAO-FORGEN 63-4/2).
Among the species introduced into Europe, Douglas fir takes pride of place. Experiments over several decades hare already supplied valid basic data for the adoption of this species in European forests. Indeed the plantations already cover very extensive areas in several countries and, in Europe as a whole, may be estimated at tens of thousands of hectares; the area is still increasing at a rapid rate.
The geographical and ecological extent of the natural range of Pseudotsuga taxifolia early set American foresters the problem of defining races and provenances. Provenance tests were established in the United States as long ago as 1911, and since then have been extended and multiplied. They have shed much light on the existence of a large number of ecological races of very varying importance to forestry. In 1949 Isaac drew up a very detailed distribution map showing Douglas fir provenance zones, accompanied by basic climatic documentation for foresters of both the countries of origin and reception.
In Europe the very variable results from the first introductions of Pseudotsuga taxifolia also attracted the attention of foresters. The problems of systematics and provenance have long been investigated in several countries. If productive capacity and rate of growth are the point of departure, resistance to disease, and especially to Rhabdocline pseudotsugae, resistance to frost, plasticity, and drought tolerance are the objects of improvement in the various countries of Europe. It has already been suggested that for north-central Europe, provenances from the coastal zones of British Columbia and Washington give the best results, while in the southern countries of Europe provenances from Oregon and from North California show better tolerance of the dry climate. Experiments on this subject are worth extending and renewing in several countries.
The fundamental problem for European foresters is now the supply of Douglas fir seed. Seed production in America is infrequent and very often inadequate to meet the great demand, both locally and from Europe. Moreover, although some enterprises send certificates of origin with their deliveries of seed, in general there are insufficient guarantees of seed origin.
The oldest plantations of Pseudotsuga taxifolia in Europe are already showing good seed production, and some countries have selected stands for the production of seed. This material is of great importance because its adaptation to local climatic conditions is already known and should thus be a point of departure for genetic improvement. Several European countries have already set up Douglas fir seed orchards, and it is suggested that this is worth developing as the best means of solving the seed supply problem.
Poplars (Populus species) ³
³ See the report Génétique du peuplier by M. Sekawin (FAO FORGEN 63 - 4/4).
The natural range of poplars covers almost the whole of the temperate zone and even extends to the subtropical zone. This extensive range, the great number of species and their phenotypic and genotypic heterogeneity demand the use of different improvement methods according to the region and the species.
In the northern part of the range, poplars of section Leuce Duby dominate (especially Populus tremula). They are rather difficult to propagate vegetatively. Improvement in section Leuce should be based on the following points:
1. production of selected seed by the normal methods used for forest tree seed, (that is, selection of seed stands and plus trees and control of seed collection and distribution, etc.);2. vegetative reproduction of stump shoots or root cuttings from selected individuals (and eventually by stem cuttings-using growth substances);
3. production of hybrids with improved capacity for vegetative reproduction by crossing with P. alba among others.
Poplars of section Tacamahaca Spach, are also of some interest in this region.
In the warm-temperate regions, poplars of section Aigeiros Duby are the most important, and especially P. nigra L. from Europe, P. deltoides Marsh. from America, and their hybrids. In these poplars, vegetative propagation generally does not present difficulties. Clone formation is easy and this permits the maximum use of the hybrid vigor or heterosis shown by first-generation hybrids.
In the southern regions, where drought resistance is an important character, poplars of sections Turanga Bunge and Leucoides Spach are found alongside P. nigra and P. alba Species of the latter section present some difficulties in vegetative propagation and it is then necessary to use the improvement methods described for section Leuce.
A study of mutations, and particularly of natural or artificial polyploids, opens up considerable prospects for improving poplars (see especially the studies on polyploid Populus tremula in the U.S.S.R. and Sweden; on the artificial polyploids of Section Aigeiros in Belgium and Italy; and on haploid individuals of P. alba in Hungary). Researchers on poplars should consider the different ecological and economic environments, but in addition to production characters, resistance to pests and disease deserve greatest attention.
Sekawin (1963) considers that future work on the genetic improvement of poplars should be done along the following lines:
1. precise definition of the objects of genetic improvement in relation to physical, economic and social factors;2. a combined big-systematic study of the poplars existing in each country and of their relationship with the environment;
3. hybridization and selection among local types;
4. international exchange of selected material and hybridization with indigenous material, together with the establishment of poplar collections;
5. studies on the inheritance of the most important characters;
6. research on polyploidy and mutation;
7. improvement of the methods of reproduction and multiplication;
8. systematic experimentation with cultivars and the clones obtained by selection and breeding.
Eucalyptus 4
4 See the report Contribution du Maroc pour l'amélioration génétique des Eucalyptus introduits dans le bassin méditerranéen by J. Claudot (FAO-FORGEN 63-4/9).
The importance of the genus Eucalyptus in providing fast-growing species needs no emphasis. As exotics these species have often shown a production capacity far above that in Australia, and each year they become more and more important in the forest economy of the world (Metro, 1954, 1963), especially in the tropical and subtropical regions.
One of the most important properties of Eucalyptus which is especially developed in certain species is plasticity. This enables the trees to develop with great vigor in very different ecological conditions from those in their natural range. Furthermore, species of Eucalyptus are able to adapt themselves to new conditions, and transmit this capacity for adaptation to their progeny.
In addition, the ease of inter- and intraspecific hybridization frequently leads to the creation of new hybrids and some of these are already being used on a large scale (examples being E. x algeriensis, E. alba and E. 'Mysore hybrid').
On the other hand, several countries encounter difficulties in using the timber of Eucalyptus. In particular, the wood derived from rapidly grown plantations may show twisted fibers, collapse and other defects.
The main obstacle to making new introductions of Eucalyptus species is the difficulty of obtaining seed of known sources of the species and the provenances required. This problem has always been discussed at meetings dealing with Eucalyptus and especially at the second World Eucalyptus Conference at São Paulo (FAO, 1961) Selection of seed stands and the formation of seed orchards in each reception region will help to solve these difficulties. Another important method is vegetative propagation and a technique effective for several species has been already developed.
Research on the improvement of Eucalyptus has three objects: to extend the culture of eucalyptus; to improve production; and to improve technical properties.
The investigations made to extend the use of Eucalyptus to new regions are based on the plasticity of certain species and provenances, the accent being placed on resistance to drought in the Mediterranean region and on resistance to cold. The selections made in France and Italy, especially in E. gunnii, E. dalrympleana, E. rubida, E. viminalis and E. bridgesiana seem to offer good opportunities of obtaining cold resistance.
The improvement of productivity is primarily based on mass and individual tree selection, and the possibility of vegetative propagation then becomes a basic consideration.
Regarding mass selection, the importance of rigorous selection of material in the nursery is stressed. This gave remarkable results at Rio Claro in Brazil. Natural and artificial hybridization and the mass production of hybrid seed also permit new types to be created and reproduced, and their hybrid vigor or heterosis exploited.
The improvement of technical properties, a subject on which Australian researchers have already achieved good results, should be based primarily on individual tree selection.
Monterey pine (Pinus radiata) 5
5 See the report Planning a genetical survey of Pinus radiata populations by M. H. Bannister (FAO-FORGEN 63-4/1).
Pinus radiata provides the most striking example of the plasticity of a species. From its natural range, which is limited to three stands of small dimensions in California and to some groups of trees on the islands of Guadalupe and Cedros (Baja California, Mexico), it has been introduced with great success to regions with very different climates, such as Chile, New Zealand, South Africa and northern Spain. Monterey pine covers the largest area of all exotic tree species.
Although the natural range of Pinus radiata is so concentrated, it appears that ecological differences can be detected between the three source islands in California, and these differences are very probably far more marked at the Mexican sites of Baja California (P. radiata var. binata).
In those countries where the species has been introduced for many years and over extensive areas it is probable that, because of segregation, migration of genes and hybridization between different genotypes, the actual population has a genetic composition quite different from any other wild population (Bannister, 1963).
According to Scott (1960), there are very good stands of Monterey pine in Australia, Chile, New Zealand, South Africa and Spain with, nevertheless, a very great range of morphological characters. Forest geneticists and tree breeders in Australia and New Zealand (Bannister, 1954; Fielding, 1957; Thulin, 1957) have already made important selection work, dealing with both seed stands and plus trees. The natural hybridization of P. radiata with P. attenuata or P. muricata, already noted in both California and New Zealand, has also been studied. In the United States and Spain some importance is given to artificial hybridization.
It would be interesting to look further into the question of provenances in relation to plasticity, with a view to establishing plantations of Monterey pine on sites with a comparatively dry, cold climate.
Mexican pines
Zobel (1961) has described Mexico as "the melting pot of the genus Pinus" Over a comparatively small area of Mexico there is an extensive series of Pinus species the range of variability, systematic position and forestry importance of which are far from well defined. Various authors have ranked as species a number of entities varying from 30 to 80 (Martinez, 1948). But it must be emphasized that within several species there is a very marked morphological variability which makes it possible to claim the presence of several "complexes of species," the evolution of which is far from being completed.
On the other hand, the importance of some of these pines in South Africa, especially P. patula (Loock, 1950), and the interest aroused in the FAO study tours in Mexico in 1960 (Morandini, 1961) have already attracted the attention of foresters in tropical and subtropical countries, who see in the Mexican pines a precious means for the rapid production of wood. The report of Hodgson gives a very important survey of the lines to follow in improving Pinus patula, which is today the most widespread of the Mexican pines.
This, therefore, is an ideal field of action by specialists in the various branches of genetics. Without doubt, the point of departure is a systematic study of the different units and an analysis of their morphogenetic variation. Ecological studies will enable the researcher to determine the existence and the extent of ecological races, which in some species with an extensive range will no doubt show very marked differences. In this respect, it is necessary to stress the importance of races comparatively tolerant to cold, because they would permit the use of some species in regions with a less mild climate (such as the Mediterranean area). The selection of seed stands and plus trees will enable seed production to be organized on a rational basis, because the demand for seed is already quite sizable. In this context, a prominent part can be played by the technical assistance programs of FAO; a highly-qualified researcher has already been sent to Mexico to cooperate with Mexican foresters in organizing seed supplies. While the fulcrum for the improvement of these pines will remain their country of origin, it must be stressed that several countries, especially South Africa, have already launched programs of improvement based on their existing plantations, some of which are more than 50 years old.
A program for the introduction of exotic trees and their improvement should be based on the following points:
Introduction of the species
1. Studies of climatic similarities between the various regions of the world; the Bio-climatic map of the Mediterranean zone recently published by Unesco and FAO (1963) provides an excellent example.2. Biological and economic analysis of the forest flora of similar climatic regions, and the choice of quick-growing species, the wood of which is readily utilizable in large-scale industries such as paper manufacture.
3. Ecological studies to distinguish the most suitable races and provenances.
4. Tests to screen out, by planting small groups in arboreta, a large number of species and test plantations (on sufficiently large areas to insure the statistical value of the results) of those species which have good prospects of success.
Improvement of the species
1. Provenance tests.
2. Selection for both quantitative and qualitative production.
3. Large-scale production of seed and cuttings for selection in seed stands and seed orchards.
It is clear that such a program will take a long time and require sizable investments. Hence it will be useful to concentrate on a fairly restricted number of species whose products have an assured economic value.
Finally, it is important to refer once more to the importance of international co-operation, which makes it possible to turn to good account the experience already acquired, and to concentrate efforts on achieving further progress.
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