CARIN EHRENBERGCARIN EHRENBERG is assistant professor, Department of Forest Genetics, Royal College of Forestry, Stockholm
THE CONCEPT OF stem quality includes external as well as internal features: morphology, anatomy, chemical composition and physiology, all of which are closely related and form a complex unit. In a more restricted sense, stem quality is defined by external or morphological properties as distinguished from the wood quality defined by internal or cellular properties. Thus, stem quality is mainly characterized by stem form and by growth and development of the branches.
A tree may be classified as good or bad depending on the purposes for which it is intended and on demands as to quality. For saw-timber production, a tall, straight, healthy tree with fine, flat-angled limbs, a narrow crown, and a well pruned stem with 1 little taper is considered a good tree. The yield and quality of timber will be satisfying and the costs for handling, barking and transport will be relatively small. The same demands on the tree are valid for pulp production, but requirements as to straightness, taper and pruning might be less exacting. For trees intended for shelterbelts or recreation areas the demands will be different.
Results reported here on breeding for improved stem quality will be restricted to the stem and branch characteristics important to quality. Results published during the period 1950 to 1963/64 will be summarized and recent results obtained after that time will be discussed. References will for preference be made to publications containing a comprehensive review of literature. The wood properties of value for pulp and timber production will only be mentioned briefly and then mainly in connexion with the effects of stem and branch characteristics on wood quality.
EARLIER RESULTS AND BREEDING METHODS
Provenance tests have revealed a great variation of stem and branching characteristics due to geographical sources. A careful test of foreign provenances and exotics is recommended in advance of transfer to and planting in unusual environments (Kiellander, 1958; Langlet, 1960, 1964; Shelbourne, 1963; Wright and Bull, 1963; Callaham, 1964).
Progeny tests with half-sib, full-sib, and selfed progenies have given valuable information on variation of stem and branch properties among natural populations, stands, progenies, and individual trees (Merger, 1955, 1960; Arnborg and Hadders, 1957; Toda, 1958; Rohmeder and Schönbach, 1959; Perry, 1960; Campbell, 1961; Squillace and Dorman, 1961; Ehrenberg. 1963; Barber, 1964; Goddard and Strickland, 1964; Johnsson, 1965; Nilsson, 1968).
The patterns of inheritance and the degree of inheritance of continuously varying characters could be studied more intensively and the variation of the characteristics assessed with greater reliability, once properly designed experiments had been established.
Improved techniques for measuring have been developed, and statistical methods for estimating genetic parameters in forest tree populations have been improved (Stern, 1961, 1964a). Estimates of potential gains from breeding and of heritability made during the period up to 1963 have been summarized and discussed by Schreiner (1958) and Hattemer (1963).
Studies of stem and branch properties, and their effects on wood quality, are reported for many of the most important conifers and even for some hardwoods (Nylinder and Hägglund, 1954; Dadswell, 1960; Campbell, 1961; Goggans, 1961, 1962; Zobel, 1961; Zobel and Haught, 1962; Shelbourne, 1963; Sunley, 1963).
The influence of silvicultural and other environmental factors on the variation of stem and branching characteristics is emphasized in all investigations. The degree of variance due to these nongenetic factors changes consistently with the traits investigated.
STEM PROPERTIES
Straightness
Straight stem is generally considered to be a desirable trait for all purposes. Deviations from straightness, such as lean, crook, sweep and twist, reduce the value and volume of the commercially useful part of the stem and may increase the costs for handling and transport. Methods used for measuring and grading defects are either subjective ratings of stem straightness or counting the number of crooks per unit length and measuring the deviation of the most severe crook or crooks. Photographs of a tree from different angles or a photogrammetric technique have also been used for quantifying bole straightness.
Stem taper
Uniformity of growth and good stem form, as expressed by the ratio between stem diameter at breast height and at a higher level, are important for volume production and properties such as wane and spiral grain.
Genetically controlled growth anomalies
Forking and fasciation in annual shoots disturb normal growth and cause crookedness of stem, development of two or more stems, and heavy branches. In the most severe cases the whole tree is twisted and bushy. Lammas shoots also give rise to crooked sterns and large branches which compete with the terminal leader and result in the formation of forked boles. The leader may retain dominance in later years, in which case the lemmas shoots produce strong branches, or it may be weakened by the competition and pushed aside, which may result in crookedness of the stem and in transforming the leader into a large branch. Ramicorn or fastigiate branches are sometimes formed repeatedly in an individual or clone.
Pruning ability
The age at which the lower branches of a tree begin to die and fall off and the capacity of the tree rapidly to occlude scars and stumps are of great importance. After artificial pruning is carried out, both the speed of occlusion and the time to regain normal growth rate are essential features.
BRANCH PROPERTIES
Branch diameter
Branches small in diameter will leave smaller knots and cause less loss of wood quality. The risk of damage by environmental factors such as snow, ice, and attacks by fungi and insects will be reduced in a fine-limbed tree. The length of the branch is usually closely correlated with its diameter. Hence, crown width and the relationship between crown width and tree height are related to branch diameter. Ocular estimates or more precise measurements have been used for grading branch size.
Branch angle
The importance of flat or wide angles of the branches for stem and wood quality is weld recognized. Branches with flat angles are more easily shed and more rapidly occluded. Trees with such branches have less knot volume than trees with steeper branch angles. Ramicorns usually have very steep angles, an unfavourable trait. The risk of damage by environmental factors of the same kind as those mentioned above is increased when the angles are acute. The angle of the branch from the centre dine of the stem usually is measured with a protractor or ocularly classified as acute, intermediate or flat.
Number of branches, number of whorls, and internode lengths
These properties are essential features with great influence on stem development and wood formation. Uniformity of growth, growth rate, straightness of stem, and knottiness are affected by the number of branches per whorl, the number of whorls per year and per unit length of stem, and the regular development of branches.
WOOD PROPERTIES
Knotwood
Volume of branch knots has been calculated by measuring knot diameter, knot angle and knot length from the pith to the outside of the tree. This feature is closely related to the formation of compression wood and spiral grain, and is one of the main determinants of quality of the wood (Wedel et. al., 1967).
Compression wood
This property is characterized by abnormal tracheid development, large intercellular spaces and an unusual amount of shrinkage. The term compression wood is applied in the case of softwoods, while the term tension wood is used in the case of hardwoods. Reaction wood, a common term for both cases, is defined by Dadswell (1960) as "applicable to the wood formed by hardwoods and softwoods, which is anatomically different from normal wood" and it "usually involves eccentric radial growth of the stem, which in hardwoods is on the upper side of leaning stems and branches, whereas in softwoods it is on the lower side."
Grain patterns
Spiral grain or grain distortions generally cause reduced strength and loss of value of a tree stem. Twists will tend to occur in material containing spiral grain. Some unique grain distortions that create attractive patterns upon the face of lumber or veneer can have extremely high values.
Tracheid length
Cell length, cell width, and cell wall thickness each affect the yield and quality of wood and influence the specific gravity.
Specific gravity
This is widely used as a criterion of wood quality and yield. It is an excellent index of the solid substance contained in a piece of dry wood (Goggans, 1961, 1962). Specific gravity values are obtained from cross sections of a tree or from increment cores.
Straightness of stem is essential for the formation of good quality wood. Crooks, sweeps and twists initiate the development of compression wood and irregular grain. The amount of severe compression wood is large in trees with severe crookedness and sweep. As the intensity of compression wood increases, tracheid length and specific gravity decrease, causing substantial losses in volume and value of a tree stem. Forking and ramicorns negatively influence the anatomy and structure of wood.
Size and number of knots, and the compression wood and grain pattern associated with knots, are mainly a result of branching characteristics such as large diameter, steep angle, numerous branches per whorl and numerous whorls. Indirectly, natural pruning ability belongs here. Early self-pruning reduces the knotwood volume in a stem since branches will be shed when comparatively thin, scars will heal more rapidly, and growth of the annual rings will regain regularity sooner. Also, pruning height, which is strongly related to total height of the trees, is important to the length of the merchantable portion of a tree stem.
Stem taper affects not only total volume production but also some properties of the sawn timber. Structure and strength of the wood especially are influenced. In trees with large taper, inclined grain is more pronounced than in trees with little taper.
PATTERNS OF INHERITANCE
Indications or proof of the occurrence of simple mendelian inheritance of stem and branch characteristics are scarce. Deformities ascribed to the effect of one or a few recessive or dominant genes are reported by Schröck (1957) and Ehrenberg (1963). Forking and fasciation of annual shoots in Pinus sylvestris L. were observed in progenies obtained after open pollination or after controlled crossings among parent trees, which displayed the same abnormal growth. The production of lemmas shoots which cause forks of a different nature is a genetically controlled feature which probably has a more complicated genetic background (Ehrenberg, 1963; Rudolph, 1964; West and Ledig, 1964).
There are some indications of a simple mode of inheritance of the form virgata in Picea abies. This snake form, at its extreme, completely lacks branches. Intermediate forms are found ranging from the extreme type to normally branched trees (Gunnert, 1962; Hölzer, 1968). Recessive genes, or a major gene causing bud and branch reduction and modified by minor genes and environmental factors, could be the explanation for this.
Stem characters such as straightness and pruning ability may be associated with sexual dimorphism, as is indicated in Fraxinus spp. (Hung, 1964) and in poplars (Muhle Larsen, 1968). If this is true, a 1:1 ratio of these characteristics could be expected. In Populus nigra L. the straight stem form is inherited as a dominant character and depends on more than two loci (Zufa, 1969).
The results obtained from studies of artificially produced hybrids between Populus nigra, P. deltoides and P. trichocarpa (Panetsos, 1969) indicate that, if the development of side branches in one-year-old seedlings is due to only one gene, which has been assumed, at least three alleles of this gene must exist. In the same material incomplete dominance of stem straightness was found and the development of an open, closed or intermediate crown form could be assigned to the effect of a small number of major genes. The influence of environmental factors on properties of this kind is great, however, and the length of time needed to obtain the F1 and F2 generations makes it a difficult task to determine the mode of inheritance.
In general, stem and branch characteristics are continuously varying traits controlled by quantitative inheritance. Additive genetic variance has been reported in all cases in which a statistical analysis of the material was possible as a result of experimental designs and methods used for assessing variation of a character. Evidence of dominant gene action (specific combining ability) is found in some cases, as for straightness and volume (Nikles, 1965). Estimates of the phenotypic and genotypic correlation of one stem character with another have been made in many species. These help to clarify inheritance patterns (Namkoong et al., 1966; Stonecypher, 1966). However, when genetic variances are being estimated, many assumptions have to be made which are never fully realized in the material investigated. This fact lends a varying degree of unreliability to calculated values.
HERITABILITY
Estimates of heritabilities have accumulated during recent years. Here the same varying degree of unreliability of estimates holds as for estimates of genetic variances (Stonecypher, 1966). A review of the published estimates cannot be made here (Strickland and Goddard, 1965; Hattemer, 1967; Sakai and Mukaide, 1967; Wilcox and Farmer, 1967; Dyson, 1969; Faulkner, 1969). Certain trends in the material will be mentioned.
The range of the estimates of heritability is wide in every characteristic investigated. It depends on experimental designs, species, age of trees, environment, and other factors.
Most estimates are obtained from young or very young trees, usually seedlings and clones from one to ten years old. Only a few of the plantations 10 to 20 years old have been evaluated. In general, estimates have been obtained from conifers, especially fast growing pine species in subtropical regions, and from poplars.
IMPROVED BREEDING METHODS AND TECHNIQUES
Many of the methods and techniques developed in recent years are fundamental and common to several research sections of forest tree breeding. An attempt to summarize the development of and improvements in the most important sections will be made here, and some techniques concerned particularly with stem quality will be mentioned.
Field experiments, specifically designed for limited and well-defined purposes, have been established for progeny testing and for provenance and population studies during the past decade. Measurements and observations from these young plantations are now being published. Valuable information is being obtained on genetic variances, on correlations between characteristics, and on genotype environment interactions. The heritability study established with Pinus taeda by the North Carolina State Cooperative Tree Improvement Programme, the International Paper Company and others, in Southlands experiment forest, Georgia, may be mentioned as such a long-term basic research project. It is especially designed for studies on quantitative genetics (Stonecypher, 1966). Mating designs and pollination techniques have been developed and improved for population studies and for progeny testing of seed orchards (Libby, 1964; Stern, 1964b; Johnsson, 1965; Andersson, 1967). The problem of clonal seed orchards versus seedling seed orchards has been discussed by, for instance, Barber and Dorman (1964), Johnsson (1964), Toda (1964), and Zobel and McElwee (1964).
Vegetative methods of propagation have been improved for many species, and new techniques for nursery management have been developed in order to facilitate the establishment of clonal seed orchards and clonal tests. These are essential parts of most tree breeding programmes (for review and references, see Squillace, 1967).
Improvements in the efficiency of selection and in procedures for estimating genetic gains have been discussed by many authors (see Stern and Hattemer, 1964; Andersson, 1965; Namkoong et al., 1966; and Squillace et al., 1967). Recommendations have been made according to the results obtained and summarized.
New methods and techniques have been developed for assessing stem quality. A photogrammetric technique for measuring bole straightness with high precision has been developed by Shelbourne and Namkoong (1965) and applied by Shelbourne (1966) in his study on the inheritance and relationships of bole straightness and compression wood in southern pines.
Much of the improvement in methods and technique is due to the rapid development of statistical methods and to the possibility of using computers for evaluating data. The background and details of these methods may be found in standard textbooks (Falconer, 1961; Li, 1963; Hjort, 1963; Williams, 1964; Wright, 1968).
Finally, the establishment of phytotrons makes it possible to control environmental factors to a large extent. The assessment of growth rate, stem and branch characteristics, and wood properties at an early age will be feasible. Comparisons will be possible, not only among seedlings and clones studied under controlled environments, but also with the same material grown under different natural environments and at different ages (Went, 1957; Helmers, 1967; Wettstein, 1967; Dormling et al., 1968).
RESULTS WITH DIFFERENT SPECIES
Recent research into stem characteristics has mainly concerned some fast-growing conifer species of high economic value. However. information on slower growing softwoods in the temperate regions and on hardwoods in all regions, but especially tropical and subtropical areas, is accumulating. Information on variation in plantations and natural stands and the degree of inheritance of stem properties is particularly needed.
Some results recorded from experiments specifically designed for studies on genetic variation, phenotypic and genotypic correlations and heritabilities may illustrate the kind and amount of information which has been obtained in such plantations. Assessments were made on randomly selected parent trees and young progenies from one to seven years old.
In Pinus taeda L., one of the most intensively studied species in this respect, a moderate heritability of straightness was demonstrated. Inheritance seemed to be relatively high for crook, but sweep was moderately to weakly inherited. Highly significant differences between progenies were indicated in progeny tests with open pollinated and control-pollinated progenies. High genetic components of variance were suggested for these traits. A moderate, positive parent-offspring correlation and also a moderately strong juvenile-mature relationship were demonstrated (Goddard and Strickland, 1964; Woessner, 1965; Shelbourne, 1966; Stonecypher, 1966).
The same degree of inheritance of stem straightness was indicated in progeny and clonal plantations of Pinus elliottii Engelm., P. palustris Mill., and P. radiata D. Don. A high general and specific combining ability for crookedness was evident in some cases (Gansel, 1965; Nikles, 1965; Nikles and Smith, 1969; Shelbourne, 1969). Similar evidence for a slight or moderately strong inheritance of stem features is indicated in other conifers, too.
Stem taper and ability to self-prune are genetically controlled features. According to Georgi and Schröck (1967), significant differences in taper occurred between one-parent progenies of different origins in a 20-year-old test plantation of P. sylvestris L. Barber (1964) estimated the heritability for inherent pruning ability ranging from 0.50 to 0.64 in a progeny test of Pinus elliottii at eight years of age. He recorded a low but significant correlation between the height of natural pruning and both the second and third-year height growth (r = 0.18 and 0.16 respectively). In a provenance test with Larix decidua L. great differences between provenances indicated a fairly strong inheritance of the ability to self prune (Vyskot, l966).
It is well known that branching characteristics are very much influenced by environmental factors, as are growth and other morphological traits. However, there is much evidence for genetic control of these properties too.
Branch diameter, which is closely correlated with branch length, varies with tree diameter, but heritability estimates indicate a moderate to weak inheritance. Branch angle is more strongly genetically controlled. Highly significant differences between progenies are recorded in progeny plantations and clonal tests, and a great part of the variation in this character is ascribed to genetic factors (Johnsson, 1965; Gansel, 1965; Strickland and Goddard, 1965; Ehrenberg, 1966).
The tendency to develop ramicorns or very acute angled large branches is supposed to be genetically controlled. The number of branches per whorl and the number of whorls are fairly weakly inherited, but quite strong genetic control is reported from very young material of Pseudotsuga menziesii (Sziklai, 1964). Correlations between branching characteristics are reported in most cases studied. For instance, negative relationships were shown between branch angle and size, and number of branches per whorl and branch size (Woessner, 1965).
Recent research in hardwoods shows great variation in each characteristic studied. High estimates of heritability for number of branches were shown for Populus deltoides (Wilcox and Farmer, 1967). Significant differences between materials in respect of stem straightness, branching characteristics and natural pruning were recorded in Populus species by Einspahr and Benson (1964) and Benson and Einspahr (1967) and in Bombax ceiba L. by Venkatesch (1969). Highly significant differences in branch angle size among trees within stands and significant differences among stands within areas were reported by Kellison (1967) in Liriodendron tulipifera. Epicormic branching was reported to be a problem in Liquidambar styraciflua (Kormanik and Brown, 1967). This trait was mainly influenced by environmental factors, but there were some indications that- it might be genetically controlled too.
In Dalbergia sissoo Roxb. stem straightness is reported to be under considerable genetic control (Vidakovic and Ahsan, 1969). There is a growing interest in hardwoods in the southern United States. A large research programme has been elaborated for studies of the species to be used in forestry (North Carolina State University, 1965-1968; Dorman, 1965). Investigations on geographical variations in some of the species have been made, and trees have been selected for future use in the breeding programme. Eucalyptus is a large and complex genus now generally planted on large areas outside its natural range. Inter- and intraspecific variations of growth, volume production, wood properties, phenological traits, and adaptation of the most valuable species are being studied (for references see Larsen and Cromer, l967), but little is known about variation and inheritance of the features.
Most of tile earlier experiments were not properly designed for estimating phenotypical and genotypical correlations, variance, and heritabilities. Neither did they render reliable information on the variation of the stem and branch characteristics in wild or in plantations. However, results obtained from earlier studies are mainly confirmed by recent results in mature stands and in young test plantations.
Early tests have given valuable information about the later development of stem quality. Results of such tests are reported, for instance, in young plantations of Pinus sylvestris (Bolland, 1967) and Pinus taeda (Goddard and Strickland, 1964; Shelbourne, 1966). A fair correlation of stem and branching characteristics in young seedlings with the same traits in the trees some years later was revealed. This indicates that it may be possible to evaluate mature stem features at an early age.
Furthermore, results brought out by progeny and clonal tests, by studies of selected mature trees, and by estimates of parent-progeny relationships indicate high possibilities for obtaining substantial gains from selection. This was discussed by Barber (1964), Nikles (1965), Nikles and Smith (1969), Squillace (1965, 1967), Woessner (1965), North Carolina State University (1966-68), Dyson (1969), Shelbourne (1969), Vidakovic and Ahsan (1969).
GENOTYPE- ENVIRONMENT INTERACTION
The influence of silvicultural factors on stem and branching characteristics was long ago recognized and documented. Site, cultivation, spacing, thinning, fertilizing and artificial pruning all affect stem quality. Changes of these factors add conspicuously to the variation of the characteristics. Change with site and climate is well demonstrated in numerous provenance tests and in progeny tests at different localities. However, test plantations specifically designed for evaluating the response of individual clones or progenies to changes in space and soil conditions are rare. Some information may be obtained from seed orchards and clonal tests, where the same clones are included in several plantations.
In a study of clones of Pinus sylvestris (Johnsson, 1965) planted on rich clay soil and on poor sandy soil at three different localities in southern Sweden, the expected variation between clones was exhibited in each test. The individual clones changed with site with respect to branch characteristics, but the ranking of the clones in these features seemed to be about the same in all three test plantations. Clones with inherently narrow crowns remained relatively narrow crowned. The response to the changing environment went in the same direction for all genotypes investigated. The branch angle was not affected by differences in soil and climate, as was shown by the nonvarying clonal means. According to Keiding and Olsen (1965), who studied two clones of Larix leptolepis planted on clayey and sandy soils at two localities, a fairly weak influence of the environmental factors on stem form was. revealed, and a very weak ortet-ramet relationship was demonstrated.
In a clonal test with Pinus sylvestris, designed specifically for testing the genotypic response to different spacing and fertilization, a similar reaction in branch angle of four clones was indicated (Andersson and Tamm, 1968). The same difference between clones in branch angle remained even after different spacing and fertilization treatments. The trees were young, four to five years from grafting, and were planted at two different spacings (1.3 x 1.3 m and 1.95 x 1.95 m). Gansel (1965) found some effects of irrigation on the formation of crooks in slash pine. In a six- to eight-year-old clonal plantation the trees in the irrigated plots showed an increased number of crooks per foot and a higher index of degree of crook compared to those on the nonirrigated plots. The results referred to above illustrate the reactions to be expected. Besides various clonal responses, even individual characteristics within a tree may differ in their response to the varying environment. The genotype-environment interactions are complicated, and much more knowledge of their nature and of the methods and techniques needed to obtain the necessary information is required. New silvicultural methods will continuously be tested and used in forestry, and must be considered when improvement programmes are being drawn up (cf. Webb and McAlpine, 1967).
Considering the importance of improved stem and branch characteristics for the production of improved wood quality, weight should be given to these properties in breeding programmes and in studies of forest tree genetics.
The inheritance of stem and branch characters has been shown to be strong, moderately strong or weak, depending on which property is studied, but genetic improvement seems to be feasible in most.
The influence of environmental factors and the effect of silvicultural treatments have been shown to vary with characteristics, with age, and with site, etc. More research is needed in this field to improve the evaluation of promising exotics and provenances.
The need for information on genotype-environment interaction has been intensively felt in recent years, when comprehensive programmes have been designed and carried out all over the world for testing and use of exotics and nonindigenous provenances. For instance, information and research projects are badly needed on Eucalyptus species now widely planted outside their natural range. The same is true for many American conifers. When transferred to other geographical areas, these species compete more or less successfully with native species on good sites or replace indigenous species in areas of unsatisfactory growth and development. In forestry, the trend of introducing exotic species or provenances is not new, but demands and needs for new material have greatly increased and accelerated recently. Problems associated with this part of forest tree breeding should be given high priority.
Closely connected with the choice of proper species and provenances is the selection of superior trees in wild populations and in plantations. The genetics of quantitative characters and population genetics should be intensively studied, as they are essential as a basis for selection.
When choosing the characteristics for which to breed, consideration should be given to the relative degree of inheritance of a character. Results obtained from clonal and progeny testing indicate that improvement can be more easily obtained in stem straightness and branch angle than in, for instance, branch diameter, which is more influenced by changes in the environment. Thus, a higher selection gain will be expected if the characteristics to be improved are strongly genetically controlled.
Clonal tests specifically designed for study of the effects of silvicultural treatments should be planted on a large scale. Progeny tests specifically planned for estimation of inheritance of stem quality and of genetic correlations between the desirable features should be established. In both cases the test plantations should be planted as near as possible to the parent trees. The validity of the estimates of the parent-progeny relationships will be increased by reducing the differences in growing conditions between the mature trees and the young progenies.
Conservation of valuable stands and trees and exploration of unknown forests are of great importance for selection purposes, for hybridization, and for preventing loss of genetic variability, which is the basis for selection. Methods used or suggested for the conservation of genes are manifold and in most cases they are expensive or difficult to handle. Natural stands could be protected in national parks or in reservation areas, and new stands could be raised intermittently from seeds of the trees included, and planted in the same areas. Individual trees could be propagated vegetatively and grown in clone banks or in botanical gardens. There is also the possibility of freezing seeds or pollen and storing them in seed or pollen banks. Results from tissue cultures indicate that plant tissues can be kept growing on artificial substrates for long periods and can even develop into normal plants under such conditions (Durzan and Steward, 1968). Methods to choose for future use should be further studied.
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