1. Breeding for growth and yield

D.G. NIKLES

D. GARTH NIKLES is office r -in - charge, Tree Breeding Section, Forest Research Station, Beerwah, Queensland, Australia. This paper is published with the permission of the conservator, Department of Forestry, Brisbane, Australia.

SELECTION OF THE more valuable species and geographic populations of crop plants and domestic animals has already been made, and current breeding practice mainly involves improving well-known varieties. By contrast, exploitation and reproduction of commercial forests mainly involve wild species, with only recent and minor use of strains selected for high yield.

Genetic selection to improve the yield of forest trees can be made at the levels of species, variety, provenance and individual tree.

Such selection may be effective because native and naturalized species, varieties or provenances do not always fully exploit the managed environment of a region. They are usually adapted to survive the extremes of local climatic and edaphic conditions rather than to produce the highest yields of products required by man. Furthermore, they may not produce the desired type of wood. Where species are unsatisfactory for these or other reasons, yield can often be improved by proper introduction, testing and selection of the best adapted and most productive populations of exotic trees.

Selection of individuals for high yield can exploit the variation among trees within populations. It is most appropriate when the best provenance has been determined (Dietrichson, 1969) and when a large population from it is available locally for selection.

Interspecific and interracial hybridization have been used to improve the yield of forest trees and are promising methods of breeding in some circumstances.

As a basis for the discussion of breeding for growth and yield in this paper, the components of yield from a commercial forest are described. Examples are given of the gains to be achieved by selection and by hybridization. Also problems in developing efficient programmes of breeding for high yield are discussed.

Yield and its components

The total production of dry matter in a plant system is termed biological yield but only a portion, its economic yield is used by man (Niciporovic 1954 Ovington 1965, 1968). The dynamic relationship between biological yield (Y_biol) and economic yield (Y_econ) of a crop in a specific environment can be expressed as Y_biol x HI = Y_econ where HI is the harvest index of Donald (1962). Breeding for increased Y_econ is possible if heritable variation occurs in Y_biol, or HI, or in both factors. The possibility of significant interaction of genetic and environmental components of Y_econ should be recognized.

Biological yield of a commercial forest depends on the inherent growth capacity of the genetic material being managed and on the environment in which it is cultivated. Examples of crop characteristics which may affect total organic production are leaf area and exposure, photosynthetic efficiency, relative growth rate, growth rhythm, rooting habit, ultimate height potential, health and response to plant density (Kozlowski, 1962; Black, 1966; Wareing, 1966; Dietrichson, 1969).

Harvest index, in a literal sense, is the ratio of the merchantable volume or weight of wood that can be taken from the forest to that produced therein. But this broad concept ignores the fact that only a portion of the wood harvested is recovered in the form of usable products. A specific term, net harvest index (HI_net), is a more useful one for the purposes of this paper. It is defined as the ratio of the amount of material recoverable after processing to that produced in the forest (biological yield).

The net harvest index achieved when a forest is exploited depends largely on the use of available wood, and on the efficiency of harvesting and processing. Considerable improvement of economic yield could often be obtained by greater attention to these aspects (Ovington, 1968). But net harvest index is also influenced by morphological characteristics of the trees, through their effect on type of utilization, ease of harvesting, and recovery of products. Variable net harvest indices would be expected among populations and individual trees which differed in stem straightness, branch angle, stem taper, wood quality and distribution of photosynthate among stem, branch, bark, leaf, cone and root portions. For example, the finely branched and straight variety of Pinus caribaea Mor. from Cuba (Slee and Nikles, 1968) and of Pinus pinaster Aiton from Corsica (Hopkins, 1960) must have higher net harvest indices than other varieties and races of the respective species.

Pinus species/site interaction at Beerwah, Queensland, Australia, as shown by the superior economic yield of

Pinus caribaea Mor.

compared with

P. taeda L.

on red earth residual soils and by the much poorer yield of

P. caribaea

relative to

P. taeda

on humic soils.

TABLE 1. - BIOLOGICAL COMPONENTS OF NET ECONOMIC YIELD

Yield type	Single trees		Stands
Yield component	Volume	Weight	Volume	Weight
Merchantable height	Yes	Yes	Yes	Yes
Basal area inside bark	Yes	Yes	Yes	Yes
Form factor	Yes	Yes	Yes	Yes
Stem quality factor	Yes	Yes	Yes	Yes
Wood quality factor	Yes	Yes	Yes	Yes
Basic density	No	Yes	No	Yes
Stand density factor.	No	No	Yes	Yes

The product Y_biol x HI_net may be termed net economic yield or recoverable yield. Net economic yield of a commercial forest is defined as the volume, weight or value of products obtainable from the wood produced per unit area of the forest during a rotation. Biological components of net economic yield are listed in Table 1. They are the broad criteria which could be used in direct selection for high yield.

Selecting for yield among species and provenances

IMPORTANCE OF SPECIES SELECTION

In many countries production by certain exotic species greatly surpasses that of most indigenous forest trees. For example, Wood (1962) has pointed out that in the United Kingdom introduced Pseudotsuga taxifolia (Poir.) Britt. yields in 50 years about twice the volume produced by plantations of the native Quercus robur L. in 100 years on sites of the same quality. There are numerous other examples of the advantageous use of exotic species in Europe (Edwards, 1963), South America (Barrett, 1969), Africa (Iyamabo, 1969), Australia and New Zealand.

Often several native and introduced species can be established successfully in a forest region, but there may be great differences among them in biological yield, net harvest index and demand for their produce. This occurs most frequently when there are considerable variations of environment within the region, resulting in important interactions of species and sites. For example, in the coastal lowlands of southeastern Queensland, Australia, plantations of Pinus species are being established. Several soil types can be recognized on which three satisfactory species have given different responses after 15 years (Table 2 and Figure 4).

Additional examples of species-site interactions are discussed by Jackson (1965). He points out that proper delineation of major sites and correct allocation of species thereto are an effective means of increasing yield. In fact, if a species is not well adapted to the planting site, there is danger of economic losses sooner or later due to gradual or sudden environmental stress.

This ideal concept of selecting the best-adapted and most valuable species for each major site may raise problems such as choice of species, site delineation, establishment, the management and harvesting of discontinuous blocks of different species, and the marketing of several types of produce. On the other hand, use of several species in a region may be a valuable safeguard against pests and diseases and changes of product requirements in the future.

TABLE 2. RANKING OF MEAN ANNUAL: VOLUME INCREMENTS OF PINUS SPECIES GROWN FOR 15 YEARS ON THREE SOIL TYPES IN SOUTHEASTERN QUEENSLAND

Species	Red earth residuals	Lateritic podsolics	Humic soils
P. caribaea Mor	1	1	3
P. elliottii Engelm	2	2	2
P. taeda L	3	2	1
Difference between ranks 1 and 2 (percent)	32	38	40

SOURCE: Data by D. Jermyn, Forest Research Station, Beerwah, Queensland, Australia.

IMPORTANCE OF PROVENANCE SELECTION

Genecological studies of many plant species have shown that characteristic local populations develop as a result of natural selection and other phenomena (Cooper, 1963; Grant, 1963; Heslop-Harrison, 1964; and Langlet, 1967). Abundant evidence proves that genetically different geographic populations of most wide-ranging tree species vary greatly in economic yield at individual test sites (Squillace and Silen, 1962; Langlet, 1963; Lines, 1965; Wells and Wakeley, 1966; Marsh, 1969; Wells, 1969).

In a review of the forestry literature on seed source environment interactions, Squillace (1969) has drawn attention to the frequent occurrence of important effects due to locality, site, photoperiod and cultural treatment. As an illustration of the joint effect of provenance and planting locality on performance, the rank of several populations of P. pinaster for mean annual volume increment at a number of test localities in South Africa and Western Australia are shown in Table 3. The Portuguese race is favoured for planting because of its consistent high yield, but the Corsican race, characterized by superior stem form and branching habit (higher harvest index), may be valuable for interracial hybridization.

Actual gains in volume production obtainable by provenance selection have been estimated in a 14 - year old test of 14 provenances of Pinus taeda L. at Beerwah, Queensland, Australia. There was a difference of 38 percent between best and second best populations. The best population produced twice as much volume as the worst.

TABLE 3. - RANKING OF MEAN ANNUAL VOLUME INCREMENTS OF PINUS PINASTER AITON PROVENANCES GROWN AT FOUR LOCALITIES IN SOUTH AFRICA AND FOUR IN WESTERN AUSTRALIA

Provenance	South Africa				Western Australia
	A	B	C	D	A	B	C	D
Portuguese	1	1	1	1	1	1	1	1
Landes.	2	2	2	2	2	2	2	2
Esterel.	3	3	3	3	4	4	4	3
Corsican.	3	3	3	4	3	3	3	4

SOURCE: For South Africa, Marsh (1969); for Western Australia, Hopkins (1960),

PROBLEMS OF SPECIES AND PROVENANCE SELECTION

The risk of using exotic species or provenances when it is uncertain whether they are adapted to local environments has been stressed by Jacobs (1964) and Zobel (1969)

1. They may succumb to unusual extremes of the environment.

2. Their growth rate and health may decline markedly after a good beginning due to unsuspected inadequacies of the site; the decline may not be apparent until a second rotation.

3. They may be attacked by insects or pathogens, especially if grown on marginal sites.

4. The type or quality of wood produced may be disappointing.

A dynamic, multistage programme of introduction and testing is imperative if local species are unsatisfactory. It is often impractical to test exotic species and provenances for a whole rotation before beginning establishment of commercial plantations. Many species and several provenances of each must therefore be established initially; most of them will be rejected in early or late stages of testing. A few will be advanced gradually to the status of major commercial species.

Problems involved in introducing and testing species and provenances are initial choice, early tests, field trials and effective cooperation.

The initial choice of species and provenances for introduction will be guided by a broad knowledge of the types of wood required, and of the ecology and plant associations of desirable species growing in donor regions of similar climate to that of the recipient region. The concept of matching climates should not be applied too rigidly, nor should all species of minor significance in donor regions l e ignored. Details of the choice and sampling of species for introduction are discussed by several workers (Wright, 1962; Callaham, 1964; Jacobs, 1964; Lines, 1965, 1967a, 1967b; Morandini, 1967; Iyamabo, 1969).

Short-term studies of variation in adaptive traits in wild populations and in progeny grown in a nursery, greenhouse or phytotron can help to limit the material for field study and to select test sites (Roche, 1968) Examples of traits affecting growth and yield that are amenable to such studies are growth rhythm (Roche 1968; Dietrichson, 1969), hardiness to frost and drought, (Day and Peace, 1934; van Buijtenen, 1966a; Bolotin 1969) and disease and insect resistance (Gerhold et al., 1966).

Multistage field testing, with replication in time and space, is essential to accommodate large numbers of species or provenances arid to sample environments adequately. There is much diversity of opinion regarding the desirable type and size of test plot, cultural treatments, experimental design, duration of the test, and assessment criteria. Decisions on these aspects must be made in accordance with the aims of the programme. Selected references can be recommended for guidance in planning field tests (Callaham, 1964; Leuchars, 1965; Lines, 1967a, 1967b; Morandini, 1967; and Iyamabo, 1969).

The need for international cooperation in genecological and yield studies of forest trees has been stressed many times, but much still remains to be done (FAO, 1969; Hagman, 1969; Larsen, 1969; Sarvas 1969). The aims of this cooperation should be to provide adequate seed collections, to ensure the preservation of gene pools sampled, to determine patterns of variation and their causes, and to formulate guidelines for advantageous transfers of seed. Exemplary 'work on rules for seed movement has been done for Pinus sylvestris L. in Sweden (Langlet, 1963), several exotic conifers in Britain (Lines, 1965), P. caribaea in Queensland (Slee and Nikles, 1968) and P. taeda in the United States (Wells, 1969). Analysis of variation by means of multiple regression, as described by Morgenstern and Roche (1969), is a promising new approach to the delineation of seed zones.

By determining and using the best-adapted, high-yielding races for each major site, costly errors can be avoided and this essential basis for achieving maximum genetic gain through population improvement can be established.

Selecting for yield within a base population

GAINS OBTAINABLE THROUGH INDIVIDUAL SELECTION

Several methods of utilizing selected individuals for the improvement of net economic yield in a base population will be considered in this section. They all depend on heritable variation in biological yield and net harvest index among individual trees.

Large differences of economic yield have been demonstrated at young ages among progeny lines (Ehrenberg, 1966; Eldridge, 1966; Webb and Barber, 1966; Orr-Ewing, 1967; van Buijtenen, 1968; Nikles and Smith, 1969; Snyder, 1969; Zobel et al., 1969; cf. Table 4) and among clones (Fielding and Brown, 1961; Burdon, 1968; Pawsey, 1968; Nienstaedt and King, 1969). This means that individual selection can be effective for improving yield. Substantial genetic gains have been predicted for several breeding methods on the basis of known or assumed values of population parameters and various selection intensities (Namkoong et al., 1966; Shelbourne, 1968, 1969; Snyder, 1969).

Realized gain can be estimated when progeny of selected trees are compared to a random sample of the base population. Substantial gains have been realized in some progeny tests (Table 4). Details of young tests are important not only because they often forecast what will follow, but because rapid early growth can be of value per se, for example in quick suppression of weeds. even if it is not sustained.

TABLE 4. - GENETIC GAINS IN YIELD REALIZED THROUGH PROGRESSIVELY MORE INTENSE BREEDING METHODS

Selection and mating method, reference	Species type stand¹	Age expt. From seed, (years)	No of parents: families	Type plot2	Percentage gains
					Height	Diameter	Volume
A. Phenotypic selection, wind pollination; intensity of selection not specified
1. Barber (1964)	P. elliottii (P)	8	11:11	M	7,6	4,9	-
2. Nikles (1966; 1969)	P. elliottii (P)	10	19:19	M	-	-	10,7
3. Woessner (1965)	P. taeda (W)	6	31:31	M	3,6	5,9	13,0
4. Nikles (1969)	Araucaria cunninghamii (P)	6,5	20:20	M	13,0	-	-
5. Brown (1969)	P. radiata (P)	12,5	23:23	M	-	3 10,0	-
B. Progeny-test selection, wind pollination; intensity of phenotypic selection not specified
1. Barber (1964)	P. elliottii (P)	8	4:4	M	12,4	8,9	-
2. Nikles (1969)	P. elliottii (P)	10	8:8	M	-	-	20,0
3. Woessner (1965)	P. taeda (W)	6	14:14	M	6,2	10,3	-
4. Nikles (1969)	Araucaria cunninghamii (P)	6,5	7:7	M	22	-	-
C. Phenotypic selection, polycrossing or partial single crossing; intensity of selection not specified
1. Kraus (1968)	P. elliottii (W)	6	18:18	M	4,6	-	-
	P. elliottii (W)	6	19:19	M	10,1	-	-
2. Goddard (1968)	P. elliottii (W)	6	9:9	L	-	-	4 39,4
	P. elliottii (W)	6	9:9	L	-	-	4 38,0
	P. elliottii (W)	6	15:15	L	4,3	6,1	4 11,8
3. Nikles (1969)	P. elliottii (P)	11,2	9:8	M	-	-	17,9
	P. elliottii (P)	13,5	6:8	M	-	-	22,3
	P. elliottii (P)	15	10:12	M	-	7,9	20,1
4. Thulin (1969)	P. radiata (P)	11	11:15	L	-	-	14,0
5. Nikles (1969)	Araucaria cunninghamii (P)	6,5	9:12	M	20,0	-	-
6. Pawsey (1968)	P. radiata (P)	12,5	21:21	M, L	-	-	22,2
D. Progeny-test selection, polycrossing or partial single crossing; 30 or 50 percent of families saved
1. Kraus (1968)	P. elliottii (W)	6	9:9	M	5 8,5	-	-
2. Goddard (1968)	P. elliottii (W)	6	8:8	L	-	-	4,5 21,3
3. Nikles (1969)	P. elliottii (P)	11-15	9:9	M	-	-	27,5
E. Progeny-test selection of the best full-sib families; 10 percent of families saved
3. Nikles (1969)	P. elliottii (P)	11-15	5:3	M	-	-	32,5

¹ Pinus species except as indicated; parent trees selected in plantations (P) or evils stands (W). - ² M = multiple row plot; L = line plot. - ³Basal area. - ⁴ Gain measured in forms of stone dry weight. - ⁵ Computed grills minimal since matings not restricted to select parents.

The simplest breeding procedure is mass selection; that is, use of wind-pollinated seed from superior phenotypes. It can give worthwhile gains if heritabilities are high, but if heritabilities are low, selection on the basis of progeny test results is much more effective than simple phenotypic selection (cf. Table 4: A1-4 and B1-4). When mating is restricted to selected parents, which is equivalent to conditions in a thoroughly isolated seed orchard, larger gains are obtained (compare Table 4: A2, A4 and C3, C5). After a further stage of selection is applied, in which only the parents having high general combining ability are used (equivalent to culling a seed orchard), greater gain is achieved (compare Table 4: (C1, C2, C3 and D1, D2, D3). A small additional increase in gain, compared to the time and effort that would be required to obtain it, was possible with P. elliottii by choosing for production the best 10 percent of single crosses (compare Table 4: D3 and El). These results are in general agreement with predictions of Shelbourne (1969) for Pinus radiata Don.

The strongest evidence of gains in yield through individual selection comes from some of the older progeny tests (Table 4: A5, C3 and C6). The experiments involved separate, replicated, multiple-row plots of commercial stock and progenies of parent trees selected for superior phenotype. Comparisons were for volume or basal area production (Table 4) and basic density of the wood (Pawsey, 1968; Nikles and Smith, 1969). The latest measurements were made at 12.5 to 15 years. By this time, tree canopies had been closed for about one half of the test periods. This means that most of the wood was produced under competitive conditions within progenies. Ages of the trees were more than one third of the likely rotations of the two species, P. radiata and P. elliottii. This suggests that the results may be reliable indications of the relative yields near rotation age. Basal area growth curves for several full-sib progenies of selected trees and commercial stock of P. elliottii (Figure 5) and photographs (Figure 6) indicate the selected material has established a distinct superiority. Moreover, the basic density of the wood of selected and commercial progeny was the same (Pawsey, 1968; Nikles and Smith, 1969). This means that gross economic yield, in terms of both volume and dry weight of wood, has been increased by selection.

Families from selected trees of P. elliottii and P. radiata in the progeny tests referred to above also show considerable improvement in stem straightness (Nikles, 1966; Brown, 1969; Pawsey, 1969). This improvement of stem quality will result in a higher net harvest index for most end uses. It is concluded that an increased net economic yield of about 30 percent to age 15 years may be expected from the establishment and subsequent culling of some P. elliottii and P. radiata seed orchards in Australia. Breeding programmes of this type are well worthwhile, since recent studies show that gains of the order of only 5 percent at harvest time are needed to justify the costs (Davis, 1969; Shepherd and Slee, 1969).

FIGURE 5.- Basal area growth in Queensland, Australia of P. elliottii commercial stock (C) and full-sib progenies of which 1 to 6 are significantly different from C and have average genetic gain of 17.6 percent at age 15.

1 ft² per acre = 0.2 m² per hectare.

PROBLEMS OF YIELD IMPROVEMENT WITHIN A POPULATION

Selecting individuals for high yield

Net economic yield of a given forest is obviously a very complex characteristic having many components (Table 1). Often, when a breeding programme has begun little is known about relative economic values, degrees of genetic control and interrelationships of the factors influencing yield. Choosing selection criteria and allocating relative weights to them must therefore be subjective at first. Usually a systematic schedule is developed at the beginning of a programme to aid the selection of superior phenotypes (Haley, 1960; Mergen, 1962; Campbell, 1964; Forshell, 1964; Pederick, 1968).

More efficient means than phenotypic selection are needed for identifying high-yielding genotypes. Progeny tests have often been established to compare the breeding values of parents, but the high cost is unlikely to be justified for this purpose alone (Libby et al., 1969; Toda, 1969). Selection within progeny tests can be made with increased accuracy by combining information on individual, family and parental performance (Falconer, 1960; Namkoong, 1966). Such combined selection will be used widely in tree breeding where suitable progeny tests are available.

Selection indices are especially useful where several complex and correlated traits are to be improved simultaneously. The construction of multiple-trait indices for rating an individual tree on its own performance is described by Northcott (1965), van Buijtenen (1966b), and Illy (1969). Worthwhile indices must be reliable, that is, based on precise estimates of genetic, environmental and economic parameters, but this requires large and expensive experiments (Stonecypher, 1966, 1969a). An alternative approach is suggested by Namkoong (1969) for the common situation in which the data needed are not available.

A 15-year-old full-sib progeny of two selected phenotypes of Pinus elliottii Engelm., which is producing straighter trees and 30 percent more volume per hectare than commercial stock (right) under comparable conditions at Beerwah, Queensland, Australia. - PHOTO: NIKLES

A 15-year-old full-sib progeny

Two selected phenotypes of Pinus elliottii Engelm

Integrating evaluation, selection and estimation experiments

Recurrent selection for multiple traits affecting yield requires the mating of many selected parents, since many progenies are needed to evaluate parents and their families, to measure realized genetic gain, to select superior trees for the next cycle of breeding, to estimate population parameters, and finally to produce an ordinary harvest. Mating systems and experimental designs which are efficient for all of these purposes should be employed in order to economize on effort (Stern and Hattemer, 1964; Schutz and Cockerham, 1966; Libby, 1969; Toda, 1969).

Descriptions of several mating and experimental designs that are being used in tree population improvement have been published (Johnsson, 1964; Libby, 1964, 1968, 1969; Braaten, 1965; Goddard, 1965; Burley et al., 1966; Stonecypher, 1966; Roberds et al., 1967; gingham, 1968). Mating systems vary in complexity and cost from the simple and inexpensive wind pollination to the complicated and costly diallel patterns for controlled pollinations. The designs differ greatly in utility for the various purposes mentioned above. A brief criticism of mating designs is given by Libby et al. (1969) and aspects to be considered in establishing field tests are described by Stonecypher (1967). The choice of mating and experimental designs should be made very carefully, having regard to the resources and objectives of the breeding programme and the biology of the species.

The importance of using a systematic mating scheme and well-executed field designs in a breeding programme cannot be overemphasized. It is basic to the achievement of acceptable genetic improvement.

Importance of knowing population parameters

Population parameters of prime importance to the breeder are genetic, environmental and interaction variances and covariances of specific traits (Gardner, 1963; Dudley and Moll, 1969). Although much progress has been made in improving forest trees without precise information on this knowledge should increase the efficiency and facilitate all aspects of the planning and execution of tree breeding programmes. Mention has been made already of the need for good estimates of genetic and economic parameters for the construction of reliable selection indices. The importance of genotype environmental interactions and the use of population parameters to choose between alternative breeding methods will be discussed in the following sections.

Published heritability estimates for yield and other traits were summarized by Hattemer (1963), but the methods of estimation of many of the ratios were criticized by Namkoong et al. (1966). These and additional estimates, again based mainly on young trees (Namkoong et al. 1966; Stonecypher 1966, 1969b; Stonecypher and Zobel, 1966; Squillace et al., 1967; Eldridge, 1969; Shelbourne, 1969), suggest that heritability values for wood density, tree height and diameter rank in that order. Stem straightness usually has a rather high heritability (Ehrenberg, 1969).

Knowledge of the magnitude and sign of genetic correlations among traits affecting yield is important in selection. For example, strong negative relationships among some characteristics may mean that simultaneous improvement of them is impossible, even if selection indices are used. Where strong positive correlations are known to exist for a pair of traits, selection can be concentrated on the one which is easier to assess.

Genotype- environment interaction

Genotype-environment interaction refers to dissimilar performance of two or more genetic entities when grown in different environments. Interactions may be large enough to warrant the selection of separate strains of trees for specific environments.

Squillace (1969) reviewed the literature on causes of interactions involving species, provenances, progenies, and clones. The most important environmental factors appeared to be locality, site and cultural treatment. Cultural conditions known to induce important interactions include site-preparation, fertilizer, irrigation, stocking and competition (Donald, 1963; Giertych, 1969; Squillace, 1969). Short-term screening of progeny lines for superior response to cultural treatment may not be reliable, and extended field trials may be needed (Goddard and Smith, 1969).

The most practical means of minimizing losses due to cryptic interactions is to use a broadly based genetic population for regeneration. Until specific genotype environment interactions are known in a particular breeding programme, selection should preferably be made under environmental conditions similar to those in which the progeny will be grown. Breeding plans should provide for testing progeny in several environments representing the major sites and under cultural practices recommended for current and future commercial planting.

Choosing a breeding method

A dynamic breeding programme should have two goals: early production of improved planting stock, and further gains in subsequent cycles. The various methods available for achieving these aims differ in intensity of selection, mating procedure, aids to selection, and the time until improved seed or rooted cuttings are produced. All affect the grain expected per year (Shelbourne, 1969). Since the cost of different breeding procedures varies also, a basis is needed for choosing the strategy having the greatest benefits in relation to cost for a given situation.

If satisfactory estimates of population parameters and the expense of various procedures are available, the gains expected from the use of different breeding procedures can be predicted, and the methods can be compared for cost and benefit (Namkoong et al., 1966; Shelbourne, 1968). Shelbourne (1969) recommended that the most practical method giving near-maximum gain per cycle should be used in tree breeding.

In choosing a breeding method, account must be taken of expected gain, biology of the species, human and physical resources available, scale of the reforestation scheme, and costs and benefits both in the initial and later stages of the programme. The expectation of rather large gains from efficient breeding programmes (Table 4) justifies intensive breeding work, but additional critical studies, like those of Davis (1967, 1969), of the economics of numerous alternative breeding procedures are needed to guide the choice of breeding method for a range of situations.

Increasing yield through hybridization

DEFINITIONS, OBJECTIVES AND EXAMPLES

Hybridization of forest trees means the crossing of genetically differing entities such as species, varieties, provenances, or selected strains. Rarely does it involve crossing inbred lines of cross-pollinating species because of production difficulties (Keiding, 1968; Snyder, 1968; Orr-Ewing, 1969). Derivatives of F₁ hybrids (F₂s and backcrosses) are included in the broad term "hybrids."

Two broad objectives of hybridizing forest trees are: to combine valuable traits of two or more populations, and to obtain heterosis or hybrid vigour (Duffield and Snyder, 1958).

Combinational hybridization is especially useful -in breeding for pest resistance and other multiple traits and in developing trees for sites to which a pure species is not well adapted. The needle-spot-resistant Thuja plicata Donn. x T. standishii Carr. and the well-formed, easily rooted Populus alba L. x P. grandidentata Michx. (Duffield and Snyder, 1958) are good examples. Several hybrids of Pinus promising combinations of superior yield with other desirable traits have been reported recently (Table 5).

Heterosis is exhibited when the mean yield of a hybrid exceeds that of its parents. It depends on the difference in gene frequencies between the two populations that are crossed and the level of dominance of alleles at loci affecting the traits concerned (Falconer, 1960). Hybrid vigour is often manifest in the cross Larix leptolepis Gord. x L. decidua Mill. (Wright, 1962; Rohmeder, 1963), in many poplar hybrids (Wright, 1964), and in numerous other interspecific combinations (Wright, 1962).

Interracial hybridization has been attempted less frequently than crosses between species, with encouraging but inconclusive evidence of beneficial results (Nilsson, 1963; Orr-Ewing, 1966; Woessner, 1968). Gains through heterosis and combinational effects may be expected, however, especially in species that have extended ranges covering a great diversity of habitats.

Sometimes backcross progenies are more promising than F₁ hybrids because of good performance and greater ease in the production of commercial seed (Libby, 1968; Slee, 1969). Advanced-generation hybrids may be expected to form the basis of future improved strains of forest trees.

GAINS REALIZED THROUGH HYBRIDIZATION

F₁ hybrids often perform very well over a wide range of environments, probably because of their highly heterozygous genetic composition (Allard and Bradshaw, 1964). But, when gains are properly measured in relation to the performance of the best alternative pure species available for a given site, it is often found that the occurrence of hybrid superiority is limited to environments in which neither parent is well adapted for high yield. These two points are illustrated by the fact that nine-year-old F₁ hybrids of P. elliottii and P. caribaea grow very well on both ridge and swamp sites at Beerwah, Queensland, but their superiority to the best parental species on the unfavourable swamp site is much greater than on the ridge site, where both parental species thrive (Slee, 1969).

Reliable quantitative estimates of gains realizable through hybridization are rare, either because satisfactory controls are not always included in tests of hybrids or the progenies are still too young for evaluation. Nevertheless, there are strong indications that F₁ hybrids or their derivatives may yield worthwhile gains under some circumstances. For example the nine-year old F₁ hybrids of P. elliottii and P. caribaea described by Slee (1969) exceed P. caribaea by 21 percent in height and 20 percent in girth (and exceed comparable P. elliottii by an even greater margin) when grown on a swampy site. The likely gain in net economic yield of the hybrids at age 11 years will be enhanced by a higher net harvest index due to better stem quality (Figure 7). Recent results from seven-year-old studies (Nikles, 1969) suggest that some backcross progenies grown on swampy sites may give gains at least equal to those of the F₁s. Since backcrosses are easier to produce than F1s, they are more likely to be used commercially.

PROBLEMS OF HYBRIDIZATION

There are several reports of variation in hybrid performance according to the geographic source of the parent populations and the individual trees mated (Nilsson, 1963; Lotan, 1967; Hyun 1969). Phenotypic selection among and within populations being hybridized may be effective for traits with high heritabilities, but extensive screening for parental combining ability is desirable for traits having little additive genetic variance. Performance testing and allocation of hybrids to suitable sites are problems requiring long-term observations.

TABLE 5. - EXAMPLES OF PROMISING INTERSPECIFIC HYBRIDS IN PINUS REPORTED SINCE THE PUBLICATIONS OF DUFFIELD AND SNYDER (1958) AND WRIGHT (1962)

Reference	Species	Traits contributed	Remarks
Jewell (1966)	P. taeda L.	Rapid growth rate	Promising for P. taeda sites with severe incidence of disease
	P. echinata Mill.	Resistance to Cronartium fusiforme
Derr (1966)	P. palustris Mill.	Good stem form and resistance to C. fusiforme	Promising for pine sites with severe incidence of both diseases
	P. elliottii Engelm.	Rapid growth rate and resistance to Scirrhia acicola
Griffin and Conkle (1967)	P. attenuata Lemm.	Drought resistance and frost hardiness	Hybrid vigour is expressed on some sites
	P. radiata Don.	Rapid growth
Hyun (1969)	P. taeda L.	Good stem form and rapid growth rate	Backcrosses to P. rigida and F2 produced by natural pollination of F1 trees are promising
	P. rigida Mill.	Frost hardiness
Slee (1969).	P. elliottii Engelm.	Good stem form and adaptability to wet sites	Backcross, F1 and F2 progenies have high potential value
	P. caribaea Mor.	Rapid growth rate

FIGURE 7. - Eleven-year-old F₁ hybrids of Pinus elliottii Engelm. x P. caribaea Mor.

are superior in economic yield to both

P. elliottii

and

P. Caribaea

at Beerwah, Queensland, Australia, but only when grown on swampy sites.

PHOTO: NIKLES

Mass production of hybrid trees is simple for those in which vegetative propagation can be accomplished readily. But problems of parental flowering times, low seedset, and poor viability of F₁ hybrid seed usually combine to make mass production of seed very expensive. While backcross and F₂ hybrid seed is often highly fertile and more easily produced by natural pollination, the performance of these hybrid derivatives must he tested widely to prove their superiority before their commercial use is warranted.

Since hybrid planting stock is almost always more expensive than that of a pure species, a hybrid needs to be capable of rather larger increases in economic yield than those obtainable through population improvement. It is of the greatest benefit when it can be mass produced economically, when it is higher yielding than the parent which it is a candidate to replace, and when it is superior to any alternative pure species. In view of these several requirements of hybrids, they have rather infrequent, specialized uses.

FIGURE 8. - A display dramatizing the results of selective breeding. This is a 21-year-old progeny test for oleoresin yield in slash pine. - PHOTO: UNITED STATES FOREST SERVICE, WILHITE

Integrating genetics and other approaches to improve economics yield

There is growing awareness of the separate contributions that can be made to yield improvement by breeding, intensive site preparation, fertilization, weed control, attention to initial spacing and periodic thinnings, protection, utilization standards, and planned exploitation. But the greatest returns from investment in attempts to improve quantity and quality of forest economic yield must come from an integrated programme employing genetic, environmental and managerial approaches. A combination of these several approaches offers the greatest opportunity for increasing the rate of forest production.

References

ALLARD, R. W. & BRADSHAW, A. D. 1964. Implications of genotype-environmental interactions in applied plant breeding. Crop Sci., 4: 503-508.

BARBER, J. C. 1964. Inherent variation among slash pine progenies at the Ida Cason Calloway Foundation. U.S. Forest Serv., Res. Pap. SE-10. 90 p.

BARRETT, W. H. G. 1969. Coníferas de rápido crecimiento para la República Argentina. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/2

BINGHAM, R. T. 1968. Breeding blister rust resistant western white pine. IV. Mixed pollen crosses for appraisal of general Combining ability. Silvae Genet., 17: 133-138.

BLACK, J. N. 1966. The utilisation of solar energy by forests. En Physiology in forestry, ed. by R.W.V. Palmer. Supplement to Forestry, p. 98-109.

BOLOTIN, M. 1969. Selection for drought resistance in woody species. Second World Consult. on Forrest Tree Breeding. FAO-FO-FTB-69-6/3.

BRAATEN, M. O. 1965. The union of partial diallel mating designe and incomplete block environmental designs. Consolidated Univ. of North Carolina, Institute of Statistics. Mimeog. Series No. 432. 77 p.

BROWN, A. G. 1969. Personal correspondence. Canberra, Forest Research Institute.

BURDON, R. D. 1968. Preliminary findings from a clonal replication trial. New Zealand Forest Research Institute. Silvicultural Rept 98. 18 p.

BURLEY, J., BURROWS, P. M., ARMITAGE, F. B. & BARNES R. D. 1966. Progeny test designs for Pinus patula in Rhodesia. Silvae Genet., 15(5-6): 166-173.

CALLAHAM, R. Z. 1964. Provenance research: investigation of genetic diversity associated with geography. Unasylva, 18(2-3): 40-50.

CAMPBELL, R. K. 1964. Plus tree selection. Weyerhauser Working Plan 15-2. Centralia, Washington.

COOPER, J. P. 1963. Species and population differences in climatic response. En Environmental control of plant growth, ed. by L. T. Evans. New York, Academic Press, p 381-403.

DAVIS, L. S. 1967. Cost-return relationships of tree improvement programs. Proc. 9th South. Conf. Forest Tree Improvement, p. 20-26.

DAVIS, L. S. 1969. Economic models for program evaluation. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-13/2.

DAY, W. R. &; PEACE, T. R. 1934. The experimental production and the diagnosis of frost injury on forest trees. Oxford Forest Mem., 16. 60 p.

DERR, H. J. 1966. Longleaf X slash hybrids at age 7: survival, growth, and disease suceptibility. J. For., 64(4): 236-239.

DIETRICHSON, J. 1969. Growth rhythm and yield as related to provenance, progeny and environment. Second World Consult. Forest Tree Breeding.. FAO-FO-FTB-69-2/3.

DONALD, C. M. 1962. In search of yield. J. Austr. Inst. agric. Sci., 28(3): 171-178.

DONALD, C. M. 1963. Competition among crop and pasture plants. Adv. Agron., 15: 1-118.

DUDLEY, J. W. & MOLL, R. H. 1969. Interpretation and use of estimates of heritability and genetic variances in plant breeding. Crop Sci., 9: 257-262.

DUFFIELD, J. W. & SNYDER, E. B. 1958. Benefits from hybridizing American forest trees. J. For., 56(11): 809-815.

EDWARDS, M. V. 1963. The use of exotic forest trees in increasing production with particular reference to north-western Europe. FAO/FORGEN 63-4/1.

EHRENBERG, C. E. 1966. Parent-progeny relationships in scots pine (Pinus sylvestris L.). Stud. Forest. Suec., 40:1-52.

EHRENBERG, C. E. 1969. Breeding for stem quality. Second World Consult. Forest Tree Breeding.. FAO-FO-FTB-69-3/1.

ELDRIDGE, K. G. 1966. Genetic improvement of Eucalyptus regnans by selection of parent trees. Appita, 19(6): 133-138.

ELDRIDGE, K. G. 1969. Variation in Eucalyptus regnans. Thesis, Dept. of Forestry, Austr. Nat. Univ., Canberra. (Unpublished)

FALCONER, D. S. 1960. Introduction to quantitative genetics. Edinburgh, Oliver and Boyd.

FAO. 1969. Report of the first session panel of the FAO of experts on the forest gene resources. Rome. FO: FGR/1/Report.

FIELDING, J. M. & BROWN, A. G. 1961. Tree-to-tree variations in the health and some effects of superphosphate on the growth and development of Monterey pine on a low-quality site. Australia, Commonwealth Forestry and Timb. Bur. Leaflet No. 79. 19 p.

FORSHELL, W. P. 1964. Genetic in forest practice in Sweden. Unasylva, 18(2-3): 119-127.

GARDNER, C. O. 1963. Estimates of genetic parameters in cross fertilising plants and the implications in plant breeding. En Statistical genetics and plant breeding. National Research Council, Publication No. 982, p. 225-252.

GERHOLD, H. D., SCHREINER, E. J., DERMOTT, R. C. & WINIESKI, J. A. 1966. eds. Breeding pest-resistant trees. London, Pergamon Press.

GIERTYGH, M. M. 1969. Growth as related to nutrition and competition. Second World Consult. Forest Tree Breeding.. FAO-FO-FTB-69-2/4.

GODDARD, R. E. 1965. Manual of progeny test procedures recommended for use in University of Florida Cooperative Forest Genetics Research Program. Part 1. Gainesville, Fla. 15 p. (Mimeo)

GODDARD, R. E. 1968. Personal correspondence. Gainesville, Fla., Univ. of Florida.

GODDARD, R. E. & SMITH, W. H. 1969 .Progeny testing for intensive management. Proc. 10th South. Conf. Forest Tree Improvement, p. 76-83.

GRANT, V. 1963. The origin of adaptations. New York, Columbia University Press.

GRIFFIN, J. R. & CONKLE, M. T. 1967. Early performance of knobcone x Monterey pine hybrids - on marginal timber sites. U. S. Forest Serv. Res. Note PSW-156. 10 p.

HAGMAN, M. 1969. Research programmes. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-8/1.

HALEY, C. 1960. Progress in the application of tree breeding in our planted forests. Austr. Forest., 24(1): 8-18.

HATTEMER, H. H. 1963. Estimates of heritability published in forest tree breeding research. FAO/FORGEN 63-2a/3.

HESLOP-HARRISON, J. 1964. Forty years of genecology. Adv. ecolog. Res., 2: 159-240.

HOPKINS, E. R. 1960. Variation in the growth rate and quality of Pinus pinaster Ait., in Western Australia. West Austr. Forests Dept. Bull. 67. 33 p.

HYUN, S. K. 1969. The growth performance of pitch-loblolly hybrid pine produced by different geographic races of loblolly pine in their early age. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69 7/7.

ILLY, G. 1969. Les indices de sélection, exemple du pin maritime. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-7/3.

IYAMABO, D. 1969. Growth and species introduction in African Savannah. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-216.

JACKSON, D. S. 1965. Species siting: climate, soil, and productivity. N.Z .J. :For., 10(1): 90-102.

JACOBS, M. R. 1964. The use of exotic forest trees. Austr. For., 28(3): 150-156.

JEWELL, F. F. 1966. Inheritance of rust resistance in southern pines. En Breeding pest-resistant trees, p. 107-109. London, Pergamon Press.

JOHNSSON, H. 1964. Forest tree breeding by selection. Silvae Genet., 13(1-2): 41-48.

KEIDING, H. 1968. Preliminary investigations of inbreeding and outcrossing in larch. Silvae Genet., 17: 159-164.

KOZLOWSKI, T. T., 1962. ed. Tree growth. New York, Ronald Press.

KRAUS, J. 1968. Personal correspondence. Macon, Ga., Southeast Forest Expt Station.

LANGLET, O. 1963. Practical results and current problems in provenance research in Sweden. FAO/FORGEN 63-3/1.

LANGLET, O. 1967. Regional intra-specific variousness. Proc. 14th IUFRO Congress, 3: 435-458.

LARSEN, E. V. Seed procurement for research. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-8/8.

LEUCHARS, D. 1965. The planning and practice of trials of exotic species. East African agric. for J., 31(1): 83-90.

LIBBY, W. J. 1958. The backcross hybrid Jeffrey x (Jeffrey x Coulter) pine. J. For., 56(11): 840-842.

LIBBY, W. J. 1964. Clonal selection, and an alternative seed orchard scheme. Silvae Genet., 13(1-2): 32-40.

LIBBY, W. J. 1968. Mating designs for second-generation selection in forest trees. Abstracts of presented papers, 11th annual meeting of the Western Forest Genetics Association, Corvallis, Ore., 1968.

LIBBY, W. J. 1969. Some possibilities of the clone in forest genetics research. Genetics Lectures. Vol. 1: 121-136. Genetics Institute, Oregon State Univ., Corvallis, Oregon.

LIBBY, W. J., STETTLER, R. F. & SEITZ, F. W. 1969. Forest Genetics and forest tree breeding. Ann. Rev. Genetics, 3.

LINES, R. 1965. Provenance and the supply of forest tree seed. Forestry, 59(1): 7-15.

LINES, R. 1967a . The planning and conduct of provenance experiments. World Symposium on Man-Made Forests. FAO-FO/MMF: 67-4b/4.

LINES, R. 1967b. comp. Standardisation of methods for provenance research and testing. Proc. 14th IUFRO Congress, 3: 672-713.

LOTAN, J. E. 1967. Early performance of Pinus contorta x banksiana hybrids. U.S. Forest. Serv., Res. Note. INT-57. 8 p.

MARSH, E. K. 1969. Selecting adapted races of introduced species. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-10/4.

MERGEN, F. 1962. Selection of superior forest trees. En Tree growth. ed. by T. T. Kozolwski. New York, Ronald Press, p. 327-344.

MORANDINI, R. 1967. Planning of species and provenance trials. World Symposium on Man-Made Forests. FAO-FO-/MMF: 67-4b/1.

MORGENSTERN, E. K. & ROCHE, L. 1969. Using concepts of selection to delimit seed zones. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/16.

NAMKOONG, G. 1966. Family indices for seed-orchard selection. U.S. Forest Serv. Res. Pap. NC-6. 6 p.

NAMKOONG, G. 1969. Problems of multiple-trait breeding. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-7/4.

NAMKOONG, G., SNYDER, E. B. & STONECYPHER, R. W. 1966. Heritability and gain concepts for evaluating breeding systems such as seedling orchards. Silvae Genet., 15(3): 76-84.

NICIPOROVIC, A. A. 1954. Fifteenth Timirjazev Lecture, U.S.S.R. Acad. Sci. (Precis by Black, J. N. and Watson D. J. en Field Crop. Abstr., 13: 169, 1960).

NIENSTAEDT, H. & KING, J. P. 1969. Breeding for delayed bud-break in Picea glauca (Moench) Voss: potential frost avoidance and growth gains. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/5.

NIKLES, D. G. 1966. Progeny tests of slash pine (Pinus elliottii Engelm.) in Queensland, Australia. Proc. 8th South. Conf. Forest Tree Improvement, p. 112-120.

NIKLES, D. G. 1969. Research reports on file. Beerwah, Queensland, Forest Research Station

NIKLES, D. G. & SMITH, W. J. 1969. Increased volume and dry-weight yield and improved stem straightness obtained through selection within a population of Pinus elliottii Engelm. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/20.

NILSSON, B. 1963. Intraspecific hybridisation and heterosis within Picea abies. FAO-FORGEN 63-2b/6.

NORTHCOTT, P. L. 1965. The comparison of tree qualities. For. Chron., 41(2): 215-221.

ORR-EWING, A. L. 1966. Inter-and intraspecific crosses in Douglas fir, Pseudotsuga menziesii (Mirb.) Franco. Silvae Genet., 15(4): 121-126.

ORR-EWING, A. L. 1967. A progeny test of Douglas-fir to demonstrate the importance of selection in forest practice, Brit. Columbia Forest Serv., Res. Note 43, 23 p.

ORR-EWING, A. L. 1969. Inbreeding to the S- generation in Douglas-fir. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-8/6.

OVINGTON, J. D. 1965. Organic production turnover and mineral cycling in woodlands. Biol. Rev., 40: 295-336.

OVINGTON, J. D. 1968. Considerations of forest productivity. Austr. For., 32(4): 191-204.

PAWSEY, C. K. 1968. Personal correspondence. Mount Burr, S. Austr., Forest Research Institute.

PEDERICK, L. A. 1968. Selection criteria - methods and records. Proc. Austr. Forest Res. Working Group N° 1, Canberra. Item 6.

ROBERDS, J. H., ZOBEL, B. J. & KELLISON R. C. 1967. Progeny testing in the N. C. State University-Industry tree improvement program. Proc. 14th IUFRO Congress, 3: 387-394.

ROCHE, D. 1968. The value of short term studies in Provenance research. Commonw. for Rev., 47(1): 14-26.

ROHMEDER, E. 1963. Experiments on forest tree hybrids in Bavaria from 1936 to 1962. FAO/FORGEN 63-2b/1.

SARVAS, R. 1969. Genetical adaptation of forest trees to the heat factor of the climate. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/15.

SCHUTZ, W. M. & COCKERHAM, C. C. 1966. The effect of field blocking on gain from selection. Biometrics, 22(4): 843-863.

SHELBOURNE, C. J. A. 1968. Tree breeding methods. New Zealand, Forest Research Institute. Tree Impr. Rept. 39. 31 p.

SHELBOURNE, C. J. A. 1969. Predicted genetic improvement from different breeding methods. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-8/16.

SHEPHERD, K. R. & SLEE, M. U. 1969. Tree breeding programmes and silvicultural practice in Australian man-made forests. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-9/9.

SLEE, M. U. 1969. Height and girth growth of slash - Caribbean hybrids in southern Queensland. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-2/10.

SLEE, M. U. & NIKLES, D. G. 1968. Variability of Pinus caribaea Morelet in young Queensland plantations. Ninth Brit. Commonw. Forestry Conf. Paper presented by Dept of Forestry, Brisbane. 50 p.

SNYDER, E. B. 1968. Seed yield and nursery performance of self-pollinated slash pines. For. Sci., 14: 68-74.

SNYDER, E. B. 1969. Parental selection versus half-sib family selection of longleaf pine. Proc. 10th South. Conf. Forest Tree Improvement, p. 84-88.

SQUILLACE, A. E. 1969. Field experiences of the kinds and sizes of genotype-environment interaction. Paper presented to a Working Group on Quantitative Genetics, Workshop on Forest Tree Breeding, IUFRO, Section 22. 23 p. (Mimeo)

SQUILLACE, A. E. & SILEN, R. R. 1962. Racial variation in ponderosa pine. For. Sci. Monog., 2: 1-27.

SQUILLACE, A. E., BINGHAM, R. T., NAMKOONG, G. & ROBINSON, H. F. 1967. Heritability of juvenile growth rate and expected gain from selection in western white pine. Silvae Genet., 16(1): 1-6.

STERN, K. & HATTEMER, H. H. 1964. Problems involved in some models of selection in forest tree breeding. Silvae Genet., 13(1-2): 27-31.

STONECYPHER, R. W. 1966. The loblolly pine heritability study. Internat. Paper Co. Southlands Exp. Forest. Tech. Bull. 5. 128 p.

STONECYPHER, R. W. 1967. Field testing and data handling in forest genetics research. Proc. 9th South. Conf. Forest Tree Improvement, p. 112-120.

STONECYPHER, R. W. 1969a. The whole tree concept. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-7/1.

STONECYPHER, R. W. 1969b. Personal correspondence. Bainbridge, Georgia, Internat. Paper Co.

STONECYPHER, R. W. & ZOBEL, B. J. 1966. Inheritance of specific gravity in five-year-old seedlings of loblolly pine. Tappi, 49(7): 303-305.

THULIN, I. J. 1969. Breeding of Pinus radiata through seed improvement and clonal afforestation. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-9/5.

TODA, R. 1969. Progeny evaluation problems - Cryptomeria. Second World Consult. Forest Tree Breeding. FAO-FO-FTB-69-7/5.

VAN BUIJTENEN, J. P. 1966a . Personal correspondence. College Station, Texas. Texas Forest Service.

VAN BUIJTENEN, J. P. 1966b . Testing loblolly pines (Pinus taeda) for drought resistance. Texas Forest Service. Tech. Rept 13. 15 p.

VAN BUIJTENEN, J. P. 1968. Sixteenth progress report of the Co-operative Forest Tree Improvement Program. Texas Forest Service. Circular 112. 11 p.

WAREING, P. F. 1966. The physiologists approach to tree growth. En Physiology in forestry, ed. by R.W.V. Palmer. Supplement to Forestry 1966, p. 7-18.

WEBB, C. D. & BARBER, J. C. 1966. Selection in slash pine brings marked improvement in diameter and height growth plus rust resistance. Proc. 8th South. Conf. Forest Tree Improvement, p. 67-72.

WELLS, O. O. 1969. Results of the south-wide pine seed source study through 1968-69. Proc. 10th South. Conf. forest Tree Improvement, p. 117-129.

WELLS, O. O. & WAKELEY, P. C. 1966. Geographic variation in survival, growth, and fusiform-rust infection of planted loblolly pine. For. Sci. Monog., 11: 1-40.

WOESSNER, R. A. 1965. Growth, form and disease resistance in four-year-old control and five-year-old open-pollinated progeny of loblolly pine selected for use in seed orchards. Supplement. North Carolina State Univ., School of Forestry. 34 p.

WOESSNER, R. A. 1968. A juvenile assessment of wide crosses of loblolly pine select trees indigenous to different geographical areas. North Carolina State Univ., School of Forest Resources. (Thesis).

WOOD, R. S. 1962. Some influences on practice in the espacement, thinning and pruning of coniferous plantations. Eighth Brit. Commonwealth Forest. Paper presented by Forestry Comm., London. 8 p.

WRIGHT, J. W. 1964. Genetic on forest tree improvement. Rome. FAO: (FAO Forestry and Forest Products Studies) No. 16.

WRIGHT, J. W. 1964. Hybridization between species and races. Unasylva, 18(2-3): 30-39.

ZOBEL, B. J. 1969. Best adapted seed sources. FAO-North Carolina State Forest Tree Improvement Training Centre, p. 60-62. School of Forest Resources, N.C. State Univ.

ZOBEL, B. J., KELLISON, R. & MATTHIAS, MARTHA. 1969. Genetic improvement in forest trees - growth rate and wood characteristics in young loblolly pine. Proc. 10th South. Conf. Forest Tree Improvement, p. 59-75.