The testing programme

During the initial work, the need for progeny-testing was soon recognised and a series of four small field tests of Sitka spruce families was established in 1953-4. Although these included less than 20 families, they were to become important in later years when the relationship between early growth and mid-rotation volume was established (Gill, 1987).

The next tests planted comprised the series of four Scots pine tests already discussed, planted in 1957. Whilst these did provide early indications of breeding values and heritability, annual planting programmes did not begin until 1966-7 for all the major species. The capacity for large scale plant production and progeny test planting grew from this time and an intensive programme ran steadily over the next 25 years.

When the primary concentration on Sitka spruce together with major testing programmes for Scots pine, lodgepole pine, hybrid larch and occasionally Corsican pine are taken into account, annual planting programmes of up to 30 hectares of conifer progeny tests involving up to 500 different families took place.

The data in the table below demonstrate the scale of the field progeny testing work during the period when it reached its peak in the mid 1980s.

Summary of average annual work related to field progeny testing, 1982-5

Tree breeders strive to obtain estimates of breeding values as quickly as possible from as small a number of trees per family as possible in order to reduce the overall cost of a breeding programme. Forest stage progeny-testing is by far the most expensive stage in terms of manpower and resources. Knowledge regarding the optimum progeny test design, the number of sites within Britain required to test a parent tree adequately and the optimum selection ages, was scarce in the early years.

Design of progeny-tests

Randomised block designs were generally used for progeny testing, but plot size varied initially between 8-tree line plots and 16-tree (4×4) square plots with between two and six replications. In 1968 an experiment was established with Sitka spruce to compare a range of linear and square plot sizes. This indicated that plot sizes of up to 10 trees were the most efficient, and accordingly an experimental design within sites of eight tree row plots with five replications was used routinely from 1977 (Johnstone and Samuel, 1978). This requirement for 40 trees per family per site on three sites (120 trees in total) became the standard design for all subsequent progeny tests from which calculation of breeding values of original plus-trees (parent trees) was intended.

Suitable progeny test sites were selected by foresters based at one of the research outstations. Site confirmation would always be required by the relevant project leader before a site could be prepared for planting. In addition to the outstations operated by the branch, the services of other outstations operated by Silviculture (North and South) Branches were also used.

Bare-land sites which had not previously been planted with trees were fairly readily obtainable until the early 1980s when areas of restocking had to be increasingly considered. It was clear that a more sensitive experimental design was required to compensate for the increased heterogeneity of a restocking site. Whereas it seemed quite possible to find bare-land sites with superficial homogeneity over the dimensions of a typical replicate (normally around 1600 m²), the random location of stumps and accumulations of brash, together with variable type of cultivation, meant that homogeneous units were vastly reduced in size on restocking sites.

The Generalised Lattice Design (also referred to as the ‘Alpha Design’) (Patterson and Williams, 1976), which sub-divided each replicate into incomplete blocks typically of just five or six plots (total dimensions 160 to 200 m²), was introduced in 1983. Plot size remained at 8-tree row plots and there were still five complete replications.

Number of test-sites

Initial trials were planted on a wide range of sites (eight for Sitka spruce, five to six for pines) felt to be representative of major planting zones for the different species within Great Britain. Combined analysis across all planting sites of height data collected six years after planting in some of the first Scots pine and Sitka spruce progeny tests indicated broad similarities between groups of sites and the number of testing sites was subsequently reduced to the three representing the most contrasting site types. For example, with Sitka spruce these were:

• Those in which the growing season is short and a cold climate additionally influences growth (sites in North-East Scotland)
• Those in which the growing season is long, low temperature rarely limits growth and frost damage is minimal (characterised by South Wales and South-West England)
• Intermediate sites in which these extremes are less prominent (the borders between Scotland and England have been commonly used).

With appropriate modification for particular species, representative sites based on these criteria have been used since 1975.

Assessment of field experiments

During the early phase of progeny-testing, a fairly intensive schedule of field assessments was used. This followed those already established in the provenance testing programme. Height was often measured at planting and at the end of the 1^st, 3^rd, 6^th and 10^th growing seasons. After this it became more difficult and expensive to measure total height of each tree, and diameter at breast height (DBH) was commonly adopted on a five yearly basis. Annual measurement of height in the Sitka spruce population study (see below) indicated that there were many changes in rank in progeny performance up to the 6th year after planting but that after this age, rankings became more consistent. Gill (1987), in a study of data from one of the original Sitka spruce progeny tests planted in 1953, found that height at 6^th year was strongly correlated with volume at 27 years. Much time and effort was therefore saved by making a 6^th year height assessment the first and definitive measure of growth rate in progeny tests. Backwards selection of plus trees for the breeding population was therefore based on selection at this age with some minor adjustments based on DBH at 10 or 15 years.

The data from the population study also indicated that there were very poor correlations between nursery height and later field performance. This led to the abandonment of routine nursery height measurement in the late 1970s.

Following breeding value estimation through progeny testing, re-selection of the best parents, based on the performance of their progeny, was carried out to create a breeding population. The very best of the breeding population would be selected to constitute the production populations which would inter-mate to give the improved propagules for the next generation of forests. The work in these areas will now be considered separately for each species.

A summary of progress in the main species appears in the table below. This table also refers to the seed origin or provenance testing programme which began as early as 1926 but only became part of the remit of the branch in 1987.

Summary of the field-trial programme for progeny testing and provenance research

Next: Production of improved material
Previous: The early years

50 years of tree breeding – Field progeny – Douglas fir

• Intermittent testing in early years
• Renewed interest in 1990s

Intermittent testing in early years

Like hybrid larch, Douglas fir breeding has had periods of rising and falling interest. Plus trees were selected in the 1950s and progeny tests were established intermittently from then until the first half of the 1970s; a total of 26 tests in 10 series. It was then deemed that insufficient planting was taking place to justify a costly breeding programme.

David Douglas’ original seed collection in 1826 was quite small and the trees derived from it yielded the first collections of British seed which was widely distributed among landowners. Later introductions in the 1850s, together with Douglas’ material, formed the basis of some of the early plus tree selections. Many of these selections were therefore closely related and clear signs of inbreeding depression often appeared in the early progeny tests, notably among full-sib crosses.

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Renewed interest in 1990s

Interest was renewed in the early 1990s when 12 experiments containing approximately 350 open-pollinated families from parent trees selected in the states of Washington and Oregon (USA) were established through a partnership with a number of other European countries. Data are still being collected from these relatively recent experiments. The objective is to analyse all the DF progeny trials for height and stem-straightness and construct breeding and production populations. It is not envisaged that breeding will proceed into a second generation.

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What’s of interest

These pages review the work performed by the Forestry Commission and Forest Research on tree improvement following the 50^th anniversary of its establishment which passed in 1998. The genetic background describes the scientific procedures of tree breeding and the technical terms used in the remaining pages. All species are referred to by their common name in English.

50 years of tree breeding – Field progeny – Hybrid larch

• ‘Hybrid polycross’ progeny testing
• Difficulties with controlled pollinations
• Planting of progeny tests between 1959 and 1994
• Some results

‘Hybrid polycross’ progeny testing

The production of hybrid material as the end product of a breeding programme has meant that progeny testing has always involved the testing of hybrid larch (HL) families derived from European (EL) and Japanese larch (JL) plus trees. All progeny test material has therefore to be produced from artificial pollination of plus trees of one species with a mixture of pollens of the other to ensure pure hybrid seed for testing. Since plus trees were often mature at the time of selection, this type of ‘hybrid polycross’ progeny testing had to be delayed until grafts in a clone bank were ready to flower. Unlike other species in which open-pollinated material could be used, there was thus a minimum delay of about 10 years between plus tree selection and progeny testing.

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Difficulties with controlled pollinations

It is difficult to carry out controlled pollinations with the larches since they flower early in spring when damaging frosts and gales may still occur, the number of full seed per cone is often less than 10 and there are problems associated with pollen storage and assessments of pollen viability.

Consequently, sowing of progeny tests is often delayed while sufficient seed is accumulated over several years. Progress was inevitably slow. Each year, in which pollinations were carried out in clone banks and untested clonal seed orchards, involving hard work in inhospitable outside conditions, the effort was often only to be rewarded with a poor harvest due to any combination of the problems noted above.

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Planting of progeny tests between 1959 and 1994

Between 1959 and 1994 a total of 71 larch progeny tests were planted. The fact that these constitute 43 series reflects that replication across sites was often less than three (many on only a single site) due to shortage of seed. This 36-year period includes an interruption between 1964 and 1973 when the breeding programme was put on hold for economic reasons. This was due to a reduction in the amount of commercial larch planting, but it was started again as the demand increased for an aesthetically contrasting species to Sitka spruce and pine monocultures.

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Some results

Despite the difficulties associated with the production of hybrid larch families, some outstanding hybrid full-sib crosses have been identified offering considerable gains over JL or EL of seed stand origins. One of the early superior specific family combinations was EL6 x JL42 which latterly appeared as a control in HL progeny tests as a measure of the standard required by new, high-performing hybrid combinations.

A small number of selected EL and JL parents have now been identified and a small breeding population has been constructed. The possibilities of mixed-species clonal seed orchards or the mass vegetative multiplication of hybrid families made by artificial pollination exist for producing a supply of improved hybrid larch.

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What’s of interest

These pages review the work performed by the Forestry Commission and Forest Research on tree improvement following the 50^th anniversary of its establishment which passed in 1998. The genetic background describes the scientific procedures of tree breeding and the technical terms used in the remaining pages. All species are referred to by their common name in English.

50 years of tree breeding – Field progeny – Lodgepole pine

• Early testing
• Planting of hybrids
• Loss of commercial value

Early testing

The first lodgepole pine progeny tests were established in 1964. Initially, as with other species, the strategy was to collect open-pollinated seed from plus trees and evaluate their breeding values using replicated progeny tests. Among the exotic conifers used in Britain, lodgepole pine is perhaps the species with the widest range of variation due to origin within its natural range. However, because the early results of provenance testing had highlighted the fast early growth rates of origins from the coastal regions of Washington and Oregon (normally referred to as south coastal origins), plus tree selection within these sources was initially favoured. Early data revealed that such parent trees still had very poor genetic quality for straightness and basal bow when planted in peat sites in the north of Scotland.

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Planting of hybrids

In the late 1960s some hybrids between widely separated lodgepole pine provenances were planted, notably at Elchies (Moray) and Shin (Dornoch). Some hybrids in these experiments showed particular promise and a detailed plan was conceived to make more widely separated inter-provenance crosses, some of which would perhaps combine in the progeny the vigour of south coastal origins with the better stem-form of north coastal or interior types.

This course of action was also advocated in a report from a visiting New Zealand tree breeder (Shelbourne, 1974), and over more than the next decade large annual programmes following these aims were carried out. Full-sib families were created involving combinations thought to have high potential both within and between individual provenances. For this purpose, large numbers of good quality phenotypes were selected within thicket stage stands and controlled pollinations were carried out on site.

The resulting full-sib progeny tests were established over 3 sites. However, this type of mating design can give no indication of the breeding value of the parent trees and thus offers no information upon which to base backward selection. Since lodgepole pine is a species which flowers 5-10 years from seed, the same progeny-test material was planted out in parallel to the progeny tests as seedling seed orchards. The aim was to rogue these by removing poor families based on progeny test results. A range of wide inter-provenance hybrids was produced together with a smaller number of within-provenance hybrids. Altogether 87 progeny tests (31 series) were planted; approximately 1,200 full-sib families over 3 sites covering a total of nearly 60 hectares.

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Loss of commercial value

Since the time when concentrated work on this species was taking place, its position in commercial forestry has fallen from that of the second most important conifer to a nurse species (for which the bushy Alaskan type is preferred) with a very limited use as a pure crop (Skeena River origin). Many of the less relevant crosses from the breeding work have therefore been abandoned, but more relevant orchards have been rogued and are now in production. No more breeding work was envisaged when the programme was formally closed following a report by Lee (1993c).

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What’s of interest

These pages review the work performed by the Forestry Commission and Forest Research on tree improvement following the 50^th anniversary of its establishment which passed in 1998. The genetic background describes the scientific procedures of tree breeding and the technical terms used in the remaining pages. All species are referred to by their common name in English.

50 years of tree breeding – Field progeny – Scots pine

• Plot design
• Progeny testing
• Breeding population

Plot design

Initially, 16 (4 x 4) plant plots designs with 3 replications had been used for Scots pine testing. This design was closely linked to the system of nursery production used for material to be planted in progeny tests (Faulkner, 1967 and see Chapter 6). Later during the 1970s, the design was modified to that in use for Sitka spruce. Row plots of 8 trees, 5 replications and 3 test-sites were then commonly used. Selection was always based on 6 or 10-year height and stem straightness; wood density is sufficiently high in Scots pine not to be considered as a selection criterion.

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Progeny testing

Progeny testing in Scots pine started a little earlier than in Sitka spruce, and by the end of the 1960s nearly 50 progeny tests comprising 16 series were established in the forest. These contained open-pollinated families collected either from the standing tree in the forest or from grafted parents in a clone bank, or from families derived from artificial pollination using pollen mixtures of known composition. A further 64 Scots pine half-sib progeny tests (19 series) were planted in the 1970s and 21 in the first four years of the 1980s (7 series). By spring 1984, the last progeny test to estimate the breeding values of parent plus-trees was planted. One or two full-sib progeny tests had been established during the half-sib testing phase and in 1987, 3 half-diallels were planted with the objective of investigating the relative importance of additive and non-additive genetic variance; data from these are not yet available.

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Breeding population

As progeny-test data were collected, a rudimentary breeding population was constructed based on height and stem straightness. The best of the breeding population at any given time was used as a production population upon which grafting of clones for new tested clonal seed orchards was based.

In 1997, a complete analysis of all Scots pine progeny tests designed to estimate breeding values was carried out and 226 plus trees were re-selected for any new breeding population envisaged in the future (Lee, 1997b). It is not expected that Scots pine breeding will continue past this first generation. It is uncertain where the relative emphasis in gain should be placed between height and stem straightness in the breeding of Scots pine. There is a slight negative correlation between the two traits (r = -0.3). In order to give flexibility for the future, three alternative breeding objectives were developed, each involving a population of 200 re-selections. Since there was found to be great overlap between the constituents of these populations it was decided to retain a single breeding population of 226 parents with which any of the 3 objectives could be pursued.

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What’s of interest

These pages review the work performed by the Forestry Commission and Forest Research on tree improvement following the 50^th anniversary of its establishment which passed in 1998. The genetic background describes the scientific procedures of tree breeding and the technical terms used in the remaining pages. All species are referred to by their common name in English.

50 years of tree breeding – Field progeny – Sitka spruce

• Estimating breeding values
• Estimation of heritability
• Estimating non-additive effects using full-sib families

Estimating breeding values

The first open-pollinated Sitka spruce progeny-test series planted with the objectives of ranking parent trees for genetic quality relative to unimproved material of the species was planted over eight sites in 1967. Until the late 1970s, families in tests were derived from open-pollinated seed collected from parent trees in situ and stored over a number of years. When all plus trees from which such seed had been obtained had been put into test, a large number, from which no such seed was available, still remained untested. All selections had been systematically archived in clone banks since 1966 and seed of these remaining untested clones was obtained from artificial pollination carried out in the banks.

A mixture of pollen from around 20 known trees was used as the male parental contribution and efforts were made to retain consistency in the composition of this mixture across seasons. These families are referred to as ‘polycrossed’ and in principle should give a more reliable prediction of breeding values than uncontrolled open-pollinated families. From the late 1970s onwards, therefore, progeny testing was based on polycrossed families.

Data for height at six years from planting started to become available in the mid 1970s, and from this time onwards a breeding population began to be identified. Generally, to qualify for the breeding population, the mean performance across all test-sites of the half-sib progeny collected from a plus-tree had to exceed the Queen Charlotte Island (QCI) control value for 6-year height by 15%. The QCI origin of Sitka spruce is recognised as the most appropriate seed origin for general use across a wide variety of site types in Britain. No similar threshold values were placed on the straightness score, which was assessed according to an objective one (best) to six (worst) system. As a cost saving measure, straightness assessments were always delayed until analysis of height or diameter assessments had revealed the most vigorous families, which were then assessed for straightness (to the exclusion of less vigorous families) together with the controls.

To start with, only height and stem straightness assessments were carried out in progeny tests. Wood density was not measured until around 1986 after which the Pilodyn^® was used. This machine measures wood density indirectly as the distance penetration at breast height of a blunt pin fired into the tree with a fixed force of six joules. Initially, as with straightness, only the most vigorous trees were assessed for wood density in this way. However, as it became apparent that there was a strong negative correlation between wood density and 15-year diameter, the practice of measuring all families in test was adopted.

When plus trees were selected, it was assumed that they were all of QCI origin although this was often not confirmed by forest records. No plus-trees of Washington/Oregon, or Alaskan origin were ever deliberately selected during these early selection years although this policy did change in the mid to late 1980s (see later).

Following analysis of data from earlier progeny tests planted on up to eight sites, family mean performance was regressed against site mean. In this way it was possible to investigate how stable the performance of each family was across sites and to use this information to allocate parents trees to breeding populations (Johnstone and Samuel, 1978).

For example, families which performed consistently well on all sites from Wales to North Scotland were assumed to derive from plus trees of QCI origin which were re-selected for the General Breeding Population (GBP). In contrast, if performance was above average on a site in Wales, average on a site in the Borders, and below average on a site in North Scotland, the plus tree was assumed to derive from an origin further south than QCI – perhaps Washington or Oregon and was re-selected for the Southern Breeding Population (SBP). Similarly, above-average performance in the North of Scotland was consistent with an origin further north than QCI – perhaps Alaskan – and the plus tree would be re-selected for the Northern Breeding Population (NBP).

The performance of the large majority of progeny was consistent with parent trees of QCI origin and the GBP soon became the main breeding population upon which most breeding, testing and production was focused. Following a re-analysis of all progeny-test data for 15-year diameter, stem straightness and wood density by Lee (1995), the top 240 parent plus trees were re-selected for the GBP. Selection was carried out using a multi-trait index selection model with the objective of maximising diameter and stem straightness whilst preventing an overall decrease in wood density.

Progress with the Northern and Southern Breeding Populations was originally slower. During the early years of progeny testing, only approximately 20 plus trees had been re-selected for each population based on progeny performance. This changed during 1971-3 when 209 plus-trees thought to be of a more northern origin were selected. Following progeny testing of these, a rudimentary NBP was formed in the late 1980s. Progeny test data for 207 further plus trees selected within material of the same origin have yet to be collected and analysed, but are likely to lead to further re-selections for the NBP.

Similarly, the size of the SBP is expected to increase significantly following analysis of data collected from progeny tests planted in 1990/91 with open-pollinated progeny collected from around 350 plus trees thought to be of Washington origin. Some of these plus-trees had been selected in North Scotland and others in Southern Ireland, but the majority had been selected by Rayonier Forestry in Washington State (USA).

In spring of 1993 it was decided that the first generation of Sitka spruce progeny testing should come to an end. Parent trees, which did not have progeny in test by this time would be discarded; there remained in fact only 22. Between 1967 and 1993 nearly 300 Sitka spruce progeny tests had been established (23 in the 1960s, 122 in the 1970s and 146 in the 1980s). Nearly 100 different series of experiments (involving the same families planted at a range of sites) had been planted; an average of 12 progeny tests or four series per year. Assuming each series contained 50 families, this equated to 200 families per year. The British genetic improvement programme for Sitka spruce was the largest in the world, the improvement programme had been active for 30 years and yet, by the early 1990s, the amount of improved material reaching the forest manager was minimal.

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Estimation of heritability

In spring 1972 a ‘Population Study’ was established over 3 forest sites. One-hundred-and-fifty parent trees which were all flowering had originally been selected in a stand of known QCI origin planted in 1935 and growing in South Strome forest (Fort Augustus) (Samuel and Johnstone, 1979). Selection aimed to represent all dominance classes of trees in the population and included 6 plus trees, 48 dominants, 61 co-dominants and 35 sub-dominants.

Following losses at germination and in the nursery, a maximum of 134 families were planted at one site with different sets of 125 families planted at two other sites; 116 families were common to all three sites. The series was designed to give estimates of additive genetic variation to at least half rotation-age. There were three complete replications at each forest site and plot size varied from 7 x 7 at Garcrogo (Castle Douglas) through 6 x 6 at Wark (Kielder) to 5 x 5 at Tywi (Llanymddyfri). Height, diameter, straightness and, more recently, wood density have been measured at regular intervals in this series of experiments. Analysis of height data up to 6 years from planting was reported by Samuel and Johnstone (1979).

The experiments were intensively studied in the mid 1990s and all height, diameter and wood density data from 1 to 23-years at the Garcrogo site analysed in depth (Lee, 1997a).

Wood density was found to be under strong control of additive genetic variance. Single tree and family heritabilities varied from 0.85 and 0.95 at just nine years old to 0.33 and 0.62 at 23 years old.

Family heritabilities for height and diameter vary little over the first half of a Sitka spruce rotation; commonly around 0.60. Early single tree heritabilities for height (around 0.35) seemed to exceed those for later diameter (0.15) and are clearly under more modest genetic control than wood density.

The optimum selection age for height was 5 years after planting, confirming earlier work (Gill, 1987). More significantly it was found that the optimum selection age for wood density could be lowered to just 9-years from planting and confirmed the strong negative genetic correlation between wood density and vigour (r= -0.70).

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Estimating non-additive effects using full-sib families

The first attempt to investigate the amount of non-additive genetic variation within a sample Sitka spruce population was carried out in 1968. All possible pair-crosses were carried out between seven standing trees (49 crosses in total) growing in a small block of Sitka spruce in Roseisle Forest (Moray). These comprise a full diallel crossing design, more commonly used in crop plants at that time as a method of quantifying the importance of several forms of inheritance in the population represented. A small and incomplete set of diallel crosses had previously been made in larch (Matthews et al., 1960), but this was one of the first fully complete diallels ever made in a forest tree species (Samuel et al., 1972).

Trees raised from the harvested seed were planted out in 1970 and 1971 to a site within Bush nursery and Tywi Forest (Llanymddyfri), respectively. Estimation of genetic parameters for height, diameter and stem-form over a 15-year period from planting were reported by Samuel (1991). This was the first information to become available on the amount of non-additive variation in Sitka spruce, although restricted by a small sample size and from an ill-defined population. It was found that at least 50% of the genetic variance for height and stem straightness could be attributed to non-additive effects, but that diameter was under predominately additive control.

Other full-sib crosses were planted from 1985 onwards but these lacked a clearly defined mating design. Although providing information on full-sib families which outperformed expectation based on parental breeding values, they contributed little to the estimation on non-additive variance.

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What’s of interest

These pages review the work performed by the Forestry Commission and Forest Research on tree improvement following the 50^th anniversary of its establishment which passed in 1998. The genetic background describes the scientific procedures of tree breeding and the technical terms used in the remaining pages. All species are referred to by their common name in English.