Genetic improvement of corsican pine

 

Summary

Novel statistical techniques more familiar in animal breeding have been used to draw together breeding value estimations of height, stem straightness and branching quality for up to 935 original plus trees planted over a range of sites between 1966 and 1987. A population of nearly 200 plus trees has been reselected. Grafted copies of the breeding population clones along with a further 68 clones selected for genetic conservation, will be planted in each of the two Corsican pine clonal archives.

The establishment of a new clonal seed orchard based on these new selections would nearly double the height gains predicted from existing clonal seed orchards whilst at the same time giving positive gains for stem straightness and branching quality. This work marks the end of the first generation of testing in the Corsican pine breeding programme.

Introduction

Whereas Corsican pine (Pinus nigra var maritima) is not a major species at a GB level and constitutes only 2% of commercial plantation area, locally it can be very important. There are 14,000 hectares of Corsican pine growing in East Anglia district of the Forest Enterprise where it makes up around 60% of the total holding. The species is also popular on other warm, dry sites along the south coast and throughout the midlands of England, the coastal fringes of south Wales and along the east coast, up to the north of Scotland.

The area of Corsican pine continues to increase as it replaces felled Scots pine (Pinus sylvestris). Corsican pine is often favoured relative to Scots pine due to its superior growth rate, stem form, better resistance to stem-rust canker (Peridermium pini) and tolerance of polluted atmosphere.

Selection and Testing

The genetic improvement of the Corsican pine grown in Britain started with plus-tree selection in the early 1960s. The first progeny tests designed to evaluate the genetic quality of those early selections were planted over 3 sites in 1966. Initially, the method of selecting and testing plus-trees was the same as the model established for Scots pine and Sitka spruce; a high selection intensity followed by occasional progeny tests over a number of sites. Progress was slow as resources were directed more to the improvement of Scots pine and Sitka spruce.

In an effort to advance the breeding work on Corsican pine nearly 900 good quality phenotypes were selected in registered seed stands growing in East Anglia during the good flowering years of 1972 and 1985. Two years following plus tree selection, open-pollinated progeny tests were planted. Tests were restricted to Thetford forest only for the 1972 selections although progeny from the 1985 selections were replicated at Thetford and Aldewood (East Anglia), and Kinver (West Midlands).

A total of 1015 plus-trees were selected from 1960 to 1985 of which 935 were subjected to progeny testing over a total of 30 test sites. All progeny in test were periodically measured for growth rate (either height or diameter at breast height), most were measured for stem straightness (on a 1 to 6 subjective score; 1 = best) but only a proportion were measured for branching quality (also on a 1 to 6 subjective scale).

Seed Orchards

There are two active Corsican pine clonal seed orchards; one (CP10) at Kinver Forest, planted in 1985 and one (CP11) at Spye Park, (Wiltshire) planted in 1990 but recently badly damaged by fire. Both orchards contain clones that were selected predominately for their superior height growth.

A seedling seed orchard has also been created at Kinver (CP12) by thinning one set of the 1987 progeny trials to the best phenotypes within the best families. It will be another approximately 5 years or so before this orchard can be expected to yield commercial quantities of seed.

Predicted genetic gains from each of these orchards are given in Table 1.

Seed Stands

Due to the irregular supply of seed from orchards, seed is also collected in good seed-crop years from a number of registered seed stands. There are 7 permanent Corsican pine seed stands registered in the eastern half of southern England (Region 40). However most planting stock derives from temporary seed stands selected to the same standards as the permanent seed stands, but registered just prior to clearfelling.

Genetic Gain Trials

Two Corsican pine genetic gains trials were planted in 1995 and 1996 in Thetford and Cannock (Staffs) respectively. These trials will allow realised gains to be estimated for British seed stands and seed orchards which in turn can be compared with predicted gains from orchards. In addition adaptability, growth rate and quality of British seed stands and orchards will be compared with seed collected in similar stands and orchards located on the continent. If continental material is comparable with British-selected sources, the availability of improved seed is much increased due to the greater maturity of orchards and more regular flowering years on the continent.

Selection and Production Populations

BLUP (Best Linear Unbiased Predictor), a statistical tool commonly used in animal breeding to remove the environmental effects due to differing quality of farm sites has been used to re-analyse all the Corsican pine progeny test data available. Where possible BLUP has been used to estimate the genetic (or breeding) value for height and stem straightness of the original selected plus-trees after removing differences due to quality of planting sites. Final selection for the breeding and production populations was based on an index of the combined traits relative to a registered seed stand control. The Corsican pine breeding and production populations are the first such tree species populations exclusively generated using BLUP in Britain.

A total of 152 of the original plus-trees have been re-selected to form the Corsican pine breeding population. These re-selected parents are heavily biased towards the 1985 plus tree selections from Thetford. This is because progeny-test replication over a number of sites allowed more accurate estimates of their breeding values; stem straightness and branching quality measurements were carried out on all trees within these tests (a measurement often omitted in earlier tests), and also because the tests were generally of good quality. An additional 47 plus-trees were selected where straightness or branching quality data did not exist or was incomplete but file notes made by research foresters suggested good quality trees coupled with good growth data. These additional trees have been included in the breeding population but will not be deployed in any production populations.

The predicted genetic gains from existing seed orchards and likely predicted gains from a new clonal seed orchard composed of the highest index-ranked plus trees from the BLUP evaluation are listed in Table 1.

Table 1 Predicted gains for height and stem straightness from the three Corsican pine seed orchards

Note: Gains are expressed as percentage improvements relative to registered seed stands. Figures for stem straightness and branching quality are not available for CP10, CP11 and CP12.

A new clonal seed orchard containing the best CP plus-trees could nearly double the predicted genetic gain available from existing orchards. As stated in Lee (1999) it can commonly take 15-years between initial grafting and first harvest of seed.

Further advice on the cost of establishing a seed orchard and the likely returns in terms of harvested seed over the life of the orchard is available from Dr S J Lee at the address shown on page 4.

Future

At present, it is not envisaged that the Corsican pine breeding programme will progress beyond this first generation of selection and testing. To ensure that genetic material is not lost, grafted copies of each reselection will be planted in the two Corsican pine clonal archives located in Morayshire and Shropshire. A further 68 of the original plus-trees will be grafted and planted in each clone bank to add to the Corsican pine gene pool should there be any future breeding activity. These latter trees will form a conservation population and will be made up of individual trees which were outstanding in one or more traits although they did not score the high overall index value required for the breeding population.

Conclusion

The first generation of Corsican pine selection and testing can now be considered complete. A breeding population of nearly 200 plus-trees has been selected and will be grafted in each of two clonal archive along with grafted copies of 68 other plus-trees with outstanding characteristics. Altogether these re-selected plus trees will form the Corsican pine gene pool in Britain. No further breeding activity is envisaged for the foreseeable future. Future data from existing Genetic Gain Trials will enable the verification (or otherwise) of predicted gains from seed orchards and the suitability of certain continental seed sources.

References

LEE, S. J. (1999). Genetic Gain from Scots pine: potential for new commercial seed orchards. Forestry Commission Information Note 27. Forestry Commission, Edinburgh.