Two birch tree species are recognised as very early postglacial colonisers in Britain. Downy birch (Betula pubescens Ehrh.) is now generally more common than silver birch (Betula pendula Roth), which latter tends to be restricted mainly to drier, freely draining soils in moderate to low rainfall climates and at lower elevations. Although they frequently occur in mixed populations, putative hybrids between the species are not reliably found. [Chromosome numbers are pendula; 2n= 28; pubescens; 2n= 56]. Morphological variation is clearly greater in pubescens and several sub-species are recognised whereas pendula has usually, clear distinguishing features in leaf, shoot and bark.
Both birch species are ecologically important for woodland habitat and visually as landscape elements but on suitable sites pendula as a forest species, is preferable for its higher productivity, better stem form and crown characters. Silviculturally, shade intolerant, it acts as a pioneer species invading forest clearings or disturbed sites, shows rapid growth rates and thus can be managed on relatively short rotations in well-thinned stands. Considerable quantities of silver birch timber are imported from Fennoscandia and N.Europe to the British furniture and plywood markets, while small quantities of home-grown timber are used in flooring. Silver birch timber can display attractive figuring but is generally acceptable as a strong, versatile whitewood, although subject to decay in exposed conditions.
The increased planting of silver birch in Britain, since the 1980’s, as a broadleaved element of native woodland restoration schemes or as amenity along roadsides, has not recognised the potential extent of environmental adaptation within the species. Further, there has been often scant regard paid to the inherent qualities of seed stands, the importance of accurate species identification and selection of appropriate planting sites.
Some earlier work had recognised the desirability of better use of silver birch. A programme of selecting ‘plus trees’ (superior phenotypes) begun by the Forestry Commission in the 1950’s was however dropped in the 1960’s, while later studies on the performance of Fennoscandian origins of silver birch confirmed such imports to be maladapted to conditions in Scotland. A breeding programme based on various selected parent trees from around Britain was developed in Aberdeen University in the 1970’s but ran out of support after 1986.
To remedy this situation and to promote the sensible use of silver birch for amenity, conservation and timber production purposes the Silver Birch group in FTT was formed in 1995. Although very little then was known of adaptive variation across Britain, in any native tree species, silver birch was a suitable trial species to determine environmental responses, because of its frequent seeding, ease of vegetative reproduction (cuttings/ grafts) and widespread natural distribution.
The Group’s aims are:
1) to estimate the inherent and adaptive variation of the species and hence the need for control of seed transfers to avoid maladaptation of planted stands.
2) develop a breeding programme for improvement of the timber quality and productivity of the species.
A dual approach was thus adopted of establishing both provenance (seed source) trials and making regional collections of plus tree scions to form grafted seed orchards in polyhouses.