BIHT

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 BIHIP 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.

Current Research

1. Provenance studies

A programme of provenance trials began in 1997 (see Table 1) and now comprises:

• Scottish trials: 4 provenance trials in Scotland and one in Ireland, testing 31 Scottish and 3 northern English seed sources collected in 1995, were established between 1997 and 2001.

• UK-wide trials: 5 provenance trials with seed sources from the whole of the UK plus a few from the near Continent; of which 2 trials are in Scotland, 1 in England and 2 in Wales. These trials planted in 2003 are testing 33 seed sources, collected in 2001, comprising: 18 English , 4 Welsh, 8 Scottish (from the 1995 collection) and one each from Norway, Denmark and France. In addition, some plus tree progenies from France were added when the trials were one-year-old.

One site in Wales, (Llandovery 41), proved unsuitable and was discarded. At the remaining sites establishment and growth have been good with the best seed sources growing at more than 1 m per year. Assessments at 1, 3, 6, and 10 years have included: height and survival at all trials; and timing of flushing / senescence and leaf size at some- see map below.

Click on the map above to see an enlarged map of birch provenances

The main conclusions from the provenance trials so far are:

• Survival: there are no significant differences in survival amongst seed sources.

• Early growth: Growth rate is highly correlated with latitude of seed source, northern provenances grow more slowly than southern ones (Table 2 ranks the ten fastest and ten slowest growing seed sources for the 1995 collection at 3-years averaged across all trials) The growth of seed sources from ‘local’ sites is usually in the middle of the performance range. Some groups of provenances are uniformly slow growing (e.g. those around Loch Rannoch) whereas northern English sources are vigorous at all sites.

• Phenology: There are significant differences among seed sources in the timing of flushing and senescence. Eastern and southern seed sources tend to flush earliest and northern ones undergo senescence earliest.

• Within provenance variability: There is very large variation in the attributes of individual trees within provenances.

• Early stem form: it proved difficult to categorise early stem form (at year 3-5) and no significant differences have been demonstrated among provenances.

The main messages for tree breeding arising from the provenance trials thus are:

• The differences among seed origins in growth rate and phenology make it desirable to identify faster growing populations for possible inclusion in breeding programmes. These will typically be more southern seed sources, which show high early growth rates across a wide range of sites (i.e. they are plastic) but they need to be assessed for stem form. The fastest growing seed source across many of the trials is Sand Hutton (Yorkshire lowlands, Table 2) but the phenotypic form in the parent stand is generally poor. However some slower growing seed sources may have good form and be adapted to particular site conditions (e.g. nutrient poor soils, regular frost incidence or higher exposure) and should not be discounted automatically. Choice of sources for breeding material needs to take account of the likely range of sites onto which improved material may be planted; and should utilise sources that perform well on similar site types.

• Despite the existence of geographically based patterns of variation, the variability in performance amongst seed sources is sufficiently large that material could be transferred considerable distances (say up to 2⁰ latitude northwards) without the likelihood of adaptation problems appearing during at least the first 10 years. Hence, in principle, seed transfer zones could be fairly large. However, the longer term performance of material transferred long distances is unknown and it is possible that infrequent climatic events could damage growth or flowering.

Although silver birch will grow on a wide range of sites, it will only produce quality timber on sites of low exposure and moderate to good soil quality.

Table 1 Provenance trials - basic data (See Table 1).

2. The tree breeding programme

Three breeding populations of 40 – 50 plus trees each have been identified within the defined geographical areas of (a) Tayside, (b) Speyside/Deeside/Donside and (c)Southern Scotland/ North of England. In addition a pilot collection carried out by Forest Research in the late 1990’s from the Great Glen area of Northern Scotland was incorporated into the programme. Collections of scions from the plus trees are grafted on potted seedling rootstocks for inclusion in polyhouse seed orchards with a view to early seed production in 3 years. The techniques are well established and follow successful Finnish forest practice. The numbers, locations and current status of plus tree collections are given in Table 3.

2.1 Selection criteria for plus trees

Plus trees are selected on the basis of their phenotypically superior stems and crown characteristics and their size and stature in the stand. Dominant and co-dominant trees are selected and the following criteria used:

1. Tree stem: straightness, circularity of stem, absence of fluting, bark characteristics.

2. Crown: persistence of leading shoot, forking, branch size and branching angle.

Scores attributed to each tree for stem form and crown characteristics are combined to give a single score for the tree in five categories [Click here for a PLUS TREE SELECTION DIAGRAM]. Plus trees are compared with each other and with up to 5 surrounding dominants in the stand, to demonstrate their superiority.

The mensurational data recorded for each selected tree are: total height, length of clean stem (to first branch) and breast-height diameter. Stand density, site variables, tree registration number and location (GPS) are also recorded.

Table 3 Regional Collections Of Plus Trees for Inclusion in Seed Orchards (See Table 3).

Related Research

The bringing together of seed sources from different geographic areas in properly designed field experiments (provenance trials) makes possible a range of investigations of the environmental relationships of silver birch. These can encompass molecular studies of, say, cpDNA, allowing inferences of the post-glacial history of the species or estimation of the physiological responses to variable climatic stimuli.

The simplest studies of phenological differences in leaf flushing and senescence have, in the case of silver birch, been the subject of a series of student dissertations. Comparative assessments of leaf size across contrasting British sites are being analysed at present, while a basic study of winter desiccation of shoots induced by shoot embolisms has been published. Recent investigations have attempted to assess the development of stem form among provenances across contrasting experimental sites and this work can be extended now with the collection of clonal ‘plus tree’ material.

An earlier study of pollen dispersal within a polyhouse seed orchard, using the Great Glen population, demonstrated satisfactory levels of cross-pollination of clones. A programme of relatively simple progeny trials for the seed produced by these breeding populations is envisaged to demonstrate their potential superiority to current commercial seedlots and allow them to attain ‘tested’ status. As more information becomes available from trials of this kind and from the provenance experiments, it will be possible to re-define the ‘regional’ breeding population and seed collection zone boundaries, for which there is as yet slight scientific evidence.

Group Membership

The research programmes of the Silver Birch Group have been designed and managed, since its inauguration in 1995, by a Steering Group who meet several times a year. The present members of the Group are:

Andrew Barbour (Chairman)

Douglas Malcolm (Secretary)

Cathleen Baldwin, Forest Research

Rob Sykes, Forest Research

Elaine O’Connor, University College Cork

David Boshier, University of Oxford

Scott McG. Wilson , Consultant

Rick Worrell, Consultant

To contact any of the Group Members please contact the Birch Group secretary Dr. Douglas C Malcolm ().