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.
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 2o 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.