Data from: Genetic diversity maintained among fragmented populations of a tree undergoing range contraction

Dwarf birch (Betula nana) has a widespread boreal distribution but has
declined significantly in Britain where populations are now highly
fragmented. We analysed the genetic diversity of these fragmented
populations using markers that differ in mutation rate: conventional
microsatellites markers (PCR-SSRs), RADseq generated transition and
transversion SNPs (RAD-SNPs), and microsatellite markers mined from RADseq
reads (RAD-SSRs). We estimated the current population sizes by census and
indirectly, from the linkage disequilibrium found in the genetic surveys.
The two types of estimate were highly correlated. Overall we found genetic
diversity to be only slightly lower in Britain than across a comparable
area in Scandinavia where populations are large and continuous. Whilst the
ensemble of British fragments maintain diversity levels close to
Scandinavian populations, individually they have drifted apart and lost
diversity; particularly the smaller populations. An ABC analysis, based on
coalescent models, favours demographic scenarios in which Britain
maintained high levels of genetic diversity through post-glacial
recolonisation. This diversity has subsequently been partitioned into
population fragments that have recently lost diversity at a rate
corresponding to the current population-size estimates. We conclude that
the British population fragments retain sufficient genetic resources to be
the basis of conservation and re-planting programmes. Use of markers with
different mutation rates gives us greater confidence and insight than one
marker set could have alone, and we suggest that RAD-SSRs are particularly
useful as high mutation rate marker set with a well-specified
ascertainment bias, which are widely available yet often neglected in
existing RAD datasets.