Weak founder effects but significant spatial genetic imprint of recent contraction and expansion of European beech populations.

Understanding the ecological and evolutionary processes occurring during
species range shifts is important in the current context of global change.
Here, we investigate the interplay between recent expansion, gene flow and
genetic drift, and their consequences for genetic diversity and structure
at landscape and local scales in European beech (Fagus sylvatica L.) On
Mont Ventoux, South-Eastern France, we located beech forest refugia at the
time of the most recent population minimum, approximately 150 years ago,
and sampled 71 populations (2042 trees) in both refugia and expanding
populations over an area of 15,000 ha. We inferred patterns of gene flow
and genetic structure using 12 microsatellite markers. We identified six
plots as originating from planting, rather than natural establishment,
mostly from local genetic material. Comparing genetic diversity and
structure in refugia versus recent populations did not support the
existence of founder effects: heterozygosity (He = 0.667) and allelic
richness (Ar = 4.298) were similar, and FST was low (0.031 overall).
Still, significant spatial evidence of colonization was detected, with He
increasing along the expansion front, while genetic differentiation from
the entire pool (βWT) decreased. Isolation by distance was found in
refugia but not in recently expanding populations. Our study indicates
that beech capacities for colonization and gene flow were sufficient to
preserve genetic diversity despite recent forest contraction and
expansion. Because beech has long distance pollen and seed dispersal,
these results illustrate a ‘best case scenario’ for the maintenance of
high genetic diversity and adaptive potential under climate-change related
range change.