Genetic diversity and fine-scale spatial genetic structure of European beech populations along an elevational gradient

Differences in environmental conditions can shape the level and distribution of intraspecific genetic variation between and within populations. Elevational gradients are characterised by strong variation in environmental conditions on a short spatial scale and provide an ideal setting to study the spatial distribution of genetic diversity. Therefore, we investigated the genetic diversity, fine-scale spatial genetic structure (FSGS) and spring phenology (bud burst) as a proxy for flowering of five European beech (Fagus sylvatica L.) populations along an elevational gradient, ranging from about 550 m to 1450 m a.s.l. in the Romanian Carpathians. Using microsatellite and genome-wide single nucleotide polymorphism (SNP) markers, we observed a slight decrease in genetic diversity with increasing elevation and low population differentiation. Furthermore, levels of FSGS decreased with elevation along the gradient. We could not detect any significant effects of spring phenological traits on the level of FSGS probably because many different environmental factors and processes vary over the years and contribute to shaping the FSGS. The slightly lower genetic diversity in high elevation populations may indicate stronger drift effects and could be due to the marginal ecological conditions and the lower abundance of beech. However, in these stands with less competing crowns and a more open forest structure, pollen dispersal might be longer ranging in this wind pollinated species which could contribute to a weaker FSGS. The knowledge about the level and structure of genetic variation along environmental gradients is crucial to inform forest and conservation management especially in the face of climate change.