Differences in fine-scale spatial genetic structure of European beech populations along elevational gradients

Limited gene dispersal via pollen and seeds typically leads to clustering of related individuals within populations, known as fine-scale spatial genetic structure (FSGS). It reflects microevolutionary processes at the local scale and can inform forest management practices for conservation and restoration purposes. The strength of FSGS varies widely between species with different life history traits but also between populations of the same species. Here, we investigated the FSGS, genetic diversity and spring phenology (bud burst) of five European beech ( Fagus sylvatica L.) populations along an elevational gradient, ranging from about 550 m to 1400 m a.s.l. in the Romanian Carpathians. Using microsatellite and genome-wide single nucleotide polymorphism (SNP) markers, we showed that FSGS was most pronounced in lower elevation populations. While FSGS results based on both marker types showed similar trends, significant differences were detected mainly based on SNPs, highlighting the higher resolution of genome-wide markers. Spring phenology started earlier at low elevations and appeared more synchronized compared to high elevations, which may contribute to differences in FSGS. We also observed a slight decrease in genetic diversity with increasing elevation. These differences in FSGS and genetic diversity could be explained by the lower density of beech and stronger interspecific competition in forest stands at high elevation. Here, the less dense forest structure may facilitate gene flow in this wind pollinated species. Future studies on other beech populations and other species with similar life history traits along elevational gradients are needed to test how common such patterns are.