Exploring the association between adaptive and growth traits and within-individual genetic diversity in common beech (Fagus sylvatica)

Common beech (Fagus sylvatica L.) is one of Europe's most widespread forest tree species. In the actual context of climate change, this species has responded through its self-regulation mechanisms, proving a high plasticity. It is important to explore the specific mechanisms driving its response to climate change, taking into account silvicultural, phenological, and genetic perspectives and their interaction. Here, we tested for association between adaptive and growth traits and within-individual genetic diversity measured as individual heterozygosity (proportion of heterozygous loci per sampled individual), based on six genomic microsatellite markers (gSSRs, genomic simple sequence repeats) and six expressed sequence tag microsatellites (EST-SSRs) for 55 beech trees. We evaluated the spring and autumn phenology of beech trees sampled along an altitudinal gradient (560 - 1150 m) and the architectural traits using a non-destructive terrestrial laser scanning method (TLS). The effect of stand density at the onset of the growing season was evaluated by quantifying the competition through the Hegyi index. The onset of bud burst and senescence, as well as the length of the growing season, varied significantly and inversely proportionally with the altitudinal gradient. There was a difference of 14 days between the individuals located at the extremities of the altitudinal gradient in the onset of bud burst, 15 days in the onset of senescence, and 30 days in the length of the growing season. We obtained a very significant and positive correlation between altitude and bud burst and a very significant but negative one between altitude and the length of the growing season. An increase in tree competition directly implied a decrease in DBH and crown dimensions, especially by neighbours closer than 4 m. Stem's slenderness significantly increased with higher competition. Our results revealed a positive relationship between individual heterozygosity and the length of the growing season, as well as with the trunk volume and DBH. Higher individual heterozygosity was associated with a longer growing season, and a precocious onset of bud burst in beech. Higher heterozygosity was also associated with considerably higher total tree biomass. The genetic diversity was inversely proportional to stem forking. TLS shows great potential in extracting beech tree biomass indicators, but we still recommend using the conventional method as a complementary method for data validation, although it is time-consuming.