Hybrid regeneration dynamics in a mixed plantation of interfertile species

Submission for the symposium S26 (gene flow to the rescue)

The European silver fir (Abies alba) plays both economic and ecological roles in maintaining the functional balance of European forests. Its current distribution in Europe is the result of recolonization after the last glaciation contributing to local adaptations across Europe. Despite this adaptive capacity, signs of decline are observed particularly in France. As a response, assisted gene flow strategies have been proposed to enhance adaptive potential by introducing drought-resistant Mediterranean fir species. These species belong to the Euro-Mediterranean Abies clade and diverged during the late Miocene to early Pliocene (3.4–5 million years ago). Despite these divergences, the seven sister species are not reproductively isolated, as evidenced by artificial crosses and naturally occurring hybrids. In France, the most commonly planted Mediterranean fir species included A. cephalonica, A. nordmanniana, and A. pinsapo.

This study focuses on a 40-year-old plantation on Mont Ventoux (France, Vaucluse) composed of pure A. cephalonica and interspecific hybrids. A total of 272 adult trees were mapped and genotyped using 60 SNPs and 15 SSRseq markers. Natural regeneration was assessed in 1,500 seedlings spanning multiple cohorts from 1999 to 2017, all similarly mapped and genotyped. To identify the nature of the seedlings, we performed clustering analyses using different types of approach (Multivariate analysis, Bayesian models, Frequentist models) thanks to the DAPC, STRUCTURE, and GENCLASS model. Additionally, parentage relationships were assessed with CERVUS and the R package “related”. Subsequently, GLM models were built to analyze the relationship between the genotypes, survival, growth, and age structure within the seedlings.

The clustering results with STRUCTURE showed that 215 of the studied seedlings are pure Abies cephalonica, while 1,265 are considered hybrids. We identified 11 types of hybrids, with 7 primarily involving Abies cephalonica. A GLM binomial survival model indicated that genotypes significantly explain seedling survival. A. cephalonica seedlings consistently had a lower chance of survival than the hybrids. Regarding growth, the GLM gamma model showed no significant effect of hybrid type on growth, but age played a significant role. Older seedlings exhibited greater growth than younger ones.

These preliminary results suggest that hybrids may exhibit higher survival rates compared to pure Abies cephalonica individuals, indicating a potential improvement in offspring fitness. In this sample, there is no clear evidence of hybrid depression.