Hybrid vigor and outbreeding depression after 100 years of replicate introduction of Caucasian beech to European forests.

Hybridization between closely related forest tree species is a major concern for assisted migration decisions. Historical introductions, such as the repeated introduction of Caucasian beech (Fagus hohenackeriana) into European beech (Fagus sylvatica) forests over 100 years ago, provide insight into potential outcomes. In two such forest stands in Central Europe, we exhaustively genotyped adults (N = 216 and 226) and surrounding seedlings and juveniles (N = 639 and 423). We found that the two species interbred successfully, with interspecific F1 hybrids comprising 10.4% and 34.2% of the seedlings in the two stands, respectively. However, their proportion decreased in later age classes, and advanced-generation hybrids were rare (<4%), despite up to three possible generations of introgression. Parentage analyses revealed a tendency toward assortative mating, likely driven by the, on average, 2.3-day earlier spring bud break of Caucasian beech, based on five years of observations. In agreement with this finding, F1 seedlings grew near adult Caucasian beeches, which acted as seed donors. To assess whether these patterns could arise from demographic processes alone or reflect outbreeding depression, we used spatially explicit individual-based simulations with mating and demographic parameters specific to the study system and sites, including selection against F1s. All simulated scenarios underestimated F1 frequencies and overestimated advanced-generation hybrid frequencies, suggesting that life-stage-specific selection may alter hybrid persistence. We conclude that Caucasian beech introductions do not pose a risk of invasiveness, and that mild seedling-stage-specific heterosis effect overridden by outbreeding depression at the adult stage is the most parsimonious explanation for the observed patterns. Our simulation pipeline can be used for other species and contexts to evaluate potential outcomes of assisted gene flow.