Effect of Genetic Ancestry on Ecologically Important Fitness Traits in Hybridizing Populus Species: Relevance for Conservation and Forest Management

ABSTRACT Forest trees and their hybrids exhibit diverse adaptive strategies, reflected in functional traits that enhance fitness under varying biotic and abiotic stresses. Hybrid zones serve as natural experiments for studying evolutionary processes and have also garnered attention for their relevance in adaptive nature conservation and forest management, particularly in response to climate change and habitat fragmentation. However, the influence of genetic ancestry on ecologically relevant traits in hybrid zones remains insufficiently studied and understood. We investigated in this study the effects of genetic ancestry on juvenile survival, clonality, vegetative growth, and leaf hairiness in hybridizing Populus alba (L.), Populus tremula (L.), and their natural hybrid Populus × canescens (Aiton) Sm. Using genetic ancestry estimates derived from restriction site-associated DNA sequencing and genotypingby-sequencing, we analyzed plant material established from either seeds collected in different years or vegetative cuttings, grown across three common garden environments. We observed consistent patterns of seed sapling survivorship in two common gardens: backcrosses to Populus tremula and recombinant hybrid genotypes exhibited higher probability of juvenile mortality after the first 3 years compared to F1 hybrids, backcrosses to P. alba, and the parental species. Genetic ancestry also influenced clonal reproduction and early vegetative growth, with hybrids genetically closer to P. alba exhibiting enhanced growth and higher clonal rooting success. Plant propagation material from older seed collections tended to grow less and exhibited lower clonality. Additionally, leaf reflectance, a proxy for leaf hairiness, varied along the admixture gradient, with increasing P. alba ancestry corresponding to denser pubescence. Finally, we discussed the implications of these findings, particularly regarding pubescence, for forest breeding and restoration initiatives.