Genetic Consequences of Tree Planting Versus Natural Colonisation: Implications for Afforestation Programmes in the United Kingdom

The United Kingdom aims to dramatically accelerate the establishment of new woodlands by 2050, yet the impact of different afforestation strategies on landscape genetic diversity and resilience remains unclear. This study integrates environmental data, whole-genome sequencing and phenotypic assessments to compare bioclimatic envelopes, genetic diversity and plant health indicators in naturally colonised versus planted populations of pedunculate oak and silver birch. We found that registered seed stands significantly under-represent the wild bioclimatic envelopes of both species, as well as those of 21 out of 39 UK native species assessed, potentially limiting adaptive diversity in planted populations. Yet, genetic diversity metrics (π, H_O and A_R) based on genome-wide markers in planted populations were comparable to naturally colonised woodlands. Planted populations exhibited higher within-group coancestry and moderate genetic homogenisation among sites, possibly reducing adaptive differentiation. Naturally colonised populations showed higher inbreeding coefficients (F_ROH) in both species, potentially due to fragmentation of source populations. Genotype–environment associations based on redundancy analysis revealed divergent selection at functionally relevant loci, indicating distinct selective pressures in commercial tree production versus natural colonisation. Health indicators revealed reduced browsing in planted trees, and differences in mildew and leaf-spot incidence, suggesting potential selection divergence between afforestation strategies. These findings support a role for both afforestation strategies in enhancing the resilience of future woodlands while highlighting pathway-specific risks of introducing unintended impacts on forest diversity.