Using large, high quality genomic datasets, we applied different metrics to determine whether planting introduces genetic bottlenecks or reduces diversity compared to natural colonisation.

Comparisons between afforestation strategies were conducted by grouping trees into comparable age cohorts: parental trees, seedlings, and saplings in the case of natural colonisation; seed sources, nursery trees, and outplanted trees in the case of planting.

Our results showed no significant difference in genetic diversity both between age cohorts and across afforestation strategies (Figure 1).

Figure 1: Levels of genetic diversity measured as observed heterozygosity (HO) in silver birch. Each bar represents a tree group from either natural colonisation or planting pipelines. Differences across groups are limited, indicating broadly similar genetic diversity across afforestation strategies.

Genomic tools also enabled us to identify levels of within- and between-group coancestry, inbreeding, and selection potentially caused by planting methods. Planted populations had significantly higher within-group coancestry, meaning trees at each site were more closely related to one another than in natural populations, which may result in a loss of genetic diversity in subsequent generations.

Between-group coancestry was also higher in several oak planted populations, potentially due to the use of a limited number of seed source stands.

This pattern reveals moderate genetic homogenisation, which could potentially reduce landscape complexity and adaptive differentiation among woodlands. In contrast, naturally colonised populations exhibited higher levels of recent inbreeding, probably due to fragmented landscapes.

Additionally, genomic analyses detected subtle signs of differential selection between planted and naturally colonised populations. For example, genes associated with flowering time, fruit detachment, and stress resistance showed evidence of selection, indicating unintended selective pressures in commercial tree production.

Health indicators, including deer browsing damage and disease incidence, were assessed at multiple sites to detect differences linked to planting versus natural establishment. Our assessments confirmed significant differences between afforestation strategies, with planted trees generally experiencing reduced browsing damage but differing levels of disease susceptibility.

For full details of our study, please read our paper: https://doi.org/10.1111/eva.70146

Our research presented several opportunities to improve management of genetic resources, which are outlined below.

Expand and diversify seed sourcing

The climatic breadth of current commercial seed stands inadequately represents the full bioclimatic diversity of wild populations. Investment in broader networks of registered seed stands and seed orchards would be beneficial for maintaining adaptive diversity.

Address fragmentation effects

Natural colonisation sites often experience elevated inbreeding. This is likely due to isolation and limited gene flow, resulting in a scarcity of parental trees contributing to the genetic pool of newly established populations. Policymakers should consider assisted gene flow, hybrid approaches combining planting with natural colonisation, and improvements in landscape connectivity to mitigate these effects.

Avoid planting populations of closely related individuals

Higher within-site relatedness in planted populations highlights the need to increase the number of regional seed sources and parental trees within commercial seed stands. This would reduce risks of future inbreeding and loss of genetic diversity. Within nurseries, there may also be opportunities to supply batches of saplings that explicitly mix offspring of different sources within a region.

Avoid genetic homogenisation

Higher between-site relatedness among planted populations at regional and supra-regional levels highlights the importance of using seeds from diverse local sources, potentially including the use of local orchards, to preserve genetic diversity across larger geographic scales and reduce the risk of genetic homogenisation.

Monitor for unintended selection

Subtle genetic signals suggest that current nursery and planting practices may inadvertently select for specific traits. Monitoring and review of nursery practices may be advisable to maintain resilience and adaptability.

Enhanced knowledge exchange and training

Collaboration between researchers, land managers, nurseries, and seed suppliers is essential to share best practices, address
genetic risks, and assist researchers in understanding the impacts of industry practice on research findings.

For further information, read our journal article or visit the Centre for Forest Protection website.

Friis, G., et al. (2025). Genetic consequences of tree planting versus natural colonisation: implications for afforestation programmes in the United Kingdom. Evolutionary Applications, 18(8), e70146.

How to cite this document

Centre for Forest Protection (2026) Genetic consequences of tree planting versus natural colonisation. Farnham: Forest Research.
https://doi.org/10.70463/OEZE6042

Research contacts

Guillermo Friis g.friis@kew.org
James Borrell j.borrell@kew.org

Royal Botanic Gardens
Kew
Richmond
Surrey
TW9 3AB

020 8332 5000

About the Centre for Forest Protection

The Centre for Forest Protection is a Defra-funded collaboration, led by Forest Research and Royal Botanic Gardens, Kew, which aims to protect our trees from environmental and socioeconomic threats, through innovative science, interdisciplinary research, expert advice and training.