New study compares benefits of tree planting vs natural colonisation for future resilience of UK woodlands

In a new study published today in the journal Evolutionary Applications, scientists from the Royal Botanic Gardens, Kew, and Forest Research, present a detailed analysis of the genetic diversity and comparative tree health of young British woodlands, revealing the pros and cons of planting versus natural colonisation. 
 
It is the first research publication of the Centre for Forest Protection, a joint venture between RBG Kew and Forest Research funded and managed by Defra. The CFP brings together expertise from the UK and the rest of the world in the joint creation of knowledge and research to protect and enhance the future health and resilience of our forests, woodlands and trees.  

As England looks to halt nature’s decline and reach 16.5% canopy cover in England by 2050, policymakers are keen to know whether natural colonisation or tree planting is the way to go. Having strong, healthy forests is a particularly salient issue in the wake of ash dieback, which has wiped out millions of trees across the country in the past decade. 
 
Dr Guillermo Friis, Postdoctoral Researcher in Plant Health and Adaptation at RBG Kew and first author of the paper, says: ‘Across the country, trees of the same species have slight genetic differences that have evolved to help them grow better in those places. We call this “local adaptation”, and protecting these small genetic differences might make the difference in helping our trees respond to climate change.’ 
 
Kew and Forest Research scientists looked at sites with naturally colonising and planted English oak and silver birch to compare young tree condition and assess the small differences in DNA between tree populations. Genetic diversity is crucial if the UK’s forests are to thrive, as although many trees will undoubtedly fail, some will have the built-in resilience to pathogens and stressors like drought that can be passed on to future generations. 
 
The good news is that both planting and natural colonisation can create a healthy and diverse forest when analysed on the local level, reinforcing recent findings in a Surrey woodland where natural selection was evolving resilience to ash dieback. 
 
Natural colonisation on its own is not a solution to the problem, however, because of the small and fragmented nature of the UK’s woodlands. The scientists found many of the young trees are quite closely related, which runs the risk of inbreeding and the drawbacks associated with that. 
 
Dr Friis says: ‘A remarkable finding of our study was that inbreeding – the loss of genetic diversity that occurs when closely related individuals reproduce – was twice as high in naturally colonised young forests compared to planted ones. This is likely due to the lack of connectivity between forest patches, often isolated by large areas of farmland. By choosing seed sources carefully, planting can help grow forests that stay healthy and adaptable over time.’ 
 
Similarly, planted sites appeared more alike than the scientists expected, when viewed on the national scale. For example, genetic variation among sites was five times higher between naturally colonised populations than between planted ones in oak, likely due to seeds being sourced from relatively few stands. Seeds of more diverse origins were used in birch planting, and this helped maintaining population differences. 
 
In a separate part of the study, the researchers found that planted trees are all too often coming from just a handful of seed sources. This is contributing to the lack of variability among planted woodlands.  
 
One proposal that will be put forward to the government and the seed supply and nursery sector is to prioritise collecting seed from a wider range of healthy parent trees from right across regions to balance genetic diversity and local adaptation when propagating saplings for planting, an approach pioneered by Kew's UK National Tree Seed Project. 
 
Dr James Borrell, Research Leader in Plant Health and Adaptation at RBG Kew, says: ‘Working closely with tree nurseries has shown that scientists and industry are really on the same page. We need more diverse seed sources to maximise the resilience of our future forests. If seeds come from the same small number of stands, then eventually we might erode the genetic diversity of our native trees.’ 
 
Dr Nadia Barsoum, Senior Forest Ecologist at Forest Research, says: ‘While on the whole we found the health of the young birch and oak trees assessed to be good, we have been finding higher incidences of leaf pathogen infections on planted birch compared with naturally colonised birch at the same locations. It is unclear what the causes of this trend might be, but a follow-on CFP project entitled SUstainable Plant Provenancing for Resilient Trees (SUPPoRT ) is building on this research.’ 

Climate predictions for the UK suggest that the trees planted today will face conditions that are drier and two, three or even four degrees warmer, without even accounting for the increased risks from pests and disease. Securing healthy forests for the future will most likely require a holistic approach of targeted tree planting with natural colonisation mixed in. 
 
Dr Friis says: ‘Moving forward, it’s not just about how many trees we plant, but making sure the ones we plant are equipped to thrive in today’s environment and that their offspring can keep pace with the environmental challenges of the future.’ 

Dr Barsoum says: “Our findings suggest that, for the UK, a hybrid approach to increasing woodland coverage - using both natural colonisation and tree planting using diverse seed sources - could be the most effective strategy for growing forests that may have greater resilience to pests, diseases and climate change.” 
 
ENDS