Planting trees isn’t enough - here’s what forests really need

Without genetic diversity, tomorrow’s forests might not survive. But we've found the solution to help them thrive in an uncertain future.

By Dr Guillermo Friis and Dr James S Borrell

A green field with rows of planted saplings and full grown trees in the background

Once upon a time, the UK had a lot more trees. In fact, much more of our landmass was made up of wild woodland compared to today.

By the 1900s, however, only 5% was left. From this lowest point, protections and regeneration efforts have since increased our woodlands to more than double what they were a hundred years ago. But we can go further, and doing so is crucial to help fight climate change, boost biodiversity and mental wellbeing.

This is why the UK is working toward reaching 16.5% tree canopy cover in England by 2050. But will simply planting more trees really help create forests that can withstand the harsher climates that are on average two, three or even four degrees warmer than today?

A woodland of large, old trees — Currently, around 13% of the UK is covered in woodland. Guillermo Friis © RBG Kew

The two approaches to forest regeneration

There are two main methods to creating woodlands. The first is tree planting, where we plant seedlings grown in nurseries. The second method is called ‘natural colonisation’, where we create the right conditions for seeds produced by the trees already living in the area to germinate and grow on their own.

But how do we know which of these strategies is the right one? And how can we make sure that the new forests we create are healthy and resilient to all the challenges they will face over the next century? To answer both, we need to think about genetic diversity.

A bed of tree seedlings in a nursery — Tree planting involves growing seedlings in nurseries. Guillermo Friis © RBG Kew

A woodland with small areas fenced off by willow fencing — Natural regeneration involves creating the ideal conditions for trees to reproduce on their own. Guillermo Friis © RBG Kew

What is genetic diversity and why is it so important?

It simply refers to the diversity within our genes – i.e the small differences in the DNA between individuals. It’s one of the things that makes us humans all a little bit different from each other. Genetic diversity is a very useful thing. The variation between different people directly reflects the difference in, for example, their abilities: while one person might be able to run very fast, another may be able to jump exceptionally high.

The same is - sort of - true for trees. When a new threat - like climate change - arrives, the variation between the trees’ genes means that some trees might struggle, but others will hopefully survive, thereby producing a new generation that are more resistant to the threat. We know that genetic diversity has already helped UK Ash trees respond to Ash Dieback disease.

So, in a nutshell, a genetically diverse forest might be more likely to contain some trees that can tolerate and survive climate change than a forest where all the trees are very similar.

Rows of planted, dying saplings — Planted saplings which haven't survived to maturity. Guillermo Friis © RBG Kew

Human planting vs natural colonisation – which is best?

Our new study tests the two strategies for growing trees. We collected DNA samples from young trees in planted and naturally colonised areas across the UK and compared the two, to see how the two methods affected genetic diversity, and how similar the resulting forests would be to the forests we have in the UK today.

The good news is that both approaches have the potential to create new forests with genetic diversity very similar to the forests we have today. But, this is only if we can address some important risks that we identified during our research.

An illustration of the differences in genetic diversity between planting and natural regeneration — When looking at how genetically diverse a forest is, it's important to consider both how diverse it is locally and how diverse forests are across the country. © Natasha Ward, RBG Kew

The key bottleneck to overcome

For the human planting approach, our data suggests that the seeds we collect for tree planting are from too few parent trees. Collecting from too few parents means that the offspring are closely related and that the newly planted forests are more similar to each other than they should be.

This could be a very big problem as over time it could start to erode the genetic diversity that we value – making trees more prone to threats. But there is a simple solution. We just need to make sure we are sourcing our seeds from healthy populations across the UK. Our scientists at Kew have already pioneered this approach with the UK Tree Seed Project.

On the other hand, natural colonisation isn’t a perfect solution either. Because our woodlands are now quite small and isolated, we found similar problems with new trees being quite closely related to each other. Over time this could lead to inbreeding, which would cause even greater problems for our woodlands. This issue could be solved by planting some seeds sourced from other locations, to boost the genetic diversity.

Illustrations of Britain, showing how diverse different types of regeneration can be — Natural colonisation combined with planting from diverse seed stocks could give us the best of both worlds. © Natasha Ward, RBG Kew

What does this mean for the UK’s 1 million hectares goal?

Fortunately, the nursery industry in the UK - who worked closely with us on this research - is aware of the problem of low genetic diversity and, with the support of the government, is aiming to identify more woodlands suitable for seed collection.

Encouragingly, there is also a growing interest in combining planting and natural colonisation approaches, to try and get the benefits of both.

Our forests are facing a challenging future. If you plant an oak tree today it might have to survive in a world that’s a few degrees warmer. But, at Kew, we're helping give our trees a fighting chance by safeguarding their remarkable genetic diversity.