Trees are excellent at taking carbon out of the atmosphere and trapping it in their trunks, roots and leaves. But what if planting them wasn't the solution?

At a former intensive dairy farm in Sussex, England, oak trees now tower up to 20 feet tall (6 metres), sucking in carbon from the atmosphere, providing habitat for birds, mammals and insects, purifying air and water, and protecting land from flooding. Alder, hornbeam, ash and birch trees are also thriving.

Twenty years ago, these trees weren't here at all. The transformation is the kind of story that many countries are aiming for with large-scale tree planting programmes, from India to the US to Ethiopia. But they might be surprised to learn of the secret to this farm's success – none of these trees was "planted" here at all.

Instead, the trees at Knepp Castle Estate in southern England were allowed to spread naturally. Birds such as jays can disperse as many as 7,500 acorns in four weeks. "Not a single tree was planted, no saplings were bought from commercial nurseries, no tanalised wooden stakes, no polypropylene tubes and plastic ties, no direct financial or carbon costs – no effort," says Isabella Tree, co-owner of Knepp Castle Estate.

The trees' growth was aided by thorny scrub that had also been allowed to grow at the farm, which acts as "nature's barbed wire", protecting the saplings from nibbling deer and the estate's free-roaming cattle and ponies.

The method described by Tree is known as natural forest regeneration. Distinct from active tree-planting, trees are allowed to grow back spontaneously, or with limited human intervention, on land where the original forest cover had been cleared for uses such as agriculture or destroyed by fire.

Trees grow from seeds blown in by the wind, carried there by animals or birds, or from plant parts such as stems, leaves or roots. For this reason, the greatest potential for natural regeneration is in areas next to an existing forest, according to the UN's Food and Agricultural Organization.

It does not necessarily involve sitting back and letting nature take its course. Some intervention, such as removing competing plants or grazing animals, may be needed to give natural processes a kickstart. This is known as assisted natural regeneration.

Far from being a new way for tree cover to increase in landscapes around the world, natural forest regeneration has taken place in countries as diverse as Norway, Brazil, Costa Rica, Nepal and Ukraine, according to research published last year by Robin Chazdon, professor emerita in the ecology and evolutionary biology department at the University of Connecticut. But this has largely taken place unintentionally, as people have abandoned farmland to move to more productive areas, or in search of jobs in cities.

Chazdon, who has studied natural regeneration for more than 30 years, questions the commonly held assumption that trees need to be actively planted to tackle climate change and biodiversity loss. "There's a perspective that humans did this damage and it's our job to fix it, and that we should govern the process, and just let nature help when it can," she says. "Another view is that forest restoration is fundamentally natural, and that humans can assist it, but ultimately it should be governed by natural processes."

In January, scientists at Royal Botanic Gardens Kew in the UK warned that tree planting was often being presented as an easy answer to the climate crisis, and a way out for businesses to mitigate their carbon emissions. But it was not as simple as it seemed. The wrong trees in the wrong place can cause considerably more damage than benefits, and fail to help people, nature or capture carbon. For example, South Africa spends millions of dollars to clear Australian acacias that became invasive after being introduced to stabilise sand dunes during the 19th and 20th centuries. Instead, the trees took over heathlands and grasslands, and lowered the water table, the experts at Kew noted. 

In a new publication, the Kew scientists say that, where new trees are needed, the focus should be on letting forests grow naturally, as long as the conditions at the site like soil quality and proximity to existing forests were suitable.

Benefits underestimated

Proponents are arguing for natural regeneration to be taken more seriously in national and international efforts to mitigate the climate and biodiversity crises. Recent research has shown that natural regeneration can potentially absorb 40 times more carbon than plantations, and provide a home for more species. It is also significantly cheaper than tree planting, with different studies in Brazil showing costs reduced by 38%, or even up to 76%.

This could make a significant difference to the costs of international ambitions to restore forests, such as the Bonn Challenge, which is targeting 350 million hectares (1.4 million square miles). This could cost $12 trillion (£8.5tn) if only active tree planting is used.

Not only that but the ability of naturally regrowing forests to absorb carbon has been underestimated by 32% by the Intergovernmental Panel on Climate Change (IPCC), according to research by US-based environmental organisations the World Resources Institute and The Nature Conservancy, which was published in the journal Nature in September.

The IPCC's rates are used by many countries to estimate the capacity of forests to absorb carbon and to report progress towards climate change goals. This strengthens the case for a greater focus on allowing forests to regrow as a climate change mitigation policy, alongside active tree planting, says Susan Cook-Patton of The Nature Conservancy and an author of the report.

The rate at which trees accumulate carbon varies up to 100-fold, depending on factors like climate and soil quality, so the researchers also produced a global map, down to a 1km (0.6 mile) resolution, highlighting areas with the greatest carbon returns from allowing lands to reforest naturally. They hope this will help decision-makers see where natural forest regrowth could have the most impact for climate change mitigation, taking the guesswork out of using the approach.

"There are lots of ways to get trees back into the landscape, including actively planting them, setting up a timber plantation, or letting them grow naturally," says Cook-Patton. "Our goal is to help people have the information they need to decide which makes the most sense."

Unrealised potential

So far, the potential for natural forest regeneration has been overlooked in national and international efforts to increase tree cover. Reasons include a lack of recognition that it is a viable restoration option; perverse incentives that favour the clearing of young tree growth for plantation development or other land use; lack of support by government agencies and other organisations; lack of incentives for local communities; and uncertainty about processes and outcomes, according to the UN's Food and Agricultural Organization.

Natural regeneration is sometimes criticised for looking "messy", says Tree. "Humans are such control freaks. The more compelling the climate emergency becomes, the more we feel like we physically have to do something, and our instinct is often to tidy up," she says.

As more policymakers, conservation organisations and members of the public see the approach in action, the mindset will change, Tree believes. She agrees with Kew's scientists that the natural regeneration of trees should be the default unless there is a specific reason for active planting.

Tree believes that funding models need radical change to recognise the benefits of natural regeneration. "The current system for establishing trees in the UK is entirely dictated by the commercial forestry model, which is all about numbers of trees per hectare," she says. However, the UK government is now considering the evidence base for natural regeneration in its new Tree Action Plan.

A simple change in the language used could make a difference, according to Karen Holl, professor of environmental studies at the University of California. "I'd like to see 'tree planting' campaigns called 'tree growing' campaigns. It's about keeping the forest standing, and allowing for natural regeneration – you don't necessarily have to plant the trees. There's an obsession with digging a hole," she says.

Cook-Patton agrees that the natural regeneration of trees may seem a bit passive and intangible to some people. Tree planting is also often easier to justify in order to get funding and to prove progress. "When it comes to climate mitigation, it's very important to demonstrate that you're doing something additional beyond what would have happened otherwise, and it's much easier to demonstrate that when you're planting a tree," she says.

She acknowledges that active tree planting could be necessary if a particular type of tree is needed to support certain wildlife species, or for timber. "With natural regrowth, you're at the whim of what is blown or carried in, so planting does give you more control over what the ecosystem ultimately looks like. But natural regrowth should at least be considered first because it can be cheaper and easier," she says.

Socio-economic issues are the greatest barriers to natural regeneration, according to Chazdon. For example, farmers in Costa Rica are given $125 (£88) a year for each hectare of the plantation they establish, but only $39 (£27) a year if that land is used to protect natural regeneration, she notes. This means that clearing young forest regrowth for plantations is the more financially attractive option.

Landowners should be given sufficient financial compensation to leave land for natural regeneration, Chazdon suggests. In the longer term, natural regeneration can payback, for example, by providing jobs in eco-tourism, she says.

Regeneration in action

In other parts of the world, the regenerative approach is already paying off. In Rwanda, a project by the Wildlife Conservation Society (WCS) in the Nyungwe Forest National Park is creating jobs for local people in regenerating around 6,000 hectares (23 sq miles) of forest lost to fires set by poachers and wild honey collectors in the 1990s.

The forest had failed to grow back by itself due to the proliferation of ferns that suffocated tree seeds buried in the soil. Previous experiments with active tree-planting were not successful, and proved expensive, says Mediatrice Bana, WCS Rwanda's project lead in Nyungwe.

In January 2020, they began using assisted natural regeneration instead, with 125 local people employed to remove ferns that had flourished in the area, to give tree seeds latent in the soil a chance to germinate by themselves. The process will need to be repeated three times a year, for three years, till the naturally growing saplings are established, Bana explains. But it is already showing signs of success, with new tree shoots visible on the forest floor across 70ha (0.27 sq miles) of the park. The government of Rwanda is now supporting the project so it can be scaled up across more than 5,000ha (19 sq miles) of the remaining deforested area.

Another country where awareness of the power of allowing trees to grow naturally is growing rapidly in Scotland. This has partly come from comparisons of the Scottish Highlands with south-west Norway – both areas are very similar in climate and geology, yet much of the Highlands is treeless, while Norway is covered in forest.

The difference is that land in the Highlands is heavily grazed by deer, while unmanaged natural regeneration has taken place in Norway, where farmland was largely abandoned in the early 20th Century as farmers migrated en-masse to the US, according to Duncan Halley, a wildlife biologist at the Norwegian Institute for Nature Research.

"They're very much used to be a presumption in Scotland that trees could not grow unless they were planted behind fences, and given the grazing pressure from deer in the Highlands that was true in most places," he says. Now that deer have been removed in the Cairngorms by the landowning partners' Wildland, RSPB and NatureScot, trees are growing at over 800m (2,640ft) above sea level, more than 150m (500ft) higher than previously, he says.

In the past year, estates in the Highlands have achieved record values, largely driven not by their potential for shooting deer as had typically been the case, but by their potential for restoration, as rich people and corporations looking to invest in the concept, pointing to a potentially radical shift in thinking, he adds.

Low-hanging fruit

Chazdon is optimistic that the concept of natural regeneration is coming to the fore. There are several existing opportunities where trees could be brought back in this way such as on the outskirts or buffer zones of protected areas, where there is no competition for commercial use. It could also be done in areas populated by indigenous people who already understand how nature regenerates due to their lifestyle of moving herds around different areas. Another option is to do it on land where cattle is kept since there is often sufficient space to keep the same numbers of cattle as well as fence off areas for natural regeneration.

Back at Knepp, thousands of wild trees are now flourishing and providing wildlife habitats and carbon storage that is not just vital, but resilient in the face of change.

"Their random appearance, spontaneously generated from seed and pollen sources near and far, mean they have astonishing genetic diversity. Nothing human beings can do in terms of planting and propagation can ever replicate the genetic diversity of wild trees," says Tree.

"This is the best hope for the survival of our trees in the face of climate change, extreme weather, pollution and disease."

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