Grantham Institute’s new lecturer Dr Bonnie Waring answers your questions about reforestation.

Large-scale tree planting is an increasingly popular component of global efforts to meet climate targets. However, forests are complex ecosystems, and poorly planned planting efforts can actually increase the amount of carbon dioxide (CO₂) in the atmosphere and increase global warming.

Ecologist Dr Bonnie Waring, who recently joined the Grantham Institute at Imperial College London, has co-authored a new briefing paper on the pros and cons of using trees to fight climate change. During London Climate Action week, she took part in a Q&A discussion with Grantham Institute Co-Director Professor Martin Siegert to explore the nuances of using trees to tackle climate change, and the other potential benefits for health, jobs and the environment. We’ve put together some of your top questions below, and recommend watching the discussion in full here:

How much carbon is in a tree?

This depends on the age of the tree, and the species to which it belongs. For example, giant sequoias can top 80 metres in height, and African baobabs can reach 10 metres in diameter. But most trees are not this big. As an extremely rough approximation, an English oak of about 30cm in diameter will contain about 500kg (0.5 tonnes) of carbon in its wood and leaves. By comparison, a passenger on a flight from London to New York is responsible for emitting about 0.2 tonnes of carbon.

How much potential is there to plant new trees on earth?

Researchers estimate that about two-thirds of land on earth could support trees. However, much of this land is currently used for agriculture. Undertaking a massive effort to promote forest regeneration or plant trees on non-croplands across the globe could sequester up to 100 gigatonnes of carbon, an amount equal to ten years of man-made carbon emissions at current rates. Yet it would take these new forests about a century to capture this quantity of carbon.

Aside from storing carbon dioxide, what other benefits do trees have?

Unlike other ‘negative emissions technologies’ (strategies to absorb CO₂ from the atmosphere, like direct air capture), protecting forests and planting trees provide many additional benefits to the environment and to society. Forests protect biodiversity – it is estimated that 80% of animal and plant life on land is associated with forest ecosystems. Trees regulate the movement of water through ecosystems, reducing flooding and cooling the local climate. Forests also have vital cultural benefits, and we know that access to green spaces is important for our physical and mental health.

What do you value more, existing forests or new forests?

It is critical to emphasise that existing forest should never be cut down to plant new trees. Dr Bonnie Waring

Our first priority should be to conserve existing forests, whether they are old-growth or young, re-growing forests. This is because cutting down forests will release more CO₂ than new trees can absorb. Why is this so? About 40% of man-made CO₂ emissions are dissolved in ocean water. If atmospheric CO₂ concentrations were reduced by planting enormous numbers of trees, some of the ocean’s CO₂ would naturally escape back into the atmosphere – a process known as offgassing – and planting new forests would only reduce atmospheric CO₂ by about 70 parts per million (ppm). By contrast, if all existing forest were cut down and burned, that carbon would pour into both the oceans and the atmosphere, raising atmospheric CO₂ by nearly 300 ppm! For reference, atmospheric CO₂ concentrations are about 400 ppm today, and were about 280 ppm prior to the start of the Industrial Revolution.

This does not mean that tree planting has no value. On the contrary, new forests provide many important benefits: in addition to helping sequester atmospheric CO₂, they generate wood products that support local economies and reduce pressure on existing forest. But it is critical to emphasise that existing forest should never be cut down to plant new trees.

When planting a new forest, what considerations should be taken?

Trees should only be planted in areas that naturally support forest – planting on grasslands, peatlands, or in tundra ecosystems can have unintended consequences that enhance warming. Dr Bonnie Waring

It is important to consider how that forest will impact the total amount of carbon stored over time. If the forest will be harvested, then it is important that most of the wood ends up in products with very long lifespans, such as building construction. This has implications for how the forest should be managed to ensure the appropriate dimensions of timber at harvest.

However, if the forest is not used for production, then the focus should be on restoring the natural structure and function of the ecosystem – monocultures of fast-growing non-native species usually don’t have the largest carbon stores at maturity. Finally, trees should only be planted in areas that naturally support forest – planting on grasslands, peatlands, or in tundra ecosystems can have unintended consequences that enhance warming.

In all cases, we must be aware of the threats the forest may face in the many decades ahead. These could include challenges associated with climate change (droughts, floods, heat waves); invasive pests and diseases; or timber poaching. To reduce these risks, it is best to plant a mixture of genetically diverse, native species, which enhances forest resilience. Close coordination with local land users is also vital to ensure that those who live nearby the forest have a stake in ensuring its continued protection.

How much does UK policy on trees adhere to this guidance?

It is important for policy makers to be in constant communication with local stakeholders and scientists, as our understanding of best practices for forest restoration is constantly improving. Dr Bonnie Waring

UK policy is rapidly evolving in light of the new ‘Nature for Climate’ fund outlined in the 2020 budget. For example, the Department for Environment, Food and Rural Affairs (DEFRA) is currently working on a new England Tree Strategy to guide efforts to protect woodland and establish new forests.

It is important for policy makers to be in constant communication with local stakeholders and scientists, as our understanding of best practices for forest restoration is constantly improving. For example, in the past, quite a few tree plantations in the UK were located on deep peats. Draining of these bogs prior to planting released more CO₂ from soils than growing trees could absorb. Further research revealed this problem and guided development of improved policies for tree planting, an example of how dialogue between scientists and policy makers can improve outcomes.

There are 3.8 million gardens in London alone. Can planting trees in gardens, or creating urban forests, have a significant impact on wider tree planting schemes?

Urban trees can make a small but significant contribution to carbon sequestration. In the United States, for example, it is estimated that urban trees capture about 20 million tonnes of CO₂ each year. Perhaps even more importantly, garden forests provide critical ecosystem services, such as cooling the local environment and providing habitat for urban biodiversity.

Does the private sector have a role to play in the tree-planting effort?

The economics of reforestation efforts are very sensitive to how much land owners are compensated for planting and protecting trees. The private sector can certainly play a role in accelerating planting efforts by subsidising some of these costs. However, this must be done responsibly, considering the ecological and social context of tree planting projects. For example, if trees are viewed simply as carbon absorbing machines, without consideration of their other environmental benefits (such as protecting plant and animal life), this may generate incentives for planting schemes that can damage the environment and worsen climate change. For example, cutting down existing forest to plant monocultures of fast-growing tree species would likely lead to a net increase in atmospheric CO₂ and a loss of biodiversity.

Do you think carbon offsetting schemes, for example, where airlines invest in tree-planting to pay back the carbon emissions flights, are effective?

The utility of offsetting schemes is controversial. At their worst, offsets allow the purchaser to continue ‘business as usual’ operations without actually delivering any reduction in emissions. For example, if CO₂ emitters pay to fund tree planting projects, but the forest is cut down after ten years, then total emissions increase. Because forests must survive and grow for many decades to achieve substantial carbon sequestration, there will always be an element of uncertainty about the longevity of carbon offsets associated with trees. It is always better to directly reduce one’s carbon footprint (e.g. by taking a train instead of flying) rather than purchasing offsets. If offsets are unavoidable, then the purchaser needs to do a great deal of research to ensure the offsets have mechanisms in place to ensure that the trees survive for many decades.

How do you ensure that trees planted in the tropics today remain as forest are not cleared for palm oil or other monocultures?

Tree planting projects, regardless of their location, will not be successful if local stakeholders are not invested in caring for and protecting the trees. This means that there should be an economic benefit to avoiding deforestation, and land used for planting should not displace other economically important activities, like the grazing of cattle. Therefore it is critical that all afforestation efforts are considered in their proper social context.

To find out more, read our briefing: What role can forests play in tackling climate change?

This briefing paper is one of a number of College projects to be awarded funding by UK Research and Innovation through the prestigious Strategic Priorities Fund. Launched in 2018, the Strategic Priorities Fund (SPF) is one of the UK’s largest, publicly funded programmes of work, supporting high quality multidisciplinary and interdisciplinary research programmes which address government priorities. The £830 million SPF facilitates collaboration across the research and innovation community and government.

For the latest news, views and events from the Grantham Institute, sign up to our weekly update newsletter.