Tree planting: Twigging the benefits

6th July 2018


P28 trees

Related Topics

Related tags

  • Agriculture

Author

tolani

Researchers are increasingly recognising the health and economic advantages of tree planting, especially in cities, writes Rick Gould.

Earlier this year, researchers reported that ‘urban forests’ in 86 Canadian cities had removed 16,500 tonnes of air pollutants, potentially averting 22,000 cases of severe respiratory illness. Then, at the start of May, the World Health Organisation (WHO) published data showing that around 80% of people living in cities are exposed to harmful levels of air pollution. Later that month, the UK government published its latest Air Quality Strategy, in which tree planting was included as one way to improve air quality.

Cleaner air is one of the many benefits that trees provide, but it is only during the past decade that researchers have begun to better understand and quantify the multiple benefits – now often called ‘ecosystem services’ – that trees give us. A detailed, global mapping of the types and numbers of trees is an even more recent achievement. This particular project revealed that our planet plays host to about three trillion trees, and has space for another 1.3 trillion. So how valuable are the world’s trees, and what benefits would an extra trillion or so bring in terms of air quality and climate change mitigation?

If levels of air pollution are not bad enough to seriously harm trees, their leaves can do a great job of mopping up pollutants. This was shown in 2009, when researchers in London examined an urban 10km x 10km grid and found that trees removed about 90 tonnes of respirable particulate matter (PM), helping to avoid two premature deaths and two hospital admissions.

The following year, a team at the US Department of Agriculture (USDA) Forest Service in Syracuse, New York, examined how much PM trees removed in 10 US cities. The results showed that the trees in each city captured pollution by the tonne, with the health benefits for the largest city valued at up to $60m (£44.9m). In 2014, the same team quantified the benefits to air quality of forests across the US, and found that trees and forests removed around 17.4 million tonnes of air pollutants in total. This effect, in turn, avoided 850 premature deaths and 670,000 cases of acute respiratory symptoms.

Green shoots

A growing number of other studies are finding strikingly comparable results. In one of these, carried out in 2017, a team from Syracuse in the US and the University of Naples, Italy, used the same methods to assess the benefits of trees in 10 ‘megacities’ – those with a population of at least 10 million. Based on current trends, 10% of the world’s population will live in 40 or so of these megacities by 2030. The researchers found that trees provided several annual economic savings: $482m in health benefits due to pollution removal; $11m as a result of the trees’ water absorption decreasing the need for processing at wastewater treatment plants; $8m from carbon capture, and $500,000 from their cooling effect, which led to lower energy usage. Moreover, the team estimated that if all 10 of the megacities were to plant available spaces with more trees, these benefits would double.

These are the rewards of planting trees in 10 megacities; how many more trees could be planted worldwide to expand these benefits? To answer this question, we need to know how many trees there are already – but until relatively recently, an accurate answer to this question eluded researchers.

Estimating global forest cover from aerial photographs and satellite images is straightforward enough. In 2015, the UN Food and Agriculture Organisation (FAO), for example, relied on such techniques, reporting that 31.6% of land was forested. By 2015, the forested area had dropped by around 1% – equivalent to the size of South Africa.

Despite improvements in technology, tree number estimates have varied wildly. In 2015, a report in the journal Nature described the first research programme to successfully characterise and map global tree density worldwide. It was published by a team led by professor Tom Crowther at Yale University in the US. The research was inspired by Plant-for-the-Planet, an organisation that coordinates and motivates young schoolchildren worldwide to plant trees.

It was started in 2007 by a nine-year-old German boy called Felix Finkbeiner, who wanted to counter climate change with a global campaign of tree planting through a network of children. He soon attracted the attention of the German Environment Ministry and then the United Nations Environment Programme (UNEP). Plant-for-the-Planet then took on UNEP’s Billion Tree campaign to encourage tree planting, and, 10 years on, the organisation’s global register of planting records more than 15 billion trees. It has now set a target of a trillion trees, and the pace of planting is rising as more people get involved – from government bodies through to small groups and individuals.

“Plant-for-the-Planet found that environmental stewardship, and especially tree planting, were great ways to get people involved in tackling climate change – but at the same time, it did not know how many trees there were,” says Crowther. He was working in functional geography, so he and a large team of international collaborators set about compiling a global map of tree distribution and density.

“We developed a model using input data from forest surveys, soil types, topography, climate data and satellite images,” he says. Data from more than 400,000 local forest surveys worldwide, providing records of tree species and forest density, was then used to validate the model, allowing the team to extrapolate the results globally. “It was a case of adding up the pixels to get a global picture,” says Crowther.

In 2015, there were found to be around three trillion trees globally, the previous estimate having been 400 billion. When combined with historical data, the team found that the worldwide tree population had fallen by about 46% since the beginning of human civilisation.

“We estimated that we are losing 10 to 15 billion trees per year,” says Crowther.

The new tree density maps were invaluable to Plant-for-the-Planet, transforming the campaign by providing a context.

Crowther now heads a laboratory of researchers at the Swiss Federal Institute of Technology in Zurich. “Working at a global scale, our aim is to provide as much data, information, guidelines and tools as possible for reforestation, in a targeted, ecologically relevant way,” he says.

For example, there is a clear relationship between diversity and sustainability. Many believe fast-growing monocultures are a good investment and effective for rapid carbon sequestration – but research says otherwise. “The world’s forests generate about $600bn annually for the world’s timber industry,” says Crowther. “If we replaced those forests with fast-growing monocultures, we calculated that they would be worth about $400bn annually.” So diverse forests are better both economically and environmentally.

The Zurich team’s research also indicates that there is room for another 1.3 trillion or so trees globally. “We estimate these extra trees have the potential to absorb 6 gigatonnes (Gt) of carbon annually, while human activities are responsible for around 9Gt of carbon emissions each year.” So the extra trees could make a massive contribution to counter climate change.

Meanwhile, Crowther is heartened by the growing pace of tree planting, especially at local level, and says it’s important to protect what’s already there. “Smaller groups and individuals collectively have a huge impact and inspire others to plant trees, but we also need to take better care of the forests we already have,” he says.

Rick Gould, MIEMA CEnV is a technical advisor at the Environment Agency. He first planted a tree in 1976, and most recently in April, with trees provided by the Woodland Trust

Digging in: organisations involved in tree planting

Image credit: Shutterstock

Subscribe

Subscribe to IEMA's newsletters to receive timely articles, expert opinions, event announcements, and much more, directly in your inbox.


Transform articles

The time is now

Dr Julie Riggs issues a call to arms to tackle a modern-day human tragedy

15th March 2024

Read more

The UK government’s carbon capture, usage and storage (CCUS) strategy is based on optimistic techno-economic assumptions that are now outdated, Carbon Tracker has warned.

13th March 2024

Read more

Multinational corporations are undermining their net-zero commitments with excessive air travel and no plans to reduce ‘the low hanging fruit’ of carbon footprints, a study by Transport & Environment has found.

13th March 2024

Read more

Large businesses across the world are avoiding climate action due to fear they will be called out for getting their work wrong, according to a new Carbon Trust report.

29th February 2024

Read more

A thought-provoking discussion on how storytelling can change the world took place in Central London last night, alongside an exclusive sneak preview of an upcoming IEMA film series.

29th February 2024

Read more

The UK’s net-zero economy grew 9% last year while delivering higher paid jobs than average and attracting billions of pounds in private investment, analysis by CBI Economics has uncovered.

28th February 2024

Read more

A consortium including IEMA and the Good Homes Alliance have drafted a letter to UK government ministers expressing disappointment with the proposed Future Homes Standard.

26th February 2024

Read more

IEMA and the Institute and Faculty of Actuaries (IFoA) have today published up-to-date guidance to help companies and individuals understand climate-related financial information.

22nd February 2024

Read more

Media enquires

Looking for an expert to speak at an event or comment on an item in the news?

Find an expert

IEMA Cookie Notice

Clicking the ‘Accept all’ button means you are accepting analytics and third-party cookies. Our website uses necessary cookies which are required in order to make our website work. In addition to these, we use analytics and third-party cookies to optimise site functionality and give you the best possible experience. To control which cookies are set, click ‘Settings’. To learn more about cookies, how we use them on our website and how to change your cookie settings please view our cookie policy.

Manage cookie settings

Our use of cookies

You can learn more detailed information in our cookie policy.

Some cookies are essential, but non-essential cookies help us to improve the experience on our site by providing insights into how the site is being used. To maintain privacy management, this relies on cookie identifiers. Resetting or deleting your browser cookies will reset these preferences.

Essential cookies

These are cookies that are required for the operation of our website. They include, for example, cookies that enable you to log into secure areas of our website.

Analytics cookies

These cookies allow us to recognise and count the number of visitors to our website and to see how visitors move around our website when they are using it. This helps us to improve the way our website works.

Advertising cookies

These cookies allow us to tailor advertising to you based on your interests. If you do not accept these cookies, you will still see adverts, but these will be more generic.

Save and close