Nature and Biodiversity

What is Arbor Day and why is it important?

The US-based Arbor Day Foundation has helped grow 500 million trees in the past 50 years.

The US-based Arbor Day Foundation has helped grow 500 million trees in the past 50 years. Image: Unsplash/Johann Siemens

Dan Lambe
President, Arbor Day Foundation
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One Trillion Trees

This article is part of: Centre for Nature and Climate

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This article was originally published on 28 April 2023 and was republished on 24 April 2024.

  • Arbor Day is an annual celebration of tree planting and is a national holiday in the US.
  • The US-based Arbor Day Foundation has helped grow 500 million trees in the past 50 years.
  • Trees can help mitigate global warming and promote health and well-being in urban areas, provided they are planted in suitable world regions.
  • The World Economic Forum's platform is part of a global movement to grow and restore a trillion trees by the end of the decade.

At the last count, there were more than three trillion trees in the world. A major study back in 2015 by a Yale University-led team of international researchers said this was around seven times more than had previously been estimated.

What’s been described by Yale as “the most comprehensive assessment of tree populations ever produced” also said that human activity such as deforestation and land-use change was causing 15 billion trees to be lost each year. It estimated that the number of trees in the world had almost halved since the beginning of human civilization.

A tree planting celebration known as ‘Arbor Day’ first took place in the United States almost 300 years later
A tree planting celebration known as ‘Arbor Day’ first took place in the United States almost 300 years later Image: Our World in Data

Trees are vital to the health of the planet and its people

Trees play a vital role in regulating climate and weather patterns by absorbing vast amounts of carbon dioxide (CO2). The United Nations (UN) estimates that forests contain around 50% more carbon than is present in the atmosphere.

When trees are cut down and forests are destroyed, the carbon they contain is released. This is a major contributor to global warming and the climate crisis. Deforestation accounts for more than 20% of global CO2 emissions, according to the UN.

Trees absorb CO2 from the atmosphere which combats global warming
Trees absorb CO2 from the atmosphere which combats global warming. Image: Global Forest Watch

And trees bring many other benefits, from helping to provide food security and protecting livelihoods, to promoting health and well-being.

A word of warning on planting trees

As well as sequestering carbon dioxide, a tree's dark leaves absorb heat from sunlight. When planted in snowy or desert areas – with light-reflecting surfaces such as snow or sand – tree leaves absorb more sunlight than their surroundings, which could contribute to the warming effect.

As part of the study, scientists mapped the planet's climate-positive and climate-negative regions for tree planting. They found that trees planted in arid, desert regions or snow-covered Arctic regions would, on balance, worsen global warming.

Arbor Day net climate impact
Image: Nature Communications

In figure a above, the orange zones show where planting trees could have a net negative climate impact, while the blue areas show climate-positive regions. A similar picture is revealed by figure b, which shows purple areas where changes in albedo (heat absorbed through tree leaves) is greater than the maximum level of carbon dioxide stored.

The positive news, however, is that most planting projects currently in operation or at the planning stage are concentrated in regions where they can help combat the climate crisis.

What is Arbor Day?

Around the world observances of the importance of growing trees have been taking place for centuries, with the first documented celebration of trees taking place in Spain way back in 1594.

A tree planting celebration known as ‘Arbor Day’ first took place in the United States almost 300 years later. In 1872, an estimated one million trees were planted in Nebraska on its first Arbor Day. More than 45 states and territories were celebrating Arbor Day each year by 1920.

Forest restoration initiatives have spread around the world, such as Kenya's Green Belt Movement, which was founded in 1977 by Nobel Peace Prize Laureate Professor Wangari Maathai.

Today Arbor Day is a national holiday in all US states. It usually takes place on the last Friday in April, but can vary to coincide with the best planting weather in different areas of the country.

500 million trees milestone

The Arbor Day Foundation is a nonprofit organization using reforestation and urban forestry to solve issues of climate change, community, and biodiversity. Our organization celebrated its 50th anniversary in 2022, and has helped plant around 500 million trees in 50 countries in that time.

Last year, our organization announced an ambitious target of growing the same number again in just five years. We will focus on areas both in the US and abroad that have the greatest need and will benefit most from new trees.

Trees are helping to solve some of the most pressing issues people and the planet face today — and the world need trees now more than ever. Our goal to grow 500 million trees is ambitious in scale, but the real impact of this work comes with the focus of where these trees will be grown.

Our Foundation uses powerful data-gathering tools to target areas and neighbourhoods of greatest need. These can highlight the communities that are most nature-deprived and also experience other socioeconomic disadvantages.

Technology can identify which areas will benefit from trees.
Technology can identify which areas will benefit from trees. Image: Arbor Day Foundation

Research has shown that trees can cool towns and cities by up to 10 degrees. This helps to mitigate extreme heat which kills more Americans than all other natural disasters combined each year. It also disproportionately affects underserved communities.

This isn't the only health issue trees can help with - neighbourhoods with more trees tend to have lower childhood asthma rates. And there's a mental health benefit too - the more connected people feel nature, the greater their sense of well-being.


What is the World Economic Forum doing about nature?

The trillion trees challenge is a global platform to mobilize, connect and empower the global reforestation community to conserve, restore and grow a trillion trees by 2030. It is part of the World Economic Forum’s drive to accelerate nature-based solutions and was set up to support the UN Decade on Ecosystem Restoration.

The platform's US Stakeholder Council, of which I am a member, is a community of US-based public and private sector organizations. It ensures operations and technical support equitably meet the needs of all the US Chapter stakeholders.

Each year, American forests and forest products capture and store around 15% of US CO2 emissions from fossil fuel combustion. But we could capture almost twice that amount if we grow more trees in a responsible way and manage our existing forests better, research suggests.

And there's growing consensus among Americans that this is the right thing to do.

A new Harris Poll survey, commissioned by the Arbor Day Foundation, also showed that 94% of people believe trees are good for the planet, 88% agreed that now is a critical time to be replanting our nation’s forests and 91% agree that trees help fight climate change.

For us, that signals people really do understand the power of trees, and realise that they provide a solution to some of the world’s greatest challenges.

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