What if all trees disappeared?

Trees are everywhere: in parks, gardens and big forests.

Birds nest in them. Squirrels climb them. What if all the trees were gone?

Look out of a window and you might see a tree. There are about 60,000 different kinds of tree on Earth, from tall pines to wide oaks to rainforest giants. Together they make up forests, which are home to huge numbers of animals and plants.

So what jobs do trees really do? What would change if every tree disappeared? This is a thought experiment: nobody is going to remove all the trees, but imagining it helps us see why they matter.

The FAO's State of the World's Forests 2020 counts about 60,000 tree species and finds that forests harbor most of Earth's terrestrial biodiversity. Trees also cycle carbon through photosynthesis, move soil water into the atmosphere and stabilize soil.

This thought experiment removes every tree and follows three threads: air and oxygen, rain and soil, and animals' homes. Sourced facts are labeled as such; consequences are reasoned ideas, not predictions.

Trees use sunlight, air and water to grow. They give off oxygen.

Their roots hold the soil. Their leaves give off water into the air.

Trees use sunlight to turn carbon dioxide and water into food, a process called photosynthesis. The carbon gets locked into their wood, and they release oxygen.

Trees also move a lot of water. Their roots pull it from the soil, and their leaves let it out into the air as vapor. A large oak tree can give off about 151,000 liters of water in a year. And forests are home to most kinds of animals that live on land.

Photosynthesis fixes atmospheric CO₂ into sugars and, in trees, long-lived wood; burning or decay returns it to the air (National Geographic). Transpiration, part of evapotranspiration, is the main route from land to atmosphere; USGS gives about 151,000 L per year for a large oak.

Forests provide habitat for 80 percent of amphibian, 75 percent of bird and 68 percent of mammal species (FAO SOFO 2020, via UN News).

Now let's pretend all the trees are gone. It is only pretend!

Follow the paths in the lab: air, rain and soil, and animal homes.

In our pretend world, every tree is gone overnight. The lab has three paths to explore: what happens to the air, what happens to rain and soil, and what happens to the animals that live in trees.

Each card says whether it is a fact scientists have measured or an idea, a careful guess. Some answers may surprise you: trees are not the only oxygen makers, but they do a great deal else.

Removing all trees is not a realistic scenario, so its consequences are reasoned from what trees do today. Facts are sourced (NOAA, USGS, FAO, National Geographic); everything about the treeless world itself is labeled idea.

One common assumption is tested on the first path: that trees supply most of our oxygen. NOAA's estimate that roughly half of Earth's oxygen production comes from the ocean changes that picture.

Trees are like busy helpers. They store carbon, make shade and hold the soil.

And they give birds, bugs and many other animals a home.

Trees connect many parts of nature. They pull carbon dioxide from the air and store the carbon in wood. Their roots hold soil together, so heavy rain doesn't wash it away. Their leaves put water back into the air, which can later fall as rain.

And forests are full of living things: insects, frogs, birds and mammals that need trees for food, shelter and places to raise their young. Take the trees away, and all of these links break at once.

National Geographic notes that clearing trees releases their stored carbon as CO₂ and leaves fewer trees to absorb it, so the gas accumulates; that tree loss leaves soil more prone to erosion; and that a closed, moist forest can shift to an open, dry one, more vulnerable to fire.

NASA lists forests, with oceans and soil, among the sinks that store greenhouse gases. These are the mechanisms behind each path; their combined effect in a treeless world cannot be calculated from them.

People plant trees and protect forests all over the world.

In some places, forests are even growing back!

Forests are being lost in some places: the FAO reports that about 420 million hectares of trees have been cleared for farming and other uses since 1990. But that is not the whole story. In parts of North America, forests are returning thanks to conservation.

Countries are working together too. The United Nations began a Decade on Ecosystem Restoration in 2021, to restore damaged forests and other wild places.

SOFO 2020 reports about 420 million hectares of forest lost to agriculture and other land uses since 1990, while noting that the target of conserving at least 17 percent of land by 2020 had been met for forests. National Geographic notes that deforestation is not always permanent: in parts of North America, forests are returning through conservation.

FAO and UNEP led the UN Decade on Ecosystem Restoration from 2021, aimed at restoring degraded ecosystems, combating climate change and safeguarding biodiversity.