Forests cover nearly a third of all land on Earth, providing vital organic infrastructure for some of the planet's densest, most diverse collections of life. They support countless species, including our own, yet we often seem oblivious of that. Humans now clear millions of acres from natural forests every year, especially in the tropics, letting deforestation threaten some of Earth's most valuable ecosystems.

We tend to take forests for granted, underestimating how indispensable they still are for everyone on the planet. That would quickly change if they all disappeared, but since humanity might not survive that scenario, the lesson wouldn't be very useful by then. As the Once-ler finally realizes in Dr. Seuss' "The Lorax," a crisis like deforestation depends on indifference. "UNLESS someone like you cares a whole awful lot," Seuss wrote, "Nothing is going to get better. It's not."

Indifference, in turn, often depends on ignorance. So to help things get better for woodlands around the world, we'd all be wise to learn more about the benefits of forests — and to share that knowledge with others. That's the goal of events like Arbor Day and the International Day of Forests, a U.N. holiday observed annually on March 21. But forests support us every day of the year, and as deforestation runs rampant around the world, they increasingly need us to return the favor.

In hopes of shedding more light on what forests do for us, and how little we can afford to lose them, here are 21 reasons why forests are so important:

1. They help us breathe.

Treehugger / Christian Yonkers

Forests pump out oxygen we need to live and absorb the carbon dioxide we exhale (or emit). A single mature, leafy tree is estimated to produce a day's supply of oxygen for anywhere from two to 10 people. Phytoplankton in the ocean are more prolific, providing half of Earth's oxygen, but forests are still a key source of quality air.

2. They're more than just trees.

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Nearly half of Earth's known species live in forests, including 80% of biodiversity on land. That variety is especially rich in tropical rainforests, but forests teem with life around the planet: Insects and worms work nutrients into soil, bees and birds spread pollen and seeds, and keystone species like wolves and big cats keep hungry herbivores in check. Biodiversity is a big deal, both for ecosystems and human economies, yet it's increasingly threatened around the world by deforestation.

3. People live there, too.

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Some 300 million people live in forests worldwide, including an estimated 60 million indigenous people whose survival depends almost entirely on native woodlands. Many millions more live along or near forest fringes, but even just a scattering of urban trees can raise property values and reduce crime, among other benefits.

4. They keep us cool.

By growing a canopy to hog sunlight, trees also create vital oases of shade on the ground. Urban trees help buildings stay cool, reducing the need for electric fans or air conditioners, while large forests can tackle daunting tasks like curbing a city's "heat island" effect or regulating regional temperatures.

5. They keep Earth cool.

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Trees also have another way to beat the heat: absorb CO2 that fuels global warming. Plants always need some CO2 for photosynthesis, but Earth's air is now so thick with extra emissions that forests fight global warming just by breathing. CO2 is stored in wood, leaves and soil, often for centuries.

6. They make it rain.

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Large forests can influence regional weather patterns and even create their own microclimates. The Amazon rainforest, for example, generates atmospheric conditions that not only promote regular rainfall there and in nearby farmland, but potentially as far away as the Great Plains of North America.

7. They fight flooding.

Tree roots are key allies in heavy rain, especially for low-lying areas like river plains. They help the ground absorb more of a flash flood, reducing soil loss and property damage by slowing the flow.

8. They pay it forward.

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On top of flood control, soaking up surface runoff also protects ecosystems downstream. Modern stormwater increasingly carries toxic chemicals, from gasoline and lawn fertilizer to pesticides and pig manure, that accumulate through watersheds and eventually create low-oxygen "dead zones."

9. They refill aquifers.

Treehugger / Christian Yonkers

Forests are like giant sponges, catching runoff rather than letting it roll across the surface, but they can't absorb all of it. Water that gets past their roots trickles down into aquifers, replenishing groundwater supplies that are important for drinking, sanitation and irrigation around the world.

10. They block wind.

Farming near a forest has lots of benefits, like bats and songbirds that eat insects or owls and foxes that eat rats. But groups of trees can also serve as a windbreak, providing a buffer for wind-sensitive crops. And beyond protecting those plants, less wind also makes it easier for bees to pollinate them.

11. They keep dirt in its place.

Treehugger / Christian Yonkers

A forest's root network stabilizes huge amounts of soil, bracing the entire ecosystem's foundation against erosion by wind or water. Not only does deforestation disrupt all that, but the ensuing soil erosion can trigger new, life-threatening problems like landslides and dust storms.

12. They clean up dirty soil.

In addition to holding soil in place, forests may also use phytoremediation to clean out certain pollutants. Trees can either sequester the toxins away or degrade them to be less dangerous. This is a helpful skill, letting trees absorb sewage overflows, roadside spills or contaminated runoff.

13. They clean up dirty air.

Treehugger / Christian Yonkers

We herald houseplants for purifying the air, but don't forget forests. They can clean up air pollution on a much larger scale, and not just CO2. Trees absorb a wide range of airborne pollutants, including carbon monoxide, sulfur dioxide and nitrogen dioxide. In the U.S. alone, urban trees are estimated to save 850 lives per year and $6.8 billion in total health care costs just by removing pollutants from the air.

14. They muffle noise pollution.

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Sound fades in forests, making trees a popular natural noise barrier. The muffling effect is largely due to rustling leaves — plus other woodland white noise, like bird songs — and just a few well-placed trees can cut background sound by 5 to 10 decibels, or about 50% as heard by human ears.

15. They feed us.

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Not only do trees produce fruits, nuts, seeds and sap, but they also enable a cornucopia near the forest floor, from edible mushrooms, berries and beetles to larger game like deer, turkeys, rabbits and fish.

16. They heal us.

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Forests give us many natural medications, and increasingly inspire synthetic spin-offs. The asthma drug theophylline comes from cacao trees, for one, while a compound in eastern red cedar needles fights drug-resistant bacteria. About 70% of known plants with cancer-fighting properties occur only in rainforests, yet fewer than 1% of tropical rainforest plants have been tested for medicinal effects. Even just walking in the woods can offer health benefits, too, including stress relief, reduced blood pressure and a stronger immune system. The latter may be partly due to trees releasing airborne compounds called phytoncides, which prompt our bodies to boost the natural killer (NK) cells that attack infections and guard against tumors.

17. They help us make things.

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Where would humans be without timber and resin? We've long used these renewable resources to make everything from paper and furniture to homes and clothing, but we also have a history of getting carried away, leading to overuse and deforestation. Thanks to the growth of tree farming and sustainable forestry, though, it's becoming easier to find responsibly sourced tree products.

18. They create jobs.

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More than 1.6 billion people rely on forests to some extent for their livelihoods, according to the U.N., and 10 million are directly employed in forest management or conservation. Forests contribute about 1% of the global gross domestic product through timber production and non-timber products, the latter of which alone support up to 80% of the population in many developing countries.

19. They create majesty.

Natural beauty may be the most obvious and yet least tangible benefit a forest offers. The abstract blend of shade, greenery, activity and tranquility can yield concrete advantages for people, however, like convincing us to appreciate and preserve old-growth forests for future generations.

20. They help us explore and relax.

Treehugger / Christian Yonkers

Our innate attraction to forests, part of a phenomenon known as biophilia, is still in the relatively early stages of scientific explanation. We know biophilia draws us to woods and other natural scenery, though, encouraging us to rejuvenate ourselves by exploring, wandering or just unwinding in the wilderness. They give us a sense of mystery and wonder, evoking the kinds of wild frontiers that molded our distant ancestors. And thanks to our growing awareness that spending time in forests is good for our health, many people now seek out those benefits with the Japanese practice of shinrin-yoku, commonly translated to English as "forest bathing."

21. They're pillars of their communities.

Like the famous rug in "The Big Lebowski," forests really tie everything together — and we often don't appreciate them until they're gone. Beyond all their specific ecological perks (which can't even fit in a list this long), they've reigned for eons as Earth's most successful setting for life on land. Our species probably couldn't live without them, but it's up to us to make sure we never have to try. The more we enjoy and understand forests, the less likely we are to miss them for the trees.