Forests cover 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 as well as 1.6 billion human livelihoods, yet humans are also responsible for 32 million acres of deforestation every year.
The United Nations declared March 21 the International Day of Forests in late 2012, part of a global effort to publicize both the value and plight of woodlands around the world. It was first celebrated March 21, 2013, nestling in between the U.N.'s International Day of Happiness on March 20 and World Water Day March 22. (It's also near tree-centric Tu B'Shevat in January 2016 and Arbor Day in April).
In honor of this seasonal focus on trees and forests, here's a list of 21 reasons why they're important:
1. They help us breathe
Forests pump out the oxygen we need to live and absorb the carbon dioxide we exhale (or emit). Just one adult leafy tree can produce as much oxygen in a season as 10 people inhale in a year. Plankton are more prolific, providing half of Earth's oxygen, but forests are still a key source of breathable air.
2. They're more than just trees
Nearly half of all known species live in forests, including 80 percent of biodiversity on land. That variety is especially rich in tropical rain forests, from rare parrots to endangered apes, but forests teem with life around the planet: Bugs 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.
3. People live there, too
Some 300 million people live in forests worldwide, including an estimated 60 million indigenous people whose survival depends almost entirely on native woods. Many millions more live along or near forest fringes, but even just a scattering of urban trees can raise property values and lower crime.
The canopy towers over a coastal-plain forest in Italy's Nazionale del Circeo. (Photo: Nicola/flickr)
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
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
Large forests can influence regional weather patterns and even create their own microclimates. The Amazon, 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.
Erawan Falls flows through a rain forest in the Tenasserim Hills of western Thailand. (Photo: Shutterstock)
8. They pay it forward
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
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.