It's hard to overstate the importance of trees. Their debut more than 300 million years ago was a turning point for Earth, helping transform its surface into a bustling utopia for land animals. Trees have fed, housed and otherwise nurtured countless creatures over time — including our own arboreal ancestors.

Modern humans rarely live in trees, but that doesn't mean we can live without them. About 3 trillion trees currently exist, enriching habitats from old-growth forests to city streets. Yet despite our deep-rooted reliance on trees, we tend to take them for granted. People clear millions of forested acres every year, often for short-term rewards despite long-term risks like desertification, wildlife declines and climate change. Science is helping us learn to use trees' resources more sustainably, and to protect vulnerable forests more effectively, but we still have a long way to go.

Earth now has 46 percent fewer trees than it did 12,000 years ago, when agriculture was in its infancy. Yet despite all the deforestation since then, humans still can't shake an instinctive fondness for trees. Their mere presence has been shown to make us calmer, happier and more creative, and often boosts our appraisal of property value. Trees hold deep symbolism in many religions, and cultures around the planet have long appreciated what a walk in the woods can do.

We still periodically pause to honor trees, with ancient holidays like Tu Bishvat as well as newer tributes like Arbor Day, the International Day of Forests or World Environment Day. In hopes of helping that spirit linger longer throughout the year, here are a few lesser-known facts about these gentle, generous giants:

1. Earth has more than 60,000 known tree species.

Jabuticaba or Brazilian grapetree, Plinia cauliflora Brazil's many native trees include jabuticaba, whose fruits grow directly on its trunk. (Photo: Adriano Makoto Suzuki/Flickr)

Until recently, there was no thorough global census of tree species. But in April 2017, the results of a "huge scientific effort" were published in the Journal of Sustainable Forestry, along with a searchable online archive called GlobalTreeSearch.

The scientists behind this effort compiled data from museums, botanical gardens, agricultural centers and other sources, and concluded there are 60,065 tree species currently known to science. These range from from Abarema abbottii, a vulnerable limestone-bound tree found only in the Dominican Republic, to Zygophyllum kaschgaricum, a rare and poorly understood tree native to China and Kyrgyzstan.

Next up for this area of research is the Global Tree Assessment, which aims to assess the conservation status of all of the world's tree species by 2020.

2. More than half of all tree species exist only in a single country.

Dragon's blood tree The dragon's blood tree is a vulnerable species endemic to Yemen's Socotra archipelago. (Photo: sunsinger/Shutterstock)

Aside from quantifying the biodiversity of trees, the 2017 census also highlights the need for details about where and how those 60,065 different species live. Nearly 58 percent of all tree species are single-country endemics, the study found, meaning each one naturally occurs only within the borders of a single nation.

Brazil, Colombia and Indonesia have the highest totals for endemic tree species, which makes sense given the overall biodiversity found in their native forests. "The countries with the most country-endemic tree species reflect broader plant diversity trends (Brazil, Australia, China) or islands where isolation has resulted in speciation (Madagascar, Papua New Guinea, Indonesia)," the study's authors write.

3. Trees didn't exist for the first 90 percent of Earth's history.

Earth is 4.5 billion years old, and plants may have colonized land as recently as 470 million years ago, most likely mosses and liverworts without deep roots. Vascular plants followed about 420 million years ago, but even for tens of millions of years after that, no plants grew more than about 3 feet (1 meter) off the ground.

4. Before trees, Earth was home to fungi that grew 26 feet tall.

Prototaxites A rendering of Prototaxites as it may have looked 400 million years ago. (Image: Mary Parrish/NMNH/University of Chicago)

From about 420 million to 370 million years ago, a mysterious genus of creatures named Prototaxites grew large trunks up to 3 feet (1 meter) wide and 26 feet (8 meters) in height. Scientists have long debated whether these were some kind of weird ancient trees, but a 2007 study concluded they were fungi, not plants.

"A 6-meter fungus would be odd enough in the modern world, but at least we are used to trees quite a bit bigger," study author and paleobotanist C. Kevin Boyce told New Scientist in 2007. "Plants at that time were a few feet tall, invertebrate animals were small, and there were no terrestrial vertebrates. This fossil would have been all the more striking in such a diminutive landscape."

5. The first known tree was a leafless, fern-like plant from New York.

Several kinds of plants have evolved a tree form, or "arborescence," in the past 300 million years or so. It's a tricky step in plant evolution, requiring innovations like sturdy trunks to stay upright and strong vascular systems to pump up water and nutrients from the soil. The extra sunlight is worth it, though, prompting trees to evolve multiple times in history, a phenomenon called convergent evolution.

Wattieza tree

The earliest known tree is Wattieza, identified from 385 million-year-old fossils found in what's now New York. Part of a prehistoric plant family thought to be ancestors of ferns, it stood 26 feet (8 meters) tall and formed the first known forests. It may have lacked leaves, instead growing frond-like branches with "branchlets" resembling a bottlebrush (see illustration). It wasn't closely related to tree ferns, but did share their method of reproducing by spores, not seeds.

6. Scientists thought this dinosaur-era tree went extinct 150 million years ago — but then it was found growing wild in Australia.

Wollemia nobilis Wollemia nobilis still exists in a few rainforest hideouts, but it's critically endangered. (Photo: Wikimedia Commons)

During the Jurassic Period, a genus of cone-bearing evergreen trees now named Wollemia lived on the supercontinent Gondwana. These ancient trees were long known only from the fossil record, and were thought to have been extinct for 150 million years — until 1994, when a few survivors of one species were found living in a temperate rainforest at Australia's Wollemia National Park.

That species, Wollemia nobilis, is often described as a living fossil. Only about 80 mature trees are left, plus some 300 seedlings and juveniles, and the species is listed as critically endangered by the International Union for Conservation of Nature.

While Wollemia nobilis is the last of its genus, there are also still other middle Mesozoic trees alive today. Ginkgo biloba, aka the ginkgo tree, dates back about 200 million years and has been called "the most ancient living tree."

7. Some trees emit chemicals that attract enemies of their enemies.

Eurasian blue tit with a caterpilar in a tree Songbirds provide valuable pest control for many trees. (Photo: Sander Meertins Photography/Shutterstock)

Trees may look passive and helpless, but they're savvier than they seem. Not only can they produce chemicals to combat leaf-eating insects, for instance, but some also send airborne chemical signals to each other, apparently warning nearby trees to prepare for an insect attack. Research has shown that a wide range of trees and other plants become more resistant to insects after receiving these signals.

Trees' airborne signals can even convey information outside the plant kingdom. Some have been shown to attract predators and parasites that kill the insects, essentially letting an embattled tree call for backup. Research has mainly focused on chemicals that attract other arthropods, but as a 2013 study found, apple trees under attack by caterpillars release chemicals that attract caterpillar-eating birds.

8. Trees in a forest can 'talk' and share nutrients through an underground internet built by soil fungi.

redwood trees at Lake Tahoe under a night sky Redwood trees rise toward the night sky at Lake Tahoe, California. (Photo: Asif Islam/Shutterstock)

Like most plants, trees have symbiotic relationships with mycorrhizal fungi that live on their roots. The fungi help trees absorb more water and nutrients from the soil, and trees repay the favor by sharing sugars from photosynthesis. But as a growing field of research shows, this mycorrhizal network also works on a much larger scale — sort of like an underground internet that connects entire forests.

The fungi link each tree to others nearby, forming a huge, forest-scale platform for communication and resource sharing. As University of British Columbia ecologist Suzanne Simard has found, these networks include older, larger hub trees (or "mother trees") that may be connected to hundreds of younger trees around them. "We have found that mother trees will send their excess carbon through the mycorrhizal network to the understory seedlings," Simard explained in a 2016 TED Talk, "and we've associated this with increased seedling survival by four times."

And, as Simard recently told CNN, mother trees may even help forests adapt to human-induced climate change, thanks to their "memory" of slower natural changes in past decades or centuries. "They've lived for a long time and they've lived through many fluctuations in climate. They curate that memory in the DNA," she said. "The DNA is encoded and has adapted through mutations to this environment. So that genetic code carries the code for variable climates coming up."

9. Most tree roots stay in the top 18 inches of soil, but they can also grow above ground or dive a few hundred feet deep.

mangrove tree on a beach in Thailand Many mangrove trees have stilt roots to help with breathing and stability. (Photo: Sayam Trirattanapaiboon/Shutterstock)

Holding up a tree is a tall order, but it's often achieved by surprisingly shallow roots. Most trees don't have a taproot, and most tree roots lie in the top 18 inches of soil, where growing conditions tend to be best. More than half of a tree's roots usually grow in the top 6 inches of soil, but that lack of depth is offset by lateral growth: The root system of a mature oak, for example, can be hundreds of miles in length.

Still, tree roots vary widely based on species, soil and climate. Bald cypress grows along rivers and swamps, and some of its roots form exposed "knees" that supply air to underwater roots like a snorkel. Similar breathing tubes, called pneumatophores, are also found in the stilt roots of some mangrove trees, along with other adaptations like the ability to filter up to 90 percent of salt out of seawater.

On the other hand, some trees do extend remarkably deep underground. Certain types are more prone to grow a taproot — including hickory, oak, pine and walnut — especially in sandy, well-drained soils. Trees have been known to go more than 20 feet (6 meters) below the surface under ideal conditions, and a wild fig at South Africa's Echo Caves has reportedly reached a record root depth of 400 feet.

10. A large oak tree can consume about 100 gallons of water per day, and a giant sequoia can drink up to 500 gallons daily.

Angel Oak tree on Johns Island, S.C. The Angel Oak, a roughly 400-year-old southern live oak on Johns Island, South Carolina, produces an impressive 17,200 square feet of shade (1,600 square meters) under its iconic gnarled branches. (Photo: Mike Ver Sprill/Shutterstock)

Many mature trees require a huge amount of water, which may be bad for drought-stricken orchards but is often good for people in general. Thirsty trees can limit flooding from heavy rain, especially in low-lying areas like river plains. By helping the ground absorb more water, and by holding soil together with their roots, trees can reduce the risk of erosion and property damage from flash floods.

A single mature oak, for example, is able to transpire more than 40,000 gallons of water in a year — meaning that's how much flows from its roots to its leaves, which release water as vapor back into the air. The rate of transpiration varies during the year, but 40,000 gallons averages out to 109 gallons per day. Larger trees move even more water: A giant sequoia, whose trunk may be 300 tall, can transpire 500 gallons a day. And since trees emit water vapor, large forests also help make it rain.

As a bonus, trees have a knack for soaking up soil pollutants, too. One sugar maple can remove 60 milligrams of cadmium, 140 mg of chromium and 5,200 mg of lead from the soil per year, and studies have shown farm runoff contains up to 88 percent less nitrate and 76 percent less phosphorus after flowing through a forest.

11. Trees help us breathe — and not just by producing oxygen.

Amazon rain forest The Amazon rainforest spans about 40 percent of South America and holds 16,000 tree species. (Photo: Shutterstock)

About half of all oxygen in the air comes from phytoplankton, but trees are a major source, too. Still, their relevance for humans' oxygen intake is a bit hazy. Various sources suggest a mature, leafy tree produces enough oxygen for two to 10 people per year, but others have countered with significantly lower estimates.

Yet even without the oxygen, trees clearly offer plenty of other benefits, from food, medicine and raw materials to shade, windbreaks and flood control. And, as MNN's Matt Hickman reported in 2016, city trees are "one of the most cost-effective methods of curbing urban air pollution levels and combating the urban heat island effect." That's a big deal, since more than 3 million people die worldwide each year from illnesses linked to air pollution. In the U.S. alone, pollution removal by urban trees is estimated to save 850 lives per year and $6.8 billion in total health care costs.

There's also another notable way trees can indirectly save lives by breathing. They take in carbon dioxide, a natural part of the atmosphere that's now at dangerously high levels due to the burning of fossil fuels. Excess CO2 drives life-threatening climate change by trapping heat on Earth, but trees — especially old-growth forests — provide a valuable check on our CO2 emissions.

12. Adding one tree to an open pasture can increase its bird biodiversity from almost zero species to as high as 80.

female black-naped blue flycatcher feeding her chicks Trees provide food, housing and other benefits for a wide range of songbirds, like this family of black-naped blue flycatchers nesting in a fork between two branches. (Photo: Super Prin/Shutterstock)

Native trees create vital habitat for a variety of wildlife, from ubiquitous urban squirrels and songbirds to less obvious animals like bats, bees, owls, woodpeckers, flying squirrels and fireflies. Some of these guests offer direct perks for people — such as by pollinating our plants, or eating pests like mosquitoes and mice — while others bring subtler benefits just by adding to local biodiversity.

To help quantify this effect, researchers from Stanford University recently developed a way to estimate biodiversity based on tree cover. They recorded 67,737 observations of 908 plant and animal species over a 10-year period, then plotted those data against Google Earth images of tree cover. As they reported in a 2016 study, four of the six species groups — understory plants, non-flying mammals, bats and birds — saw a significant biodiversity boost in areas with more tree cover.

They found that adding a single tree to a pasture, for example, could raise the number of bird species from near zero to 80. After this initial spike, adding trees continued to correlate with more species, but less quickly. As a stand of trees approached 100 percent coverage within a certain area, endangered and at-risk species like wildcats and deep-forest birds began to appear, the researchers report.

13. Trees can lower stress, raise property values and fight crime.

spring at Shinjuku Gyoen National Garden, Tokyo, Japan Urban trees, like these at Tokyo's Shinjuku Gyoen, offer more than just ambience. (Photo: Wayne0216/Shutterstock)

It's human nature to like trees. Just looking at them can make us feel happier, less stressed and more creative. This may be partly due to biophilia, or our innate affinity for nature, but there are also other forces at work. When humans are exposed to chemicals released by trees known as phytoncides, for example, research has shown results such as reduced blood pressure, reduced anxiety, increased pain threshold and even increased expression of anti-cancer proteins.

Considering that, maybe it's little wonder trees have been shown to raise our evaluations of real estate. According to the U.S. Forest Service, landscaping with healthy, mature trees adds an average of 10 percent to a property's value. Research also shows urban trees are correlated with lower crime rates, including things from graffiti, vandalism and littering to domestic violence.

14. This tree has been alive since woolly mammoths still existed.

Methuselah bristlecone pine Methuselah, a bristlecone pine, has been living in this spot for 4,848 years. (Photo: Rick Goldwasser/Flickr)

One of the most fascinating things about trees is how long some can live. Clonal colonies are known to endure for tens of thousands of years — Utah's Pando aspen grove dates back 80,000 years — but many individual trees also stand their ground for centuries or millennia at a time. North America's bristlecone pines are especially long-lived, and one in California that's 4,848 years old (pictured above) was considered the planet's oldest individual tree until 2013, when researchers announced they'd found a another bristlecone that sprouted 5,062 years ago. (The last woolly mammoths, for comparison, died about 4,000 years ago.)

To intelligent primates who are lucky to have 100 birthdays, the idea of a brainless plant living for 60 human lifetimes evokes a unique kind of respect. Yet even when a tree does finally die, it still plays a key role in its ecosystem. Dead wood has huge value for a forest, creating a slow, steady source of nitrogen as well as microhabitats for all kinds of animals. As much as 40 percent of woodland wildlife depends on dead trees, from fungi, lichens and mosses to insects, amphibians and birds.

15. A large oak tree can drop 10,000 acorns in one year.

The nuts of oak trees are hugely popular with wildlife. In the U.S., acorns represent a major food source for more than 100 vertebrate species, and all that attention means most acorns never get to germinate. But oak trees have boom and bust cycles, possibly as an adaptation to help them outfox the acorn-eating animals.

During an acorn boom, known as a mast year, a single large oak can drop as many as 10,000 nuts. And while most of those may end up as a meal for birds and mammals, every so often a lucky acorn gets started on a journey that will carry it hundreds of feet into the sky and a century into the future. For a sense of what that's like, here's a time-lapse video of an acorn becoming a young tree:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.