11 surprising facts about trees

There are around 3 trillion trees on Earth, according to a global study led by researchers from Yale University. That includes over 60,000 known tree species, more than half of which are endemic — meaning they're found in only one country. Brazil, Colombia and Indonesia are home to the most tree species. The bad news: there are 46% fewer trees today than at the start of human civilization.

Trees clearly can't uproot themselves and move, but their population centers can shift over time in response to climate pressures. A study looking at 86 trees species between 1980 and 2015 in the eastern United States found that 73% moved west, where rainfall is increasing. Others headed to the poles, apparently to escape heat. On average, they moved about 16 kilometers (10 miles) per decade.

Trees not only give us shade, they can also mitigate extreme temperatures by transpiring — absorbing the sun's radiation and releasing water into the air through their leaves. Urban areas can become sweltering "heat islands" in summer. But a 2019 study from the US found that tree canopy cover of 40% or more could lower summer temperatures in cities by as much as 5 degrees Celsius.

Trees draw CO2 from the atmosphere and are therefore crucial in the fight against climate change. They can also use their leaves to filter particulate matter and toxic gases like nitrogen dioxide and sulphur dioxide from the air. A recent UK study found that silver birch, yew and elder trees could reduce particles at rates of 79%, 71% and 70% respectively.

Trees can reduce our stress levels and help us feel happier and healthier. Several studies have shown that spending time in nature, or even just looking at trees or flowers through a window, can lower blood pressure, boost the immune system, improve sleep, reduce depression and anxiety, and even speed up recovery after surgery.

Forests have their own communication systems — almost like an underground internet —that allows trees to swap nutrients and send warnings about drought or disease. They interact via networks of soil fungi, known as mycorrhizal networks. Research by ecologist Suzanne Simard has shown that paper birch (pictured) and fir trees use this system to send water, carbon and nutrients back and forth.

Trees can't flee if their leaves are being devoured by a hungry herbivore. But what they can do is release chemicals — volatile organic compounds — into the air to warn nearby members of the same species there's a threat in the area. Studies show that other trees respond by boosting their own production of anti-herbivore toxins, which, in the case of acacias (pictured), makes their leaves bitter.

When besieged by bugs or parasites, some species, including apple trees, and tomato, cucumber and lima bean plants, release compounds into the air to alert the attackers' predator. Most often, these predators are insects. But a European study showed that trees infested with caterpillars also put out chemical signals to attract caterpillar-eating birds, such as the great tit (pictured).

Trees are the oldest living organisms on Earth. One individual can survive hundreds, even thousands of years. According to the OldList, an officially dated record of ancient trees, the oldest known living individual is a bristlecone pine in California's White Mountains. Named Methuselah, it's around 4,850 years old. Its exact location is kept a secret to protect it from vandals.

A photograph can't really do justice to the world's tallest trees: redwoods. The tallest known living specimen is a coast redwood called Hyperion measuring 115.85 meters (380 feet) — more than Big Ben or the Statue of Liberty. The giant, discovered in 2006 in California, is believed to be several hundred years old.

California is also home to a giant sequoia named General Sherman, thought to be the biggest living tree in terms of volume. It stretches to a height of 83.8 meters and is 7.7 meters in diameter. The title of the world's widest tree goes to the Arbol del Tule (pictured), a Montezuma cypress in the Mexican state of Oaxaca. It has a diameter of 11.6 meters and circumference of 42 meters.