Arbor Day 2018 Facts: The Oldest Tree in the World, How Forests Improve Human Health

Trees are an important part of ecosystems around the world. Most people know that they breathe life into our atmosphere by sucking up carbon dioxide and exhaling our beloved oxygen, but trees have many other benefits as well.

Trees absorb harmful airborne particles, cool the air and provide shade. They also serve as homes for other creatures and, of course, make the area more beautiful. If you appreciate all the work trees do for the planet, here are some facts that will help you celebrate Arbor Day on April 27.

Why do we need trees?

Research has shown that trees help people on an individual basis. Among various health benefits, being surrounded by trees can reduce stress, lower blood pressure and boost a person's mood, give a jolt to the immune system, and enhance sleep quality. Although forests are a good place to get these health perks, parks in urban and suburban areas will also do the trick.

Why do tree leaves change color?

Leaves get their green color from chlorophyll, which trees produce to absorb energy from the sun. Chlorophyll and solar energy combine with the carbon dioxide and water it is absorbing in the process of photosynthesis, the tree gets the nutrition it needs. But as the days get shorter in the fall and the winter, trees produce less chlorophyll, taking away the green from their leaves and leaving behind colors like red, orange and yellow.

Bristlecone pines in California's White Mountains are among the oldest trees in the world. GABRIEL BOUYS/AFP/Getty Images

What is the oldest tree in the world?

Trees have been around for hundreds of millions of years, but the longest-living tree still standing today is believed to be 5,062 years old. It is a Great Basin bristlecone pine—whose species name is Pinus longaeva—which lives in California's White Mountains. The second-oldest, a 4,845-year-old, is from the same species and lives in the same spot.

How old is Arbor Day?

The holiday of the trees goes back to the late 1800s. It all began in Nebraska, shortly after that territory became a state, according to the Arbor Day Foundation. A man named J. Sterling Morton, who had moved from Detroit, proposed taking a holiday to plant trees in 1872 and the idea soon caught on. Other states started celebrating Arbor Day on the last Friday in April, although some also designate Arbor Day on other times of the year based on when it is best to plant trees.

Is Arbor Day celebrated worldwide?

Countries across the globe have their own days for planting or celebrating trees. In Israel, for example, people celebrate the Jewish holiday Tu BiShvat. The name translates to the date on the Hebrew calendar, the 15th day of the month Shevat, which falls in January on the Gregorian calendar. In Germany, close to the same time as the American holiday, there is Tag des Baumes, or Day of the Tree. Japan has its version in the spring known as Greenery Day.

How many trees do we need?

There are tens of thousands of tree species to choose from. A single acre of forested area can pull 6 tons of carbon dioxide out of the air and put out 4 tons of oxygen on an annual basis.

Trees line the Yellowstone River in Yellowstone National Park in Wyoming. REUTERS/Jim Urquhart