Old growth forests are forests that are in their natural state, having not been damaged by human activities such as logging, clearing for agriculture or road-building. All of the world’s rainforests still in their natural state are old growth forests. So are boreal forests, which span the entire globe in cold climates in the Northern Hemisphere, mostly in Canada and Russia.

Old growth forests are made up of trees that are hundreds and sometimes even thousands of years old. In fact the oldest living thing on earth is a bristlecone pine tree in Nevada, which is 4,700 years old. That means it was already alive when the Egyptians were building the pyramids! However, there are more than just ancient trees in old growth forests. They support a complicated web of life that includes an unimaginable number of younger trees, plants, mammals, birds, insects, bacteria and all kinds of other life.

Millions of plants and animals call old growth forests their home. Tropical rainforests are full of jaguars, toucans, parrots, gorillas, tarantulas and all kinds of other animals you’ve probably never heard of like aye-ayes and okapis. There are so many fascinating animals and plants in tropical rainforests that millions haven’t been studied or even named yet, and others are still waiting to be discovered. Scientists often find hundreds of different species of insects living on a single tree. In fact, over half of all of the earth’s animal and plant species live in tropical rainforests. Tons of animals live in old growth forests in temperate and boreal regions as well, such as beavers, black and brown bears, deer, moose, caribou, foxes, raccoons, skunks, rabbits and many bird species.

Many people live in old growth forests too. In fact, indigenous, or native, peoples have lived in forests for thousands of years. Today, hundreds of distinct indigenous groups with their own languages and cultures still live in old growth forests around the world. Indigenous peoples rely on the forest for food, medicine, shelter, and clothing. They live what is called a sustainable existence, meaning that they use the land without doing harm to the plants and animals that also call old growth forests their home. As a wise indigenous man once said, "The earth is our historian, our educator, the provider of food, medicine, clothing and protection. She is the mother of our peoples."

One of the most important things old growth forests do is help to prevent global warming, or the rising of the earth’s temperature. Global warming has the potential to cause the widespread extinction of plant and animal species, the flooding of coastal areas, and an increase in dangerous storms like hurricanes. It also threatens to destroy forests, which evolve slowly over time and would have difficulty adjusting to a fast change in the climate. However, protecting old growth forests can help prevent global warming. Forests remove carbon dioxide, which is a gas that is the main cause of global warming, from the air and store it inside of themselves. Rainforests also recycle their own rain, making lots of clouds in the atmosphere that keep some of the sun’s heat from reaching the ground. This helps to keep the planet cooler. When rainforests are destroyed, these clouds disappear and the carbon that all of the plants were storing goes back into the atmosphere. This traps more of the sun’s heat here on earth, causing the weather to be much hotter.

Some logging companies that make money from cutting down trees sometimes say that an old growth forest is ‘dead’ and that it should be cut down and replanted with young trees. These forests are not ‘dead,’ but have in fact reached a natural balance. A forest is a living system, with never ending cycles of growth and decay. In an old growth forest, these natural processes haven’t been disrupted for thousands and even millions of years. Every part of the of the old growth forest ecosystem has an important role to play in keeping the forest healthy. Dead trees, both on the ground and standing (know as snags), provide homes for many kinds of animals, especially those that build nests in the hollow areas of the tree. Dead wood is also the habitat of lots of insects that play an important role in the food chains of other animals. When a giant old growth tree dies and falls to the ground, perhaps in a windstorm, it creates an opportunity for young trees to receive sunlight and grow. This mix of healthy giant trees, younger growth and fallen trees and snags is what makes up a natural forest.