Sunlight filters through a forest in Union Wood near Ballygawley, Ireland. (Photo: Mark Carthy/Shutterstock) 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 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 ox...