NO MATTER WHERE they are located or what types of trees grow in them, Earth's forests are a valuable natural resource. From the massive canopies of rain forests in Costa Rica to the great pine forests of the French Alps, the trees that grow in forests form complex ecological systems that help keep the planet's climate and atmosphere in balance.

More than 30 percent of earth's land surface is covered with forests, and they play a vital role purifying the air—so much so that forests are often called the "lungs of the earth." Like all green plants, trees "breathe in" carbon dioxide. They function as carbon sinks, or reservoirs that are capable of storing vast amounts of carbon, and they "exhale" oxygen back into the atmosphere. Forests also cleanse the air by absorbing impurities such as dust, smog, and other forms...