Russia, owing to its gigantic size, has diverse ecosystems, including polar deserts, tundra, forest tundra, taiga, mixed and broadleaf forest, forest steppe, steppe, semi-desert, and subtropics. About half of Russia's territory is forested, and it has the world's largest forest reserves, which are known as the "Lungs of Europe"; coming second only to the Amazon rainforest in the amount of carbon dioxide it absorbs.