Trees give us shade, construction materials, are turned into boats, prevent mudslides, clean the air, produce fruits to mention a few things that came to mind.
Other facts I learned from the official site:
-Forests are home to 300 million people
-Forests cover 31% of land area
-Forests are home to 80% of our earth biodiversity
Into the woods for Green Day # 170
(* Illustration is photo of "Corydalis cava (+ Robinia pseudoacacia) - mixed forest (48°09' N 16°57' E)" taken by HermannFalkner on March 20, 2011 and shared via Sokol stream on Flickr)