Why are trees crucial to human's survival?

How do trees clean the air?  How do trees breathe?  Do trees die in the winter when they shed their leaves? For free home activities sign-up at www.WhoSmarted.com See omnystudio.com/listener for privacy information. See Privacy Policy at https://art19.com/privacy and California Privacy Notice at https://art19.com/privacy#do-not-sell-my-info.