This is surprisingly difficult. There are quite a few songs about the joys of nature, of which "Country Life" ("I like to rise when the sun she rises, Early in the morning...") is probably the best-known, but few that are really about environmental preservation.

There are a fair number about environmental damage, such as "Mighty Mississippi" (about the 1927 Mississippi flood) and "The Miramichi Fire" (Laws G24) and "The Johnstown Flood" (Laws G14), but not much about preventing it.

Of course, there is always "Woodman, Spare That Tree." :-)