Today I saw this photo taken in Mexico City by the artist Gabriel Kuri which shows a tree by a bus stop which is completely covered with chewing gum. I also recalled having read about a photograph by David Byrne of a tree covered in gum in Mexico City (possibly the same one).
I googled "chewing gum tree" and found this flickr group of 92 photos of various trees covered with chewing gum (including the one photographed by Kuri and possibly Byrne).