Some problem trees (from around the world):
"Problem-cause-effect tree diagram" from Our People, Our Resources
Another tree, from Eric Mar's Asian Studies and Activism website.
How-to description of the problem tree activity, with a link to an image (near the top), from Dublin radion station Near 90.3 FM's Community Media Participatory Learning Manual
Description of the problem tree activity, from a BBC article about women organizing in Malawi.