Some problem trees (from around the world):

"Problem-cause-effect tree diagram" from Our People, Our Resources
http://www.iucn.org/themes/spg/Files/opor/fig5_1c.html

Another tree, from Eric Mar's Asian Studies and Activism website.
http://userwww.sfsu.edu/~ericmar/tree.html

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
http://www.nearfm.ie/plm114.htm

Description of the problem tree activity, from a BBC article about women organizing in Malawi.
http://www.bbc.co.uk/cumbria/faith/features/anita_payne/06.shtml