Does my Bamboo Harvester produce 5 gold for a nonhuman if i have 3 forests of Shinomen in play?
When one of your Forests is paying for a Nonhuman card: it produces one additional Gold.
multiple Forests of shinomen in play
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Yes, they are cumulative.