In the army we had something called a JEWT; Jungle Exercise Without Trees.

You'd designate an area of the North Yorkshire Moors as jungle, then have some one mark out clearings and such like with white mine tape. Finally tell everyone to ignore people who are more than a few yards away and not in a clearing.

Simple! And cheap, you don't have to by all those expensive trees. OR, you could do desert warfare, now that's really easy to do!