RE: A question of trees on the battle field....
For our backyard set up, we took downed branches (about 1-2" thick) and just screwed them to scrap 1x4s and 1x6s.DFifferent lengths and individually. They were cut about 2' high, so they make a nice forest, but they are far too heavy to transport like that. They also could be knocked down by the tanks, which made for some cool battles, ie. knocking trees down to block roads, but like the real thing you (or at least Squidward) couldn't predict which way they'd fall.
I would think 1/4" ply would make nice bases for the Xmas tree parts, set in groups.