I played a course last weekend that also had these christmas trees all over. It was like having little pockets of OB everywhere on the course, because these were so dense, if you hit your ball into it, you couldn't even find your ball, hence technically one should play it as a lost ball.
However we also just played them as a lateral hazard...
And its beyond me why if they insist on having these trees on the course, they can't cut the lower branches off...