I'd use a particle system in blender but could be too heavy for poser, the sapling tool in blender was broken for a while I'm not sure if its fixed yet (I rarely use it )

I'd aim for "proper" leaves on a close tree and intersecting planes on a distant tree to keep the poly count down  but it does depend on the comp you have and the detail you need, poser gets so sluglish with higher polycounts