partical system without mesh contact

i want to make a forest scene for my swing game, but i want a forest scene with different trees positions every time; so i thought partical system, but i can really have the tree branches going into each other, so how?
by the way is there a way so create simple tree ( a cylinder with smaller cylinders coming out perpendicular to it)

You still can’t use blender’s particle system in the game engine as far as I am aware. The best way to do this I think would be to create some sort of random level generation system which would use a grid plane to place tree objects at random vertices. You would need to use python for this. Another way you could do this without python is to add an empty, then animate its position to where trees can spawn. Then add a random sensor connected to an add object actuator to add objects at random times.

Example of the 2nd method:
https://dl.dropboxusercontent.com/u/38384163/blender_treespawner.blend

This is just a basic example. You can create your own, more complex and fine-tuned version of this concept.