Based on the wonderful thesis on LSystem animation by Nicholas Hampshire which can be found here nccastaff.bournemouth.ac.uk/jmacey/MastersProjects/MSc09/Hampshire/userGuideFull.pdf I created this simple implementation of a tree. The tree is created using particles connected with springs. Bend forces keep the shape of the tree intact.

This test doesn't do much fancy, but it shows how the tree keeps its shape when external forces are applied.