While at MPC in 2002, I wrote tools and did fx animation for many shots in the Whomping Willow sequence for Harry Potter and The Chamber of Secrets.
The tree model in Maya consisted of a main character-like structure, controlled by character animators. From this structure grew many smaller secondary branches too numerous to animate by hand. I wrote a hierarchical cantilever beam solver to simulate the secondary branches. The system was inspired by a Siggraph paper on hair simulation, the main difference being that the forces and torques in this system are accumulated recursively through the hierarchical structure, instead of linearly as in a strand of hair.
The system could respond to standard maya fields, custom plugin fields, and collisions. For example, the branches bounced off the car and ground, while gravity and turbulence were applied to help give the tree a little life while it was static.
As the physics system knew about the accelerations along the branches, it could emit leaves and twigs based on the whomping impacts. Those leaves in turn were instanced to particles, and were made to flutter to the ground realistically with a plugin, called flutter, that calculated drag forces on the instanced geometry as it fell.