Zachary,
I do plan to put a tutorial together on how to do this. I basically define the ultimate shape of the tree with spline paths. One defines the trunk. Others are "path" constrained to the trunk. Still others are contstrained to these branches and so on. Each branch is basically a tube with a bone assigned to each spline ring. The bones are constrained to a spline path. To make the branch "grow" I adjust the ease for length and scale the bones for thickness. Leaf clusters are done as self-contained groups, each with its own bone for scaling and constraint to a path. To make the work manageable and a lot easier to animate, I used these basic components to make three basic "characters": a trunk, a long branch, and a short branches. I then replicate the branch models a few times and change the shape of the spline paths and the angle of the leaf groups to create a "cast" of different characters I can assemble into a complete tree. I create a full growing action for each character so that I only need one or two slide posers to animate each branch. Each branch must also be made transparent at the very beginning of the growth action so it remains hidden until its time to "sprout". As efficient as this approach is, it is still a lot of work though. Trees have very simple anatomical structure, but a whole lot of it!
Bill Gaylord