This shouldn't be too hard to do with any pro level animation package. Break it up into 3 scenes. First make your summer scene. For
the transition through fall, use the summer scene to start. I am afraid that you'll have to make the leaf objects that fall as separate objects in most packages. Animate their fall to the ground.
As for the color shift, the easiest way if you can't animate colors is to make a texture map that fades to fall colors from summer and animate
it on your geometry. Using maps on the lights will help as well. Then for your winter scene, use the fall scene as base. Make snow "objects" that start scaled to zero
and grow throughout the scene mimicking the way that snow accumulates in patches. The real time will be spent in making the snow objects that grow everywhere in
the scene. Use the same trick from fall to get the lighting and colors right. The final thing that is really going to sell the scene is to duplicate the "freakishly fast" look
of time lapse motion photography. Whether you animate the objects to move faster or play with the frame rate of the rendered frames is up to you. Remember however that
to sell photorealism, your background has to be just as good as the tree itself. Include some back action that the audience will never be concious of, but will feel that "something
isn't right" if it is missing. Have you watched some stock time lapse footage? If not, do so before you plan your scene - it will really help.
Supa-Fly Animan Guy