There are several different ways to build trees in a 3D package: starting from the simpler low poly objects such as the billboards used in video games (simple planes mapped with tree images on a transparent background) to slightly complex objects where a trunk mesh is attached to a foliage mass made of little alpha textured planes, each one representing a leaf or even a twig, to more complex and heavy meshes where every little branch and leaf is actually modeled.
For this two-parts tree shader recipe, we will use a model coming from the many environment assets of the CG short film Big Buck Bunny, the second Open Movie produced by the Blender Foundation. All the movie assets are free to be downloaded, distributed, and reused, even for commercial projects, because they are licensed under the Creative Commons Attribution 3.0 (http://creativecommons.org/licenses/by/3.0/).
The general shape of the tree and of the leaves as well is pretty "toyish". This is because...