Ben Amend has published a tutorial on tree creation in Blender on his site, Blender Smoothie. It is based on the Sapling addon and consists of three parts that each look into different aspects of the process.
In this first part of the tree creation tutorial series, learn how to use the sapling addon to generate a realistic 3D tree model.
In this second part of the Blender 3D tree creation tutorial series, learn how to set up the textures and shaders for the leaves and bark.
In this third and final part of the tree tutorial series, learn how to apply gravity to the leaves for a more organic appearance.