Physically based rendering
SpeedTree 8 added a new physically based renderer (PBR) in the Modeler viewport. PBR attempts to more accurately model the way light works in the real world, allowing you to use measured surface values of real-world materials and have them look correct in any lighting situation.
With this change, the material settings and lighting setup have been changed from previous versions of SpeedTree.
Additional reading
The field of physically based rendering is always evolving, and there is much research into making highly realistic, but fast, algorithms. The following pages include information on the ideas behind PBR, why things work the way they do, and how to feed a PBR art pipeline successfully.