After exporting a mesh from the SpeedTree Modeler, you can use the included Maya plugin to load it into Maya and recreate the materials as closely as possible.
To install it, in Maya, open the Windows→Setting/preferences→Plug-in Manager menu item. Select the “Browse” button along the bottom and select the script, which can be found at <apps install>/scripts/Maya/SpeedTreeImporter.py.
Note: For the plugin to auto load on the next run, you will need to also check Auto Load next to it in the Plug-in Manager and make sure that you add a line to your Maya.env file similar to the following, so Maya can find it again: MAYA_PLUG_IN_PATH=<apps install>/scripts/Maya
Once the plugin is installed, you will now have new file types available for import in Maya: SpeedTree, SpeedTree for V-Ray, and SpeedTree for Mental Ray, SpeedTree for Redshift, SpeedTree for Arnold (aiSurface), SpeedTree for Arnold 5 (aiSurfaceShader), and SpeedTree for Renderman.
When one of these file types is chosen, it will load a SpeedTree Material (.stmat) file. You don't need to worry about the format of the actual mesh exported from the SpeedTree Modeler, as that information is stored in the stmat file. The only difference between the three importers is the type of materials made after the mesh itself is imported.
If you wish to change how the materials are created, or add completely new material types, the plugin's python script has been designed to be easily extensible in this manner.
All you need to do is add or change one of the classes that inherits from SpeedTreeImporterTranslatorBase. You will need to have a “description” member and a CreateMaterial() function that does the actual creation of the material and returns a shading node, based on the information in the SpeedTreeMaterial class that is passed in. Hooking the material up in the correct place on the mesh is all handled for you elsewhere.
For examples, please see the SpeedTreeImporterTranslator, SpeedTreeImporterVRayTranslator, SpeedTreeImporterMentalRayTranslator, SpeedTreeImporterRedshift Translator, SpeedTreeImporterArnoldTranslator, SpeedTreeImporterArnold5Translator, and SpeedTreeImporterRenderman classes in SpeedTreeImporter.py.