There’s tons of ways to reduce triangle counts in the SpeedTree Modeler, but a big one that’s often overlooked is the Simplify tool. Simply put, Simplify reduces the number of polygons in your model by replacing complex geometry with images mapped to simple meshes. This means that you can start with a high-detail tree (2-300K or even 1M+ triangles) and use that geometry to render custom texture maps to use as the basis for a lower-detail model.
Case in point: The 300,000+ triangle model below was used to generate much lower-detail versions. Rendering and placement of the new textures is done automatically, with some artistic input from the user. You can see that simplifying at different levels has a dramatic effect on both the appearance and polygonal complexity of the result.
This feature is ideal for mobile and lower-end platform development and will be included in our UE4 and Unity 5 offerings.
For details on how this works, take a look at our Simplify documentation.