There is at least one way to export terrains and plants(trees) with leaves; whether it is a satisfactory way remains to be seen. I used a larger hardwood tree I'd designed in Carrara and thought I'd see if I could export it successfully and import it with leaves into another 3d program.

I went into the plant editor and clicked to put checkmarks in the Full Detail Mesh and Show Leaves boxes. Then, while I was still in the plant editor, I clicked on Edit in the main screen>
Convert to Other Modeler. From there a dialogue box opens and gives you the choice of using either the Primitive, or Vertex Modeler. I clicked on the Vertex Modeler...Big Mistake! Carrara locked up completely and when I went into Task manager to see what was going on, Carrara was using more and more memory until it stopped dead at 688,368K. My Task Manager even froze up, until I opened up another Task Manager Window and shut Carrara down.

I must have been feeling brave because I went right back(after doing a restart) and tried the other option, the Primitive Modeler. Everything went swimmingly it seemed. I exported the converted tree as a.obj file and it didn't take Carrara too long to do the conversion. When I looked at the .obj model I was a little shocked to see that the file size was 190MB!

I don't have XSI, or any other rendering app. on hand to test with, so I used Bryce5.1 to import the mesh to see what it would look like. After a half-hour the mesh finally imported successfully but without textures. The .obj file had a 399kb mat. file, but it was all black, so I assigned some
default Bryce materials to the various tree parts.
Here is a Carrara render of the original source tree...