You could create a simple cylinder and use a transparent texture, static objects will not use semi but should work with full transparency, and place this within the trunk.
Alternately you could separate the trunk and leaves, make the leaves dynamic with no collision and the trunk static.
Using the collision mode above may depend on which order you link the trunk and leaves before export from the modeler you could try switching this out if you want to do it this way.
Just an aside, you shouldn't need to delete bin and dbo files for any texture or script changes you make but its important for any model/texture file name changes or any changes to the mesh itself.
The bin file contains only reference to which texture/model/script is used and the dbo is a conversion of the .x model so they are not affected by changes to texture or script unless the name of the texture/script file is changed in some way.
Hope this makes sense. It may not seem like much but when developing or creating media the time lost deleting bin and dbo with every small change adds up
