Carrara has the ability to save any trees you create into the
object browser, located in the drawer on the left side of your screen.
To do this, create a tree, and return to the Assemble room.
Next, open up the Sequencer drawer at the bottom of the screen. Also
open up the Object Browser drawer on the left side of the screen.
Next, open up the Sequencer drawer at the bottom of the screen. Also
open up the Object Browser drawer on the left side of the screen.
Drag from the TreeDruid row in the Sequencer...

...to the My Objects section of the object browser.

When the File Info dialog opens, you can give this particular
tree a name.

Voila! It's now in your Objects Browser, and you can
drag the tree into your scene for reuse later.
