Symbolic: In this part if the allegory, the focus is on the branches that were in the nethermost parts. These branches carry the genetic information from the mother tree, but not the direct strength of the roots. The branches from the original tree will therefore be added into the new trees in the nethermost region.
While the gathering of Israel is the event that is described by the image of the nethermost branches coming in to the main tree, the preaching of the gospel to all the world is the event imagined by the transferal of the branches from the main tree to the nethermost region. Thus in the end, all will be brought back together, with the unique benefits of the historical covenant brought together with those of the adopted covenant. In this last period, the gospel will come back together as one, with no separation into different parts of the world.