Symbolic: The botanical process is that the branches from the nethermost region that were part of the original tree will be brought back to the original tree. The focus of the nethermost branches was most recently the Lamanites and Nephites, but there is no reason to exclude any of the nethermost branches including those that were described s being in the poor and poorer places.
The event to which this refers is the gathering of Israel. The process of bringing scattered Israel back to the main roots is the image and reference. In the case of the Lamanites and Nephites, however there is an additional possibility, which is that the reason that this grafting will be beneficial is that the Book of Mormon comes as part of this re-grafting, and the words of those far flung branches of Israel will bring strength back to the original tree.