The allegory begins with the efforts to save the tree. The tree is the house of Israel, and the various elements of the tree will represent different aspects of the history of the house of Israel. The first, and most important part, is that the olive tree is old and has begun to decay. This is a theme of many prophecies of Israel just before the Assyrian invasion. We have the record of Isaiah preaching that the house of Israel has strayed from Jehovah. Clearly, Zenos preached the same message in the north.
The efforts to save the tree begin with the first level of care. It is pruned and fertilized. That suggests that there are religious reforms attempting to return to a more correct form of worship. In the southern kingdom, we have the reforms of Hezekiah as an example of this pruning and nourishing.
The result is that there is some success, but most of Israel continues. The top of the tree is the larger amount of the leaves and branches, and therefore the location where there should be fruit. That continues to perish. More is required.