The imagery of the poem now returns to the original theme of the vineyard. The root of the vineyard is Israel, specifically the historic Israel. Because Israel has turned away from their God, the vineyard is burned to stubble.
Anthropological note: One of the practices of ancient agriculture is the "slash and burn" technique, where a portion of the field would be cut down and burnt over. This technique is actually a means of replenishing the soil, and is used to combat the effects of working a field until the nutrients are drained from the soil. In the agricultural practice, the burning removes that which was there but is done to prepare the land for the plantings and harvests to come. Particularly in the context of the allegory of the vineyard, the burning should not be seen as a destruction (though old stubble, old non-producing vines are clearly destroyed) but rather as a process of creation, where new will rise from old.