Verse 30 is a typical Isaianic reversal. Rather than the first-born of the rich, it will be the first-born of the poor who shall have food. Rather than the wealthy being safe, it will be the needy who "lie down in safety."
Using the image of the "root" to symbolize the past rulers of Palestine, the Lord notes that they will be killed with famine. The use of famine here is an intended reversal of the "first-born of the poor shall feed." It is probably that the famine is the result of warfare, for while the "root" is killed with famine, the "remnant," or descendants, will be killed, presumably by the sword.