There are two themes in this verse. One is of the completion of the destruction, and the other is the hope of future deliverance.
There is some difference in the translation revolving around the concept of the "return."
Gileadi:
And while yet a tenth of the people
Remain in it, or return,
They shall be burned…
NIV:
And though a tenth remains in the land,
It will again be laid waste…
It appears that the sense should be that the remaining tenth will also suffer "burning" rather than have this refer to a small return that will occur in the future. This also fits with the current known history of the destruction of the northern kingdom, for none "returned" after their dispersal to Assyria as the southern kingdom will after their coming captivity in Babylon.
The "tenth" that remains is not a randomly chosen number, but a symbolic one. This tithe of the people will remain after the rest are carried out. While it cannot be certain, it is possible that this "burning" may be related to the poetic burning seen in chapter 15:24. That burning was analyzed as a possible burning of renewal, and perhaps this tenth may also be undergoing a burning of purification, for the next phrases give hope coming from this seen of utter destruction.
The ending sentence of verse 13 is also perhaps clearer in other translations:
Gileadi:
But like the terebinth or the oak when it is felled, whose stump remains alive, so shall the holy offspring be what is left standing.
The reference here is much clearer to an ancient world in touch with the natural world around them that for modern man. The terebinth and the oak are referenced because the readers would have understood the message that Gileadi phrases as "whose stump remains alive." Each of these trees, even when felled to a stump, can give off new shoots and grow again. Thus the Lord is telling Isaiah that although this terrible destruction will surely come, there will be a very small remnant that will remain, and will be faithful. While this verse does not describe the regrowth of the trees, the ultimate redemption of Israel is the clear message.