Putting ashes on one's head showed mourning (note the change from the judgment in Isaiah 3:16–25). Mourning would turn into joy. In Hebrew, the words for crown of beauty and ashes form a wordplay.
oil of joy: A face covered with oil showed blessing and well-being (see Psalms 45:8; 133:2).
The remnant would begin as small shoots. They would seem weak and unimportant at first.
But over time, they would grow strong and large, like great oak trees (see Isaiah 60:21).