Isaiah speaks of the trees as if they were people who could rejoice. This emphasizes that it is such a great thing that God has stopped the king of Babylon that even nature would rejoice if it could. Alternate translation: "It will be as if even the cyprus trees and the cedars of Lebanon rejoice over you" (See: [[rc://en/ta/man/translate/figs-personification]])
Why they rejoice can be stated clearly. Alternate translation: "rejoice that God has made you powerless" (See: [[rc://en/ta/man/translate/figs-explicit]])
Being laid low represents being made powerless and unimportant. Alternate translation: "Since you have become powerless" (See: [[rc://en/ta/man/translate/figs-metaphor]])