Malbim: If a tree was not cut, just its strength to nurture ceased due to old age, and its roots aged, its stump will die and dry. It cannot sprout again by itself, for it has no moisture or nurturing. It can live again via water, since the stump died but the root is still alive, just it is weak.

Devar Eliyahu (9): Shoresh is below; its place is called Eretz. Geza are the small roots tangled in the land. Its place is called Afar. Both of these have hope. If it will be cut, afterwards it will grow, as if it was planted anew. Also if it died, and water comes to it, it will sprout - unlike man.