The apple tree symbolizes eternal youth. It is derived from the Norse myth of Idun. She is the goddess of spring, of eternal youth. She was never born, and thus, she can never die. She has a tree with magical golden apples. They give youth and health to whoever eats them. All the gods would always come to Idun to eat her apples.
At a certain moment, Idun was abducted by the god Loki, who brought her to the giant Thiassi. The gods suddenly became older and older, because they couldn’t eat Idun’s apples. Everything slowly started dying - winter was coming. Eventually the gods forced Loki to bring Idun back, and so he did. It resulted in a fight with the giant Thiassi, who was eventually beaten.
This myth is very similar to the Greek myth of Persephone, the goddess of beauty, who was abducted by Hades and taken to the underworld (where she became Hecate, the goddess of the underworld). For as long as she was in the underworld, everything on earth died and it became winter.