Why did Frodo feel he had to leave Middle Earth at the of ROTK?

[Zigûr]

Originally Posted by Lotrelf

One final question- What about the Trees, did they ever recover? If not, how can the effects of Morgoth's darkness be said temporary?

The Trees never recovered. After Yavanna retrieved the fruit and flower which would become the Sun and Moon, "then the Trees died, and their lifeless stems stand yet in Valinor, a memorial of vanished joy."

The reason we don't consider Aman to be Marred like the rest of Arda is because unlike the rest of Arda the matter of Aman is not combined with Morgoth's spirit, something he did in Middle-earth. So the Trees were destroyed, but they and the land at large were not actually corrupted.