The Trees fell during the First Age; the Downfall of Númenor and the Changing of the World happened towards the end of the Second Age, almost three thousand years later, and involved Sauron, not Morgoth, who'd been defeated and exiled from the world at the end of the First Age.

As for Legolas, he was a Sindarin elf born in the Third Age. He never saw the Trees, and had never been in Aman at the time of The Lord of the Rings.

The only named characters in The Lord of the Rings (other than ones mentioned in songs and legends) who had ever seen the trees were Galadriel, possibly Celeborn and Glorfindel, technically Gandalf and Saruman (provided the Istari count as the same people as the Maiar they were back in Aman), and, maybe, Tom Bombadil and Goldberry, but who knows, really, with those two. (I don't think Sauron ever was in Aman, at least during or after the time of the Trees).