How could it be? Growing up, there was a large horse chestnut tree that was a meeting point for all the kids in the neighborhood. It was such a huge part of our lives that it became an icon for our childhood, as several others have agreed with me as adults. It's gone now, as it grew old and diseased and someone cut it down. But it was a very significant tree for many people in the town for many years. I doubt, however, that it, or so many other trees that had similar impact on people, would ever make a Wikipedia list. There are just too many trees.

It can never be. There are many notable trees, but some of them will never have a Wikipedia article.

No, but you can add anything missing if you have a source!

Does the tree have a Wikipedia article about it? If not you can add it. If it does, you can add it to the list.

Wikipedia allows anyone to edit and contribute! (although many users don't know that and a smaller than miniscule amount of users actually do.)