With apologies to those who have not played Merion.
This must sound like a strange topic coming from someone who, for the most part, believes that less-is-best when it come to trees on golf courses. Even when they are well away from the fairway and/or green, they still restrict air flow and light. This is especially problematic for good green conditions.
However, Merion has done a good job of removing my hated Stupid Trees over the years and leaving those that are legitimate bogey makers following a poor shot.
Removal Examples: The trees on the right edge of the quarry that blocked the right half of the 16th green from the right half of the fairway are gone. The trees that form the chutes off the 10th and 12th tee boxes have been trimmed back. The trees at the top of the quarry on #18 are gone. If you hit your tee shot where they used to be, you end up on the wrong side of the hole (and in the rough), anyway.
Retention examples:
What I like about the trees that remain is that they obviate the need for deep rough (which I abhor) and they almost always make the recovery shot some kind of bail out.
1) Tall pine trees to the right of #1;
2) Pine trees left of the fairway on #1;
3) Thee big trees between #2 and #5 - very important for making the recovery from a big hook off #2 quite difficult;
4) Tall pines on the right of #4 at the top of the hill;
5) Overhanging trees on the right of #7;
6) Pine trees to the left of #8 near the left bunker;
7) Grove of trees across the creek to the left of #11;
Pine trees to the right of #14 near the 15th tee box;
9) Tall pine grove short/right of the 15th green;
10) Grove of trees right of the fairway bunkers on #16.
Without any of them, the hole in question would require either deep rough (yuck) or some other deterrent (I don't know what they would be).
I don't see Necessary Trees very often. Merion seems to have quite a few. Fortunately, they were not removed when the East Course was restored to 1930.