Right, that makes sense, thanks.

As well as deleting the trees, though, you also deleted the piece of grass around them which was already mapped, and then replaced it with something much more coarsely detailed.

Firstly, this loses history of others’ contributions to the map (https://wiki.openstreetmap.org/wiki/Keep_the_history), and secondly it’s losing geometric detail.

If the deletion was accidental, note that the map editor has an undo button to allow you to undo mistakes while editing.