How do I pick the top object if it is overlapping itself?

I have a game where you click on one tree, it disappears. The goal is to click on all the trees to win.

The problem is that some of the trees are overlapping each other, so when I click on a tree, all the tree sprites underneath it also got clicked. Note that all these trees are essentially the same referenced sprite called "Tree" and on the same layer.

I want to be able to pick the top one on the layer, how do I do that?