Rendering order in a 2d game

Greetings JGO.

I’m currently making a 2-dimensional game in java using lwjgl. It contains objects(such as trees) which should cover whatever they stand in front of(determined by their y-coordinates). This is done to create the illusion of a 3rd dimension. Example seen in this image from the game, where the player is placed ‘behind’ the tree:

I am however adding entities to the game(mobs, items etc.) and these are stored in a list.
My question is: How do i figure out in what order to render the list of mobs/entities and the trees?