I would like to help you, but i am a bit confused, but I am giving it ago.
So you wanted to have grass then a tree behind that grass, I am guessing.
The way you can get this done is my layers, have two different layers which both have tile maps on them, I would copy and past the same size and position of both so they are the same, then when making your tree look behind the grass, draw the tree on the lower layer, then continue drawing the game on the higher layer.
If you still are confused I can make a video explains it better.
Good Luck!!!