I actually rather like the tree--you did a good job preserving the layering and feel of the canopy in the final version--but it's also out of perspective.  Your grass is in the 3/4 / top-down view so often used in classic RPGs, while the tree is viewed side-on as it would be in a sidescroller.  This makes it look sort of like it's lying down on the ground, which is a bit odd. 

This is a much better effort  ,

Your tree is constructed well, although it does look like you might have traced it or at least been a little slavish in your reproduction. Copying things is how you learn and ability with a pencil is invaluable but try to figure out why your reference looks the way it does and apply that information to the creation of your work.

Using tiles in profile (from the side) in a top down game is unusual, not to say it can't work. Have a look at some screenshots from popular RPG's and look at the way perspective is handled. If you're seeing the grass from above shouldn't you be looking at the tree in the same way? 

You're right that attention needs to be paid to what to draw and what to leave out when working at a small size. This is something that will come with experience, learning from your mistakes.

Try drawing out what it is you want to achieve on paper (the whole scene), then transferring those ideas to pixels.

Good luck