ACtually, the shading is very similar. The light is usually upper right, but can be just above the tiles in order to lower the count of 8x8 tiles that are needed.
Now, onto critique. The first tree looks decent. It has a nice outline, and details are fairly solid. One thing that can be improved is the dimensions. I have not measured it exactly, but it looks to be about 26 pixels wide. That will do if the tree is only going to be used a few times, but if it is a border tree, try making it between 29-32 pixels. The details, though fairly good, could be touched on a little more, to add more detail. The palette works well with this tree, but the top could be a little darker. Now for the smaller tree. It has a good outline, but it needs a lot of improvement. For one, the details need completely reworked. It looks like just a bunch of lines drawn inside the tree. Try making thse lines look like leaves, and shade accordingly. The top of the tree also shares the same problem with the bigger tree. For that third thing, to be comlpetely honest, I have no idea what it is. Now the bush looks great. The only thing I would do to it if I were you would be to add a small trunk to the bottom of it. The sign looks fine the way it is, except for one thing. Add some depth to it, which can be accomplished by adding two rows of pixels to the top of it. Now onto the teepee. It looks great, except for one majer thing: perspective. The teepee, right now, appears to be right in front of you. The correct perspect would be to see the top and the front of it. All you really need to do to it is round off the bottom.
Overall, good attempts. Don't let my comments get to you, I am a perfectionist. Continue spriting tiles!