Welcome and be warned! Pixel art is addictive as hell
Wow I really like your style
I normally don't like single pixel noise but you pull it off and you've got the perfect amount of little highlights scattered around the place without going overboard.
If you stick around you will learn all sorts of other cool techniques but I hope you don't loose your cool style in the process.
Crits:
Lose the nasty cyan background colour and go for grey or low saturation colour at 50% bright to get the best balance in your work.
Colours are great but pure grey is horrible. Look up "hue shift". Do you understand HSB mode in colour picker? I personally prefer this mode as it gives so much control.
Stone highlights are to strong but suits the spade handle so I'd say add a colour.
Some of your highlights conflict with your light source. Look at the vines, they look awesome but the highlight needs to move a few pixels left and shading on tree trunk should be adjusted accordingly.
Some of your shadows don't line up also.
IMO you should use the shadow angle on the gravestone (45 degrees up and right) as this will give strong light and dark to your objects and cast the shadows partially behind objects so they consume less tiles and cause less interference than going sideways.
Give the tree some bigger roots to make it more interesting. If you have layers, draw a grid over each tile them some ellipses around the trunk base to help you line up the perspective. Its a common mistake that the bottom of a tree looks kinda flat like cardboard cutout where it meets the ground. Use the grass blades to blend the roots in use dark green to suggest bumps where the roots are going into the ground.
Top of tree is all wrong in terms of shading and u have a bad reference.
Refer to something like this where the lighting is offset...
https://s-media-cache-ak0.pinimg.com/236x/00/9e/1a/009e1aeb06100c534290c79d8f0a3b7e.jpgSome other good references with strong lighting (important) to get a feel for the clumps but obviously you need to adjust the light source to your scene...
http://fc07.deviantart.net/fs70/i/2014/230/e/7/tree_on_street_png___dsc_0557_by_annamae22-d7vofp7.pnghttps://s-media-cache-ak0.pinimg.com/originals/2b/31/b4/2b31b4035aad78ed526633ea454ff7a4.pngNow its good ur attempting to make clumps but I don't think you're going to get away with rendering the leaves using noise.
Who knows you might pull it off but usually it takes time to draw the leaves using a bunch of little controlled clusters of pixels which cast little shadows onto each other.
Here's an example of leaf clustering I did not so great and wouldn't suit your scene but hopefully u get something out of it (i forgot to pause the last frame sorry)
http://i.imgur.com/1W1rBw5.gifThis guy does it much better obviously but looking at his stuff can bring a tear to the eye
http://fool.deviantart.com/art/Dragon-rider-149899562