Hi Sam,
A couple of observations. The tree is the point of focus, there aren't enough values in the tree. It's an old tree because it's twisted which you did well but the surface is missing age; so you need more values by adding more dark tones. The leaves also need more value. Add some negative space to the leaves.
It looks like you drew this drawing with the same pencil. which limits you when adding depth.
Get rid of the horizon line it doesn't put the mountain in the background. The mountains aren't interesting enough to justify being in the drawing. Maybe put something in the mid ground between that tree and mountain; like an old cart or smaller trees in the distance. Because it looks like tha mountain is up against the tree.
You didn't create a light source. That can be remedied by putting a shadow of the tree on the ground.
Also the grass needs values its too even. Add some dark and light patches. Dark in the background and light closer to the foreground
Add some clouds and it will help put the mountain in the background and adds interest.
Also if you can, purchase this book: Drawing Nature by Stanley Maltzman Here is the link on Amazon.com
http://www.amazon.com/Drawing-Nature-Stanl...9901&sr=1-1That's alll this guy does is draw nature with pencils. Truly amazing stuff. It will give you the tools you need to kick it up a notch.
Hope this helps,
Don't give up!
Respectfully,
peterd