Roots don't do that. Your thickness of the trunk at the tree should be barely larger at the base. The angle along the left side looks parabolic, but at the very most, I'd use a rational like smooth.
http://www.analyzemath.com/Graphing/GraphRationalFunction.html
http://www.analyzemath.com/Graphing/GraphingQuadraticFunction.html
http://www.fs.fed.us/r10/spf/fhp/hazard/evaluation.htm [pictures of interest]
Looks very nice, however this would not work on actual NES, because you have more than 3 colours per 16x16 blocks (the actual limitations of the hardware).
The light blue color in the clouds jump out a lot. Maybe trade that color for shade you could also use on the frame to give it a shinier look?
Updated
The tree is large on the base because it is supposed to be as ancient tree, so its really big