HMm, it could use a little colour though. That's easy enough, make a linear scale from 0 at the bottom to 1 at the leaves and interpolate between a nice brown and a green.
That's looking a little better now! It could do with a bit of tweaking, the green is coming in a bit too early, but that's all right I can apply a function to the linear scale to get one that has a gradient slow, then fast at the end.
The next step was to increase the feeling of 3D - a nice ground plane would do this nicely:
After that what I plan to do is add grass (maybe) and clean up the code (a lot). Still, it's not bad for a graphics system that only uses straight lines.