I, too, cheated at pixel art in order to make an edit in an attempt to demonstrate something!
Above is a hideous and horrendously rushed attempt to show you how I "build" my trees in a volumetric, 3d-feeling way. I'm six or seven billion miles away from natural-feeling volumetric trees, but hey, that's a battle for another day. On with the C&C.
First things first. I chucked this into graphicsgale to see what you had going on, ninja colour wise. I noticed that you kinda sorta built a treetrunk and branches inside your tree, which is AWESOME... except for the part where they don't show up at all. They're a shade of almost-near-pitch-black-but-not-quite that graphicsgale is clever enough to pick out, but the human eye isn't. I'm not sure if that was intentional or not, but in either case it'll probably need to be addressed eventually.
Second, I'm willing to bet the reason your screenshot has a dark and dingy feel to it is because you seem to be in the habit of working on black backgrounds. When in doubt about what background your object will be on in the game, go with a 50% gray. Pure black or pure white will affect your perception of values!
Ignore whatever the heck I did to the trunk, but do pay attention to the fact that I was working with a definite light source. You seem to be shading most of your things by putting the highlight at the highest point. Though this does work to a certain extent, it is probably going to cause you some problems when you get to large objects (such as your friend, the tree) that need a light source to determine any kind of shape or volume.
In games, having the light beam straight down from heaven (as NaCl shows) is common and wise.
Hueshifting from a cool colour (Blues, deep greens, etc) colour to the colour of your lightsource (sunlight is yellow!) is almost a pixel art "cheat" to making things look good. Vegetation especially seems to be affected by the yellowness of sunlight, and will feel a lot more like vegetation with a very yellow-green highlight.
Yup! That's a bunch of stuff to think about! You've got a bunch going for you in your screenshots and trees already (I actuallly very much like one of your dirt tiles! No joke!) and I'm looking forward to seeing what your tree will turn into!
-edit-
Good glory, that's an embarassingly ugly tree! Adrian, I apologise for the ugly tree. I tried, but it was very early in the morning, and I'd forgotten to sleep. That's my excuse, and I'm stickin' to it
