the foliage bothers me, but i like the trunk.

It would help tremendously if the darker shadows were applied to the bottom of the leaves

 Thanks for all the help and references! I see what you mean. The color choices do make it look flat.

There is some promise here just some extra work will make this really shine.

I have two personal tweaks that I would suggest. First, most of the color values are a little too close together which make it seem flat, give it some more range from highlights to shadow at it will really pop. The other suggestion I have is to give the tree a more interesting shape and form, right now it resembles a lollipop. If it bulged out in different ways it will make the silhouette more eye catching. As for tutorials, there are as many ways to draw trees as there are people to draw them. I like these examples, you may take from them what you like.

http://www.pixeljoint.com/pixelart/65898.htm

http://www.pixeljoint.com/pixelart/10466.htm

http://www.pixeljoint.com/pixelart/17613.htm

Here is a tutorial from Kiwinuptuo for a reference.

http://kiwinuptuo.deviantart.com/art/Pixel-Art-Tutorial-Tree-196679114?q=gallery%3AKiwinuptuo%2F27054576&qo=2

Good luck!

 The trunk is so-so but what could really use the most work on would probably be the leaves (though I can understand how you said you made this awhile ago).