Oh, goodness, the tree. o_o That tree probably took as long to paint as the girl did. (I know, I'm supposed to be being encouraging here, aren't I?) And of course I can't find the tutorial I learned to do trees from... I'll see if I can describe the process myself.
Basically, the deal with tree branches is to approach them as if they have knees. Trees have knees. Whenever a branch comes out they'll be knobbly. Imagining that you're working digitally, you want to draw a rough tree with a solid brush... probably with size pressure sensitivity instead of opacity, and sort of build the general shape of the tree... the trunk, and spread the branches up and out, depending on what kind of tree you're drawing. I usually try to do a couple large branches and then some smaller ones branching off... no real pattern, I just mostly wing it.
Anyway, every branch and turn is like drawing a knee. Knobs. Trees are bumpy. There shouldn't be straight lines.
Once I was happy with the shape of the tree and tidied up the base unshaded sillhouette of it, I made a new layer overtop of that as a clipping mask (so that it wouldn't go away from the basic brown doodle). I did the shadows of the branches first, because that was based on the light direction and switched back to a bigger fuzzy brush to sort of get a general feel of the bright and dark places on the tree. Once I'd mapped out where the light was going to fall on the tree, I made a new layer over that (not on a clipping mask) and switched back to a hard (opacity sensitive instead of size sensitive) brush that was about the general size of the style of bark I wanted... and drew a ton of lines. Every line individually. It took FOREVER, but I couldn't find a bark brush that looked decent and didn't know how to approach making one that would do what I wanted. So really, the only suggestion I have would be if you're working digitally, set the brush to opacity pressure, and if you're using Photoshop, remember that [ and ] change the size of the brush and you can pick up colors with alt, and it might save some time to draw birch bark instead of creepy tree bark because birch bark just involves doing a few lines around and the odd larger black mark, whereas creepy tree bark involves drawing about a million lines.
I took a look at your gallery and since it seems you do draw digitally and with a similar style to me, here's my suggestion.
Approach bark like you would hair, if it was really short, straight hair. It's a whole bunch of lines that shade a tube. Just remember that trees are not straight, and they are bumpy, rough, and knobbly.
That's basically how I approached it... and that's not even touching on leaves! Hope that helps!
And thanks! I really appreciate the kind words.