The forests look flat on top, and the presentation of the 'trunks' seem lacking, somehow.

Why not try Tolkien styled forests. Here's a link to an old map of mine that uses what I call Tolkien style forests, I called the Slave Trader's Map. These weren't brushes, I actually drew that little forest entirely by hand, tree by tree. Although this style of forest is completely different from yours, perhaps you'll want to maintain the simplicity of your style, but pair up your verticle lines closer to appear as wider trunks, then vary the space between trunks.