Reckon there will be a better person to answer this than lil o me. But I wanted to ask. Are you using a sharp stick with the ole wet n dirty stuff or a digital tablet ?
There was a tutorial by Delgondantelius which was fab and all about using inks and so on for mapping and specifically covered trees. So you can do an advanced search on that if I dont beat you to it and edit that link in.
Edit -- [Award Winner] Tutorial & Guide for Hand Drawn Maps (Pen & Ink w/ Photoshop)