Trees - Digital pencil sketches

I was a bit fed up with studying anatomy those days, therefore I decided to challenge myself a bit and to fight my worst enemy : vegetation. Starting by its most iconic leader : trees I searched for books, and after looking at several of them, my choice went for "On Drawing Trees and Nature" by J. D. Harding. This is quite an old book, since it was published in 1846 ! It focuses on pencil drawing, but I have good hopes that later I'll be able to translate its techniques to digital painting. The first tree is referenced from the book and the two others are from imagination.