sketchbook drawing : tree studies

I've been working on tree studies to improve accuracy seeing the negative spaces, and placing the lines exactly as I see them. But with this, I've realized my overwhelming need to also be able to generalize mass shapes into more abstract groupings that convince of their nature through value shifts. In order to find more help in thinking of the masses, I created a couple copies of etchings by William Seltzer Rice. Although mine are not etchings, I used a charcoal pencil which helped force me away from details:

And from these studies I see just how powerful the drawing becomes with large masses of great contrast. My finished work has been lacking in the large dark masses, and the difference is quite obvious. Must keep working at it...