After my Great Uncle passed away in 2005, his immediate family allowed to me to have some of the books and magazines that were in his studio workspace. Some of the more interesting things I was able to get were a few issues of 
The Journal of Commercial Art and 
American Artist from the late 1950s.
Here is an article 
Ernest Watson wrote for 
American Artist - 
Special Summer Issue - June - July - August 1956. It covers a few topics on pencil sketching, but I edited it down specifically to what he wrote  about drawing trees.
