The ability to paint trees in Transparent Watercolor has been a basic skill set in many Watercolor painter's repertoire for a couple hundred years or so.
In my own case I learned how to paint in Transparent Watercolor by working from Ted Kautzky's now famous "Ways with Watercolor" book that my mom got for me on my 16th birthday.
But I soon was so enamored by Kautzky's style that I eventually bought every book he ever did. And it was this book here shown with green cover that I used for a long time as I developed and refined my abilities.
Sketching is my first effort to process my artistic observations. During my travels in the Himalayas I'd take every opportunity to try out subjects in my Artist Journal using watercolor.
Watercolor painters traditionally spend time actually studying trees and learning how to paint them. I refer to this as a "Species Specific" skill... where an artist learns enough about trees to be able to paint various species effectively. Kautzky often made trees the focal point of his paintings, as you see here in this painting below of fall light illuminating Maple Trees in New England.
Trees often came into focus in the paintings of John Singer Sargent. It is instructive to see how he'd often focus his skills on such complex subjects as the now famous "Palm Studies", shown here at bottom. It's as though he simply worked the challenge of such a complex and difficult observation of light, shade, design, and values. There's no particular focal point... such very complex brushwork capturing the play of light & form on this challenging subject.