After the bird guides were complete in the early 2000s, I searched for another topic to delve into. I wanted to work on another field guide, and one that would be challenging and also a field where I thought I could make a contribution.
Unexpectedly, I found that tree study had a lot of similarities to bird study. They are both around us all the time, and we see them multiple times as we go through an ordinary day. They can be identified from a distance, change with the seasons, display different forms in different conditions, etc. And I had always been a little frustrated by the existing tree guides.
You can read more about my approach and the philosophy behind the guide here: A Modern (holistic) approach to Tree Identification