Update. Mom emailed to ask why the oak was felled. I reviewed my pictures, including one I took the morning after the first picture, which clearly shows the oak as severely damaged. In fact, even the first picture shows the damage.
Another trip to Folsom, this time by car. Although the drive is long, there are some benefits — for example, being able to strap my bike to the back.
This morning, I rode my bike to work along Lake Natoma. I had to stop a moment to look at the oak trees across the field. Beautiful!