Mia Zinn was a member of her middle school ecology club, had planted a reflection garden and had implored public officials to preserve a local woods, and when she became terminally ill, she wanted to become a tree. So, the day after she died, her father, Chris Zinn, visited Serenity Ridge Natural Burial Cemetery and Arboretum in Baltimore County, a 45-minute drive from the family’s home in Abingdon, Md. He was drawn to a wooded area that opened up to a wedge of western sky.