Andy Dahl plans his vacations around visiting trees, much to his family’s chagrin. He uses his shotgun to shoot off witch’s brooms—genetic deformities that grow high up in trees—so he can see what they look like up close. His oldest daughter knew the Latin genus and species of trees when she was 3 years old.

Dahl, the University of Iowa’s arborist, has loved trees for as long as he can remember, when he gaped at nine majestic and ancient bur oaks growing in his parents’ yard in Washington, one of which he got stuck in once while climbing and needed to be rescued from.

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