The Wisconsin Department of Natural Resources identifies the victim as 10-year-old Allison Spahr of Cary, Illinois.
The girl, her father and her brother were camping at Devil's Lake State Park in Baraboo when the tree fell early Sunday.
The girl and her father, 51-year-old Kevin Spahr, were trapped while her 13-year-old brother, Kyle Spahr, was able to climb out.
Park superintendent Steve Schmelzer says a thunderstorm had passed through the area, and that may have weakened the tree, which otherwise appeared healthy.
The boy had minor scrapes and bruises, and the father was treated and released from a hospital. The girl died at the scene.