The San Francisco Chronicle reports the boy was sharing a tent with another boy in a San Mateo County park when the 40-foot oak tree fell on them before dawn Wednesday.
Assistant County Manager David Holland says the boy was taken to an intensive care unit in critical condition. The other boy suffered minor cuts and bruises.
No names were released, but officials say the two were camping with members of the boy's extended family. The tree's branches hit the family's other tents, but no one else was seriously hurt.
There were signs of rot in the tree, but officials say the outward appearance of the tree didn't indicate there was a danger.
Memorial Park is about seven miles east of Pescadero and 50 miles south of San Francisco.
Information from: San Francisco Chronicle, http://www.sfgate.com