California Code, Public Resources Code - PRC § 5090.38

No owner or other person having legal control of property in the vicinity of any lands in the system is liable for any actions of any type resulting from, or caused by, the user of an off-highway motor vehicle who is trespassing on property outside the system; and no owner or other person having legal control of property in the vicinity of any lands in the system is liable for any one's actions of any type commenced on, or taking place within, the boundaries of lands in the system.