Christmas trees can be dangerous for cats, especially if they contain tinsel or are not secured properly. Tinsel can be swallowed and cause intestinal obstruction or blockage, which can be life-threatening. Cats may also try to climb the tree and knock it over, which can cause injury to the cat and damage to the tree. Additionally, the tree itself may contain toxic substances or sharp branches that could harm the cat. It's important to ensure that the tree is properly secured and monitored at all times when cats are present.