Yes, Christmas trees can be poisonous to cats if they are not properly maintained or if the cat ingests any part of the tree. Cats are curious creatures, and they may be tempted to nibble on the tree or even chew on its decorations. The sap of the tree can be irritating to their skin and mouth, and the sharp needles can cause internal injuries if ingested. Additionally, some Christmas trees are treated with toxic chemicals that can be harmful if consumed. It's important to keep the tree away from cats and to supervise them if they are near it.