Christmas trees are not poisonous to cats. However, some parts of the tree can be hazardous to cats. Christmas trees can contain pine needles, which can be toxic if eaten, and the sap can cause skin irritation in cats. In addition, the water in the tree's base can contain fertilizers and other chemicals that can be harmful to cats if ingested. Therefore, it is important to keep cats away from the tree's base and to make sure that any broken branches or needles are cleaned up immediately.