PASADENA, Calif.— Although Christmas tree fires are rare, they are very serious when they do occur. According to the U.S. Fire Administration, one-third of home Christmas tree fires are caused by electrical problems and one-quarter start when the tree is placed too close to a heat source, such as a fireplace, woodstove, radiator or space heater. It’s important to place your tree at least three feet away from heat sources, and make sure you don’t block room exits in case...