Box 2-1579 - Someone set a Christmas tree on fire in alley next to building. BFD responded and Engine 22 extinguished fire. Companies went back in service. Several Christmas trees/dumpster/trash fires in District 4 had been extinguished in short period of time prior to this fire. District 4 chief requested BFD fire marshal to respond to investigate fires. Chief and fire marshal were on the ball with their follow-ups. Fire marshal rechecked walls inside building and discovered walls were now hot in basement and 1st floor. Previously there had no indication of fire when inside walls had been checked. Fire marshal requested Box 1579 struck for fire in high rise. Engine 33 Ladder 15 1st due. Engine 33, in alley, stretched line into window and proceeded to find and extinguish fire in basement. Difficult to access room on fire due to steel door, narrow hallways, building alterations, lighting, heavy smoke. Fire had spread rapidly through walls up to 4th floor. Many lines were needed and put in operation very quickly to stop advance of fire. Excellent job done by operating companies pulling walls and ceilings and extinguishing fire with heavy smoke. Easily could have progressed to greater alarm except for initial actions of District Chief and fire marshal and actions of operating units. Also fortunate fire did not occur later at night when most occupants would have been asleep.