On March 22, 2002, approximately 1530 central standard time, a Piper PA-12, single-engine airplane, N3687M, sustained substantial damage when the airplane struck trees during final approach to the Terrell Municipal Airport, Terrell, Texas. The airplane was owned and operated by the flight instructor under 14 Code of Federal Regulations Part 91. The flight instructor and the private pilot, who was receiving instruction for his tailwheel endorsement, received minor injuries. Visual meteorological conditions prevailed for the local flight, and a flight plan was not filed. The flight originated from Terrell approximately 1430. The flight instructor reported that he had flown the airplane with the private pilot earlier that day for an accumulated flight time of 3.1 hours during the completion of the private pilot's biennial flight review. No aircraft discrepancies were noted during the flights. Subsequently, the airplane was refueled for the tailwheel endorsement flight. The private pilot was performing a short field landing to runway 35 when tree branches hit the fuselage, and the right wing struck the trees. Subsequently, the aircraft slowed, nosed down, and turned right. The flight instructor recalled applying left aileron and left rudder which produced little or no effect, and the airplane hit the ground. The flight instructor stated that he believed that the airplane encountered a sink rate which he did not detect. He further stated that "the deterioring flight path went undetected" by him since he thought the airplane had cleared the area of the trees, which caused him to "under utilize visual clues more to the sides of the approach path." The flight instructor reported the wind was northerly at 5-15 mph. The private pilot reported the wind from the northwest at 5-10 mph and "bumpy." The FAA inspector, who responded to the site, found the airplane approximately 400 feet short of the runway. There were numerous tree branches broken at a height of approximately 60 feet agl. The main wreckage was found approximately 100 feet beyond the trees. The wing spars and ribs were damaged, and the firewall and fuselage were buckled. A gouge was found in the leading edge of a bent propeller blade.