The 1,202-hour flight instructor collided with trees while attempting a go-around during an instructional flight. The training flight had been in the closed traffic pattern for Runway 35 (3,603-foot long, by 60-foot wide). According to the flight instructor, the single-engine airplane touched down and began to veer off into grass on the left side of the runway. The flight instructor advised the student to "add some power and turn back onto the runway;" however, the student responded by applying full power while simultaneously pulling back on the yoke. The airplane lifted off the ground and impacted trees that were located on the left side of the runway. The flight instructor reported that at the time of the accident, the wind was from 050 degrees at 9 knots, gusting to 12 knots. The investigator-in-charge calculated the density altitude at 3,311 feet.