The pilot reported that the turf airstrip had a "sunken area" which created a "ramp." During takeoff, when the airplane encountered this portion of the airstrip, it became "airborne prematurely." The airplane subsequently settled into ground effect; however, the left wing impacted a small tree near the end of the runway. The collision caused the airplane to pitch "downward and left" and the pilot was unable to maintain altitude. The airplane came to rest in a muddy rice field off the end of the runway with damage to the wings and fuselage.