According to the pilot, during the takeoff roll, the airplane encountered a bump in the grass runway that forced it to lift off the runway several knots under rotation speed. The airplane descended to the left, impacted trees off the left side of the runway, and came to rest in a swampy area resulting in substantial damage to the wings. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation. Federal Aviation Administration inspectors examined the runway after the accident and found it to be in satisfactory condition, with no evidence of bumps.