At 10:00 central time, a Smith 60-601P struck trees during initial climb from runway 28 at Fillmore County Airport. The pilot and passenger reported no injuries. The pilot reported, I initiated the takeoff on runway 28, accelerated to 100 MPH, rotated & lifted off. The gear and flaps were raised, and the aircraft stopped accelerating at about 112-115 MPH. I lowered the nose to pick up some speed, but it still wouldnt accelerate. I could see the trees approaching west of the airport and knew I didnt have climb speed, but also knew there was no choice but to pull up & hope I didnt stall. I pulled up sharply, but clipped the tops of the trees, the airspeed was still at 115 MPH IAS with 12+ degrees nose up, and the airplane climbing rapidly. I then looked at the GPS ground speed read out, and realized that it was at 140 Kts. After assessing things, it became obvious that airspeed was not reading correctly. The pilot also reported the pitot had recently been the home of a mud dauber nest.