The pilot was departing a confined pad amongst some small trees. He made a vertical take-off to about 20 feet and then commenced forward movement. The aircraft then began to descend and the pilot increased the collective pitch. The blades entered an overpitched condition and during the subsequent attempted landing the main rotor struck a tree. The pilot had flown in and out of the DEPARTURE pad a number of times, on the day of the accident, without encountering any difficulty. Immediately prior to the accident he had refuelled the aircraft to what he believed was approximately half tanks. Subsequent investigation indicated that the tanks were three quarters full. Temperature at the time of the accident was about five degrees above that for the pilot's earlier flights. The pilot was aware that a power check should be carried out at each lift off but he did not check the excess power available on this occasion. When the aircraft moved from the hover into forward flight there was insufficient power available to maintain rotor rpm when the pilot increased the collective pitch angle to stop the descent. It is possible that the terrain and wind conditions may have caused some mechanical turbulence which could have added to the amount of power required to transfer from the hover to forward flight.