The pilot was to land the helicopter in a clearing which was some 80 metres in diameter and surrounded by trees up to 20 metres tall. He conducted a 40 knot power check some 200 feet above the trees and concluded that he would be able to hover the aircraft out of ground effect with 1-1.5 inches Manifold Air Pressure (MAP) margin. The helicopter was landed in the clearing without incident. To depart the clearing one hour later, the pilot set 31" MAP/3100 rpm (full power is 32" MAP/3200 rpm) and climbed the helicopter at about a 60 degree angle. He reached tree-top height and had just started to accelerate the aircraft when it began to settle. The pilot immediately applied full power and full collective pitch but was unable to arrest the descent. Although the engine rpm decayed to below 2500, the pilot was able to manoeuvre the helicopter into a confined area and land adjacent to a creek bed. Just prior to touchdown, the main rotor blades hit the trunk of a dead tree. No fault was found with the helicopter which might have contributed to the accident. The pilot had limited experience in high weight operations into confined areas. The pilot described the wind as light but varying substantially in direction at times. The power check conducted by the pilot would have been valid only if conditions at takeoff were exactly the same as those existing at the time and place of the check i.e. 200 feet above the tree tops and one hour before the accident. There is no guarantee that conditions were the same and 1-1.5 inches MAP was not sufficient buffer in the circumstances to ensure safe operations. By climbing at 60 degrees, the pilot was committing himself to the takeoff without any fall-back option. The correct technique required the helicopter to be climbed vertically in a "towering lift" type DEPARTURE.