The pilot reported that due to the density altitude and the weight of the helicopter, he could not gain sufficient airspeed during the takeoff to climb. As the helicopter approached a tree line he initiated a left turn to avoid the trees after receiving a "low rotor" rpm warning. The pilot lowered collective and added throttle just before impacting rough, uneven terrain, which resulted in substantial damage to the helicopter. The pilot stated, "…it [the helicopter] was just too heavy and it was too hot, and the terrain was not level for a run on landing." The density altitude at the time of the accident was calculated to be 10,612 feet.