The pilot reported that he took off into the north towards trees about 50 metres from the lift off point. He accelerated the helicopter through translational lift speed but had to climb soon after to aviod the trees, which were some five metres high. Once above the trees, the helicopter began to descend and lose performance. The pilot applied full collective pitch but was unable to prevent the aircraft descending into the trees. The pilot reported that winds in the area had been predominantly from the south for a number of days. Although conditions at ground level at the time of the takeoff were fairly calm, there may have been a southerly wind component above tree-top level. This, coupled with the relatively high operating weight of the helicopter might have been sufficient to cause the loss of performance described.