The pax deplaned and the plt took off on a short flt to a nearby arpt to refuel. After climbing for about 30 to 40 sec and accelerating to aprx 100 kts, the plt started to adjust the pwr for lvl flt. At about that time, the eng lost pwr. The plt attempted to restore eng pwr, but was unable. He turned to land in a large field that he had just overflown. During autorotation, the eng stabilized at idle rpm, but would not respond to throttle movements. The plt saw power lines in his autorotation path and elected to cross over a creek and a row of trees and land in another field. However, the helicopter had insufficient alt to clear the trees. Subsequently, it hit the trees and crash landed on a creek bank. After landing, the eng was still running and was shut down by using the fuel shut-off sw. The aft fuel cell was punctured during impact; therefore, the amount of fuel on board could not be determined. The eng operated during a post-crash check, but impact damage prevented operating it at high rpm. No preimpact part failure or malfunction was verified.