The pilot of the turbo prop airplane reported that, after starting the airplane with a ground power unit (GPU) he exited the airplane. He disconnected the GPU and removed the wheel chocks. The airplane started to roll down an incline. He jumped into the airplane through the aft cargo door and attempted climb into the pilot's seat, to no avail. He pulled the fuel to cut off and the airplane impacted the trees. The pilot further reported that he had set the parking brake before starting the airplane, but either failed to set it completely or bumped it when he exited the pilot's seat to disconnect the GPU. The airplane sustained substantial damage to the both wings and right wing lift strut. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.