THE PILOT STATED THAT JUST AFTER TAKOEFF, AND JUST ABOVE THE TREES, THE HEADWIND CHANGED TO A TAILWIND AND THE AIRPLANE WOULD NOT CONTINUE TO CLIMB AND IT IMPACTED THE TREES. HE FURTHER STATED THAT HE WAS TAKING OFF UPHILL, AND HE DID NOT USE FULL POWER FOR THE TAKEOFF AND CLIMB. HE SAID AFTER MAKING A PARTIAL POWER RUN UP, AND THE AIRPLANE STARTED MOVING FORWARD, HE FORGOT TO INCREASE THE POWER FOR THE TAKEOFF.