SHORTLY AFTER TAKEOFF, THE PILOT NOTICED BLACK SMOKE TRAILING FROM THE LEFT ENGINE. SUSPECTING A FIRE, HE ELECTED TO LAND STRAIGHT AHEAD IN AN OPEN PASTURE. THE PASTURE WAS WET AND SLIGHTLY DOWNHILL MAKING BRAKING INEFFECTIVE. THE AIRPLANE STRUCK A LARGE TREE WITH THE LEFT WING, SPINNING THE AIRPLANE 180 DEG INTO A FENCE ROW. NO EVIDENCE OF AN INFLIGHT FIRE WAS FOUND ON THE LEFT ENGINE.