PILOT OF BELL 206B CONDUCTING FORESTRY OPERATIONS EXPERIENCED PROBLEM WITH FIRE DISPENSING APPARATUS BEING SLUNG BENEATH AIRCRAFT WHILE RETURNING TO HELIPAD FOR FUEL. PILOT LANDED, RELEASED EXTERNAL LOAD, AND GOT OUT OF AIRCRAFT TO RELEASE ELECTRICAL CONNECTION. UPON RESUMING HIS FLIGHT HE WAS UNAWARE THAT A CABLE FROM THE RELEASED LOAD WAS CAUGHT ACROSS HIS RIGHT SKID. THE COMPANY MECHANIC WHO WITNESSED THE ACCIDENT STATED THAT HE ATTEMPTED TO ALERT THE PILOT AS HE TOOK OFF BUT WAS UNABLE TO GET HIS ATTENTION. THE PILOT STATED THAT HE MADE A CLIMBING RIGHT TURN AND LOST CONTROL OF THE AIRCRAFT AS THE EXTERNAL LOAD WAS INADVERTENTLY LIFTED AND IMMEDIATELY BEGAN SEVERE OSCILLATION. THE AIRCRAFT IMPACTED SMALL TREES PRIOR TO GROUND IMPACT. THE HELICOPTER SUSTAINED SUBSTANTIAL DAMAGE AS A RESULT OF THE ACCIDENT. THE PILOT, WHO HOLDS AN AIRLINE TRANSPORT RATING, WAS SERIOUSLY INJURED. VISUAL METEOROLOGICAL CONDITIONS PREVAILED AT THE TIME OF THE ACCIDENT.