A group of researchers and foresters who lived in a disclosed location in central louisiana has been infected with a gas from the trees they were researching. This intel causing them to show aggression, paranoia and hallucinations which gradually caused violent psychosis. From the size and amount of the body parts that were found, their strength and pain thresholds must have given them incredible resistance to the physical trauma a chainsaw can deliver.
Further research was abandoned when investigators suffered the same fate. Speculation is that the "enchanced" trees produced an undetectable by-product of oxygen that, with sufficient exposure, turns people in homicidal "zombies." Thus creating a area that we now know as "THE TREE FARM MASSACRE"