Quite simply, the "scar" is referring to the line of demolished trees, the plane made, when it crashed.
Just to be clear, picture the plane crash-landing. For maybe 300 meters, it would make a sort of roadway, a strip of ripped-up jungle.
He's referring to that "thing" as a scar.
(That book is really boring, BTW :) And I find that particular sentence very badly-written. The "bath" wouldn't be "all around" you, the whole things a bit poorly-put together. he should have said "as the plane crashed it formed a scar in the jungle, twenty feet wide, a hundred feet long, and as tall as the trees. this scar, this hundred foot scar, as deep as the trees themselves, it was a bath of heat, and he was sitting in it. he realised he was sweating, he realised his mouth was open. he pictured himself as if from high, a tiny immobile human in the middle of this great gash in the skin of the forest. and then all the boys started killing him.")