Sentence:  ... he resorted to the practice of “<WSD>niggering</WSD>,” as it is called: which is simply laying light pieces of round timber across the trunks of the trees and setting fire to them at the point of contact; by which means the trees are slowly burned through. 
Meaning of the word marked with <WSD> tags:  To clear land by laying light pieces of round timber across the trunks of the trees and setting fire to them at the point of contact, by which means the trees are slowly burned through.