Sentence:  Forest fires have often been started from wantonness; for the sake of making a big blaze, fires will be carelessly left by guides, or will be smouldering in the <WSD>duff</WSD>, where it will burn for weeks. I have seen the smoke from fires in the duff even after the snow has fallen. /  Local term for the vegetable growth covering the forest ground of the Adirondacks. under the spruce trees, the falling needles accumulate to considerable depth, forming the "spruce '''duff"," a peculiar and interesting variety of forest humus. 
Meaning of the word marked with <WSD> tags:  Decaying vegetable matter on the forest floor.