Sentence:  Grey-coloured woods covered a large part of the surface. This even tint was indeed broken up by streaks of yellow sand-break in the lower lands, and by many tall trees of the pine family, out-topping the others--some <WSD>singly</WSD>, some in clumps.... 
Meaning of the word marked with <WSD> tags:  In a single or unaccompanied manner; without a companion.