disc▪ The value ‘(white) poplar’ is not mentioned in DRS at all. Lane mentions it in the lemma ḥawar (s.v. ḤWR), saying: »‘a certain kind of tree’ : the people of Syria apply the name of ḥawr to the ‘plane-tree’ (dulb); but it is ḥawar [… In one source] it is said to be »‘a certain kind of tree of which the gum is called kahrabāʔ ’; by the modern Egyptians (pronounced ḥawr) applied to the ‘white poplar’ [= the value now lexicalized in WehrCowan1979]; ‘a certain kind of wood called al-bayḍāʔ [the white one] because of its whiteness’.«
▪ Since there are no direct cognates, and given the oscillation in pronunction between ḥawar and ḥawr, a definitive statement about the etymology of the term can not be made. If it depends on ‘(marked contrast between black and) white’, one will have to conform ↗ḥawar. For the whole picture, see ↗ḤWR.