if males are purged at sight .
If that's done, it would have to be done by a pervasive secret police, because as far as even Dineru (who was chief of her clan for a long time) knows, only females are ever born in the roots of the Silverroot Trees.
I think that what the Third Queen (the Reincarnator who became the Elf Queen in the Mythological Age) did was not only purge the males, but also set up the Silverroot Trees as a kind of artificial womb system, that grows new elves.
So before the Third Queen, elves reproduced sexually like Humans and other Demon Races, but after her, they can only reproduce via the Silverroot Trees.
Yes, I know they are born from the silver trees but it doesn't state anything else. That specific part of what you said doesn't exclude the light elves, it said elf tribes.
So what you're wondering is if the same Silverroot Trees (the same species of tree) can produce Light AND Dark Elves, correct?
That perhaps some difference in the environment (soil composition, or even the ambient magical energy due to a nearby population of Elves) will aspect a group of Silverroot Trees to produce predominantly (or only) Light Elves or Dark Elves?
Sort of like how the same plant can have different colored flowers based on the acidity of the soil (Hydrangeas being one of the most common example)?
Since the Light Elves in Chapter 1 cut down the Silverroot Trees near Dark Elf villages that produce Dark Elf babies, I think that the trees, once they start being able to produce elves, get locked in to producing one type of elf.