Also, having items to show scale helps :)
birds, fallen trees, animals, anything the eye can relate to and apply scale.

Also, up close, use "heroic" trees, single trees with their polygon count increased if necessary, to look good.

usually, real forests as opposed to plantations, will have bare areas, due to fire, specific tree species that poison off others, or stony ground etc.
You can also see areas, like say where a stream runs, that have different trees or bushes due to more water.

The variety helps break it up, and helps scaling, IMHO.

:)

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