The river shrinks and black crows gorge on bright mangoes in still, dustgreen trees.
The author uses “…in still…” to describe the “…the (dustgreen) trees.”
Since the author is describing the trees, in still is an adjective.
Using the Cambridge dictionary, still is defined as:
staying in the same position; not moving:
So the author is describing “…the (dustgreen) trees” to be not moving.