UV Surfaces can use only channel 1, due to the way that surfaces are handled in Forest:
To speed up the scattering process, surfaces are pre-processed in a common cache, storing multiple data including the UVW coordinates.
Since this cache is shared between all Forest objects, initially we don't know what map channels will be neeed (assuming each Forest object may use different channels), so we limited it to map channel 1.
A similar issue occurs with the use of multiple UV channels for the Distribution Map.
No promises, but i'm taking a look to this, and we'll try to remove these limitations.