Design a gradient material in PS, use a transparency mask in Bryce. Use a sphereical gradient on a sphere applied to object top, or a linear gradient on a 2d plane which you could tip for adjustment.

 Personally, I would use gradient layers in Photoshop, much more control over color & opacity. Plus one less component in Bryce willl reduce render time.

Have fun!!
ICM

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Ockham's razor- It's that simple