Here's two images from the same scene, the first is fine but add some volumetric rain and Bryce appears to render the boundary box of the tree as well.

Odd - anyone know why?

If that tree is a photo with an alpha mask, sometimes volumetrics interfere with proper display of alphaed photos. One of them bugs. Maybe you can add postworked rain or something?

You can try to add rain in Photoshop after the final render.First duplicate the layer then try in this order: filter-noise-add noise, then blur-gaussian blur, then image-adjustment-treshold, and finaly blur-motion blur. Usualy its the way for making the metal, but it could work. Hope it will help you ;)

I think this problem is when an volumetric object and an alpha channel object intersect. Is the tree inside the volumetric cube (or whatever was used)? To get around this you can try "sandwiching" the tree between two vol objects, so that the none of them overlap. This quick image shows a simple alpha tree with a cube surrounding it (left side) and 2 cubes just in front and back of the tree (right side). There may be something else going on here but this sems to work for me. Best; Dan

I have the same problem when using trees that are on a 2d plane. You'll find if you move the tree or rain away from each other it might fix that little but annoying problem for you.