You can't force Bryce to do anything really.. I have play a lot with the Distance rendering, and I think its a WIP for Corel. I think they need a patch to get rid of many little glitches. I have done 4 distance renders and never noticed that fact, but I went back and looked at some of mine and I see what you mean.... I played with it a little to see if I could get an results that were exceptable. If I get your question you want the "foreground tree" to have a transparant look to the leaf. The only way I was able to get that look somewhat was to really make it transparant and add a little bit of light near the object. Hope that helps..