Like so many things in Bryce the trees are not exportable to other apps, but as Delrino said (if you stay in Bryce) you can get some really nice ones with some effort. Bryce uses metaballs to create the trees, which is also why they can look so realistic - even close up. Bryce5 trees render much faster than polygonal trees because they are using metaballs, and this also allows you to build small forests of them with very little speed penalty. Treedruid standalone or the Cararra plug-in creates pretty nice trees that you could export into Cinema4 - that might be an alternative you could look at.