Bryce won't export trees. Bryce exports rocks. Select rock. Press "E" When the edit window comes up press and hold "Ctrl" then press "D". Pick what type of export you want from the menu that pops up. Anything that can export except terrains will export using these commands. To export terrains select the terrain and press "E" When the Terrain canvas comes up click on the triangle on the top right corner. A pull-down menu pops up and the last command is export. If you use clipping on a terrain when you export you still get the entire terrain.