You can multi replicate and use a disperse like 2d 3d with rotate or resize. The problem is resize is often too extreme and all the trees still have exactly the same branching. You also don't want to use 3d rotate and stand the trees on end or flop them over on their sides. I do a multi replicate of about 50 with all the scaling at 100%. I do a 2d disperse to get the trees spread out at random. I highlight about 5 trees and edit the gravity or randomness just a few clicks off the default settings. I displace these up Y so I know they are done and select 5 more. Some I also rescale a few percent. When I have gone through all of them I select all.I press the down arrow to drop them on the terrain, then I go get a snack because it's going to take several minutes to calculate all the drops. Once the trees are down they will all stop on the terrain as soon as the root tips touch. Displace them all Y down a little more untill the roots are buried. Once you are satisfied they are all tucked in you can then deselect them. Look for the occasional straggler that hit the top of another tree when dropped and is stuck in the air. Shift it over and drop it again till they are all planted. FYI you cannot export Bryce Trees.