My trees average from 1.5 to 7 megabytes of data, obviously a lot by most standards, so you wouldn't use this method for a large number of trees. Only the one closest to camera and featured needs to be this detailed. The nice part of doing your trees this way is the tremendous control you have in composing the tree exactly as you like.

Tree Image One shows the general construction of one of my trees (actually it's the "Windswept Bonsai" in the Bryce 3 gallery). Both front and side wireframe views are included. Because the terrains used for the leaves have flat bases, which I don't want to be seen, I tilt these leaf terrains at an angle toward camera, so the flat bottoms are not noticed, and so the scene's sunlight shines nicely on the leafy mass. But from a side view, you can see that these leaf terrains don't actually surround the branches, but rather just stand out in front of them. So this tree was built to look great from one general front view.

To better understand the tree construction, I highlighted only the terrains, and then only the symmetrical lattices, so you can more easily see which is which.

Tree Image Two breaks the tree down into it's basic parts, showing you the trunk, a sample branch, and a leaf terrain. Each is shown highlighted in red in the wireframe view, so you can see what part of the tree I'm talking about, and then that piece is shown as it appears in the Terrain Editor as both a grayscale image in the terrain canvas and as the perspective view preview shape.

Building the tree is obviously the hard part, but once you do all this work to make a really nice tree, don't forget that it needs the proper lighting to really look great. Even in daytime scenes, I like to add accent lights to the tree to give it a little extra sense of intrigue. Generally the lights come from the same general direction that the sun does (it would look strange to see highlights on the shadow side) but I use an angle different than the sun so only a few parts of the tree get the extra light. I also usually color the lights yellowish or orangish to give the tree bark color some variation in it's color, not just variation in it's light and dark.

Tree Image Three shows the two lights I set low to the left of the tree and slightly behind it, so only a thin rim of light would show on the left edges of the trunk. And so you can see what the lights actually do, I rendered the scene with lights on and off, so the two images could be put side by side.