Tree Tweaker is a CraftTweaker add on that allows you to add custom trees to world generation. The shape, height, leaves, logs, frequency, biome, and more can all be customized through zen script. Trees can use modded blocks and be set to generate in modded biomes.
Example Tree Script:
import mods.treetweaker.TreeFactory;
var jungle = TreeFactory.createTree("jungle");
jungle.setTreeType("JUNGLE");
jungle.setLog("minecraft:bone_block");
jungle.setLeaf("minecraft:brown_mushroom_block");
jungle.setGenFrequency(7);
jungle.setMinHeight(10);
jungle.register();
More detailed information and additional examples can be found on the Tree Tweaker Wiki
