Update: It seems that nowhere in the generation code does it trigger that event. I have solved what I wanted to do by overriding decoration generation of trees with the following:
@SubscribeEvent
public void decorateBiomeEvent(DecorateBiomeEvent.Decorate e) {
if(e.getType().equals(DecorateBiomeEvent.Decorate.EventType.TREE)) {
Random random = e.getRand();
World worldIn = e.getWorld();
BlockPos chunkPos = e.getPos();
Biome biomeIn = worldIn.getBiome(chunkPos);
int k1 = biomeIn.theBiomeDecorator.treesPerChunk;
for (int j2 = 0; j2 < k1; ++j2) {
int k6 = random.nextInt(16) + 8;
int l = random.nextInt(16) + 8;
WorldGenAbstractTree worldgenabstracttree = biomeIn.genBigTreeChance(random);
worldgenabstracttree.setDecorationDefaults();
BlockPos blockpos = worldIn.getHeight(chunkPos.add(k6, 0, l));
if (worldgenabstracttree.generate(worldIn, random, blockpos)) {
System.out.println("Tree at " + blockpos.toString());
worldgenabstracttree.generateSaplings(worldIn, random, blockpos);
}
}
e.setResult(Result.DENY);
}
}