How do I Override biome.getBigTreeChance(gets a generator) in a vanilla biome

[GiantNuker]

How do I go about replacing/modifying it? 

[GiantNuker]

I tried that already and it did not work.

[Daeruin]

It might help if you tried explaining what you are trying to accomplish and what you've already tried. "I've tried and it does't work" doesn't give us much to go on.

[GiantNuker]

Sorry, when I use BiomeDecorateEvent.Decorate I cannot get an algorithem working to place the trees so they are not overlapping,

If there is a good way, please tell me.

[GiantNuker]

Look at how vanilla does it...

Vanilla does not do it.

[Daeruin]

Look at vanilla WorldGenTrees class. That first block of code in the generate method is basically scanning a section of the world in the shape of a vanilla oak tree to see if there is anything else in the way. Something along those lines might work. It would probably be harder for big trees. Or look at WorldGenAbstractTree. There are methods that define what kinds of blocks can be replaced by a tree. You might need to create your own tree generator class that extends WorldGenAbstractTree and overrides those methods.