I want to add some trees to minecraft and for this purpose I need to know, how to generate them into the World properly, one time when generating a new Chunk, the other time when growing saplings.

 

Can you tell me how to do this?

You'll need to create a world generator and register it using GameRegistry.registerWorldGenerator(). Your custom sapling would then use that world generator in its overridden growTree method to replace the sapling with a tree.

 

I suggest looking into the following classes:

• net.minecraft.item.ItemDye
  
• net.minecraft.block.BlockSapling
  
• net.minecraft.world.gen.feature.WorldGenTrees
  

 

Relevant methods regarding the sapling are markOrGrowMarked() and growTree().

 

I suppose things changed with 1.7.x, and since I have not updated yet, I cannot give further leads. However, I guess it's still not as easy as simply creating a new BlockSapling and overriding those two methods. You'll have to examine the applyBonemeal() method of the ItemDye class to find the best way...