To configure the biomes in your world, you have to edit the biomes.yml.
Now the're two ways:
In this biomes.yml you will find for every Biome an Config-Part. Here were using the Config-Part for the Biome JUNGLE:
biomes:
JUNGLE:
overlay:
chance: 6
blocks:
- 38:0;10
- 37:0;5
- 31:1;60
- 31:2;50
- 18:3;25
- 18:0;10
trees:
chance: 350
trees:
- JUNGLE_TREE_1
- JUNGLE_TREE_2
- JUNGLE_TREE_3
- JUNGLE_TREE_4
- JUNGLE_TREE_5
- JUNGLE_TREE_6
structures:
chance: 160000
structures:
- DUNGEON
layer:
'1':
- 3:1;15
- 172:0;5
- 2:0;170
- 1:3;2
'2':
- 1:0;30
- 3:0;110
- 1:3;8
- 1:1;8
'3':
- 1:0;40
- 3:0;80
- 1:3;10
- 1:1;10
'4':
- 1:0;70
- 3:0;40
- 1:3;15
- 1:1;15
'5':
- 1:0;120
- 3:0;10
- 1:3;25
- 1:1;25
But what does this Values mean?
overlay.chance defines the chance to spawn an overlay-block. The Chance will be 1 to overlay.chance. So lowering the chance-value in the config will increase the spawn-chance.
overlay.blocks contains all Blocks that could be overlayed. The block-values are build as following:
(ID):(SubID);(Chance)
The Chances in this Block-Values define the chance out of all Blocks. That means the chance is Chance to All Block-Chances.
trees.chance defines the chance to spawn an tree. The Chance will be 1 to trees.chance. So lowering the chance-value in the config will increase the spawn-chance of Trees. The Chances will define the chance to x-z-Blocks, that means a chance of 256 will spawn one tree per Chunk, an chance of 512 will spawn one tree per 2 Chunks and so on...
trees.trees is an list of all Trees, that can spawn in this biome. An list of all trees can be found here.