How to replicate:
>Click on 'Single Player' from the main menu
>Click 'Create New World'
>Click 'More World Options...'
>Cycle along the 'World Type: [...]' until you reach 'World Type: Customized'
>Click 'Customize
>Drag the 'Biome: [...]' slider from 'All' to 'Plains'
>Click 'Done'
>Click 'Create New World'
>Let in generate

If you explore around, you will find Forest Biomes. I understand this may be for the players to be able to get wood, however, there should be an option to disable this Forest Biome generating so that the world will be entirely Plains Biome. If this is not an intended thing, then this is a bug that needs fixing.

Solutions? :
>Fix the bug
>Add option to disable Forest Biomes generating