Pumpkin/assets/datapack/data/minecraft/worldgen/feature/trees_birch.json

{
  "type": "minecraft:random_selector",
  "default": "minecraft:birch_bees_0002",
  "features": [
    {
      "chance": 0.0125,
      "feature": "minecraft:fallen_birch_tree"
    }
  ]
}