Structures Will Spawn in Wrong Biome

So I'm working on a mod right now and I've got these tree structures which are only designated to spawn in certain types of forest. For my "Thin Oak" Trees, I've selected Forest, Plains, and some others. For "Thin Spruce" I've selected Taiga and others. But I haven't selected my custom biome for either one. They will both spawn in my custom biome, "Arctic Tundra", although I did not select that for either tree. If anyone can help with this that would be great. Thank you!