Saplings can grow inside of another tree.

Every Sapling Except Pale Oak and Dark Oak can grow inside another tree.

Link to video https://1drv.ms/v/c/925dc423bcf93690/EaTE8z5ML4VNmK4J7fI7qCQBzC9W-UBpMq9jhW2WcYv6gw?e=TP0A0W (file was too big, so I uploaded it to OneDrive.