Hello ,
Thank you for posting your question in the community. The Pivot point editing allows you to control how to place elements and you can reference the tutorial video here: https://youtu.be/lAD5Gr2s1XM
When changing the rotation of the tree you will need to place then rotate the object, once rotated you can move it by selection one of the 3 axis or planes, but do not click on the center yellow dot as that will try to use the collision and default Z axis - thus changing it back to vertical. I hope this makes sense and if you have any additional question please let me know.
Kind regards,
Vincent B.