So, as you noted, there are several things going on with this edit that make this unusual.
Firstly, "Sequoiascape" is a beuatiful art installation at the Fresno Yosemite International Airport lobby open to the public that is a replica of several Sequoia trees that are found in the Sierra Nevada Mountains near the airport. As such, I marked these reproductions as both natural=* and man_made=*. I did not want to tag the materials it is made of because that would be too complex. Please let me know if you think it should be just one but I believe both tags are warrented in this unique instance. 
I added a node as you reccomended and changed the layer of this relation in https://www.openstreetmap.org/changeset/153511130 let me know if anything else looks off to you. Have a great day!