Core Pre-Rolled Steel Garden Tree Rings Black

I have a small palm tree that is already mature/grown.......can the bolts be removed and screwed back ;so that the tree ring can be placed around tree?

Yes. The tree ring arrives in curved section and then you bolt together around the tree.

Why are specs in metric? Are you hiding the fact that these tree rings are only 2.6 inches tall?

The specs are in metric because the rings come from the UK and they use the metric system.  I do try to put the US system info on each product listing as well, but obviously, I did not on this one.  Regardless, no, I am not trying to hide the fact that they are 2.6 inches tall.  Most people can do a quick conversion as you have done.

While 2.6 inches doesn't seem like a lot, most people don't realize that too much mulch can actally damage a tree rather than help as it is supposed to do.  A great article on this can be found at Why you shouldn't over mulch trees.

Many people find that burying the 2.6" ring so that just the very top sticks above ground allows for easy mowing around the tree.