Tree grates! Perhaps you do not know what they are. Tree grates are tree cages that keep street trees from escaping and protect them from the elements.
"Tree grates have been used for many years to help integrate trees into the urban hardscape. Tree grates provide space for the tree while allowing pedestrian traffic over the tree planting area. They help to suppress weeds and trash accumulation in the tree planting area." [ironsmith.cc]
Many tree grates are made of iron and while there are many more-or-less functional-only tree grates in the city, there are also places that have really cool, beautiful tree grates.
Here are a few: