I just got back from three weeks in Southern France and Italy. Two takeaways: first, the London plane trees look so much better then in San Francisco (see photo)! And second, we have so much more species diversity then in most European cities. I think it's because urban trees in Europe are largely planted by the municipalities. Some bureaucrat decides that elms are the way to go, and bang - hundreds of elm trees, one after another. In the city of Nimes, France, near where we stayed, the entire city was planted with Chinese hackberries. And in Rome, it seemed like the entire city had only three or four species. It does create architectural unity, I guess - but I prefer our diverse tree anarchy.