This photo was taken four days ago. Spring is in such a pellmell rush right now that the scene has already changed, but for a few days anyway you could spot every Norway Maple in town if you knew what to look for. Their flowers and emerging leaves made a distinctive flush of yellow-green, just ahead of most other trees. If you were keeping your head down, you could still see their tiny flowers fallen onto the sidewalks.
Like the barberry bush, the Norway maple is an exotic that can be very invasive. It pops up along the fencerows of people's yards, and before they know it, they have a large bully in their yard, pushing up into the canopy of more favored trees, making such dense shade and grabbing so much soil moisture that nothing can grow underneath it. Though it has long since lost favor in the horticultural trade, its self-seeding and capacity to tolerate shade insures it a place in Princeton's default landscape.