The sidewalks and pavement of Princeton tell a lot about what sorts of trees are growing overhead. If you see lots of red when you look down, there's probably a native red maple above that has just dropped its small flowers.
Even when past the flowering stage, the trees still have a reddish tint this time of year, as the seeds (called achenes) begin to grow out of the red flower stems that remain on the tree.
A spent flower cluster colored pea green indicates a Norway maple.
This time of year, it's possible to travel down the street identifying every Norway maple in sight, with their distinctive shade of green. Norway maples were a popular exotic tree to grow at one time, but their very deep shade and allelopathic tendencies (they release chemicals from their roots that suppress growth of other plants) make it hard to grow anything underneath them. They have also proved very invasive in some parts of the country.
A collection of prickly balls on the street mean a native sweet gum tree is nearby.
These sweet gums line the 206 side of the Community Park soccer fields.
It's not the best tree to grow in a yard if you like to romp barefoot in the grass, but they can be impressive in other ways. When I lived in Ann Arbor, Michigan, where they are uncommon, a horticulturist at the university took great pride in a small sweetgum he had planted. The beautiful paneling in my parents' home was said to be sweetgum.
In natural areas, they tend to grow in floodplains, sprouting thickly in open areas.