Answer: If this is a newly planted tree, you would simply use an organic mulch to cover the area over and around the tree's rootball. A groundcover would steal moisture and provide root competition for the new tree, so the mulch is theoretically better for the tree. If periodically raked or topped up it also looks tidy.
The mulch should be several inches deep and be spread out in a flat layer. It should not touch the bark of the tree. It should extend outward for several feet in all directions to allow space for spreading roots below ground. The mulch helps keep the soil moist, holds down weeds, and also feeds the soil as it breaks down over time. As the tree grows, the area taken up by the base of the trunk will expand and you will have a smaller and smaller area to mulch.
If you really want to plant something there, you might look at a groundcover such as Ajuga. This traditional groundcover is easy to remove if it spreads where it is not wanted, it also tolerates both sun -- while the tree is young -- and shade later on as the tree grows. As with the mulch, you need to keep it several inches away from the bark of the tree.
Ajuga is usually widely available as plugs or small inexpensive plants in the spring. Your local professional nursery staff may have other suggestions based on a more detailed understanding of the planting site and your design goals. Your local street tree commission may also have suggestions (or ordinances.)
Enjoy your street tree!
Q&A Library Searching Tips