Styphnolobium japonicum (formerly Sophora japonica) trees are blooming.Related post: 3 ways to identify a Styphnolobium japonicum in NYC.
Don't water a tree's trunk! This practice can lead to fungal decay at the root collar. Also, sprinkler irrigation can lead to "surface compaction created by water hitting the soil surface and dispersing the soil aggregates" (ISA Arborists' Certification Study Guide, 2001, 40).
I think the tree (Zelkova serrata) pictured is dead, but it is being watered (with a Treegator® Slow Release Watering Bag). So, perhaps the tree dropped its leaves because the soil is very dry because of the current heat wave. It does not help that it is located within a hot plaza. I could not determine if the cambium is green -- a way to gauge if the tree is alive.
These trees are overwrought! There is a tree guard to protect the tree trunk (from gnawing horses -- a historic reason for installing trunk guards), a tree pit guard (to discourage lax dog owners, among other things), and a car door barrier.
Tree protection zone installed for construction work at the 6th Ave/W. 3rd playground. The International Society of Arboriculture recommends protecting at least one foot (1') from the trunk for each inch (1") of trunk diameter. The protection zone pictured above does not adhere to this standard but it is protecting the trunk and exposed soil surface. What is New York City's policy for tree protection for trees in/near construction sites? Can I find the information online? Related post: Manhattan's parks as green infrastructure.