Here’s the follow up picture from Friday’s puzzler:
As you can see, there’s a street light near the lower half of the maple. (I cleverly hid it behind the utility pole in the first photo.) The green part of the tree never received the message that days were getting shorter, since the street light is bright enough to interrupt the dark period necessary to initiate dormancy.
This is one of my favorite phenomena unique to urban environments. It doesn’t appear to hurt the tree, although the green leaves will die before the tree can scavenge their nutrients. If more of the tree were affected, I’d be more concerned.
Congratulations to Planting Oaks, who nailed this right off. And kudos to those thoughtful alternative explanations, all of which could logically have had an impact on color change.