It is about 3 weeks after X-mas and I assume almost everyone is discarding their tree. That adds up to a lot of trees.
I am not sure who is responsible but most of them have been piled up at the ends of many blocks. So in the residential parts of Park Slope about half the corners have a pile of trees on them. That means some intersections have no trees and some have a pile on every corner.
This is 8th Ave and Union looking South
Below is a Google Street View during warmer times. It looks like they were about to film a movie. Must have been a day of the week ending in "y"
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A couple of blocks away an interesting thing was done with some of the smaller trees.
They were hung across a rope diagonally across the intersection.
I was not the only person who thought this was photogenic