This is an interesting estimation case I met several days ago. I am wondering how to solve it. My initial thoughts was to firstly segment the areas where trees are planted, open area (mainly streets in my assumption) and closed areas.
For open areas, I may estimate how many streets are there in new york, their average length, and the distance between each tree, in order to estimate the total trees by the streets.
For closed areas, I will continue to segment them into major public areas that contain trees, like schools, hospitals, parks, and assign an average number of trees to each segment.
I think it's not a case that is meant to make us akaward, as long as we can provide a convincing framework. So I am not sure whether my framework is MECE, and I am looking forward to hear from everybody here.