I really, really like the picture, but the foreground tree doesn't stand out much, and there are trees behind it. Even though it's wonderful, it just doesn't seem to fit the single tree category
I see the "single tree" that you were using to meet the challenge, but to my eye it is lost in the other background trees and doesn't command enough attention to make the image fit the challenge well. However the image overall for me is technically a nice one, I like the sky, the overall mood of the image and the use of monotone.