Entropy/frequency of a natural image
Entropy would be a good metric in order to separate trees. However I would suggest you to calculate the entropy of gray values within a rectangular window. Consider this image:


After entropy filter, I could obtain this one:



Note that the trees (and also other informative areas) look white and it's possible to segment them out with a simple threshold. Try and see whether it also works for your case.

Due to the possible pre-computations, this could be optimized to run in real-time.