So far, other than Yamato, the other person who knows Wood Release is Hashirama Senju. I thought that Wood Release was one of the Senju Kekkei Genkai's, but then Yamato ended up having it. Assuming that he isn't a Senju (which he isn't), why does he have Wood Kekkei Genkai, and how is he able to use it so well?
Yes, it is true: Yamato is not a Senju. By the way, Yamato's real name is Tenzo. Anyhow, when Yamato was just 6, he was abducted by Orochimaru as one of his test subjects. Orochimaru wanted to see if he could make Yamato have Wood Release abilities by putting Hashirama DNA into him. The plan worked, and he could use it easily. But then Hiruzen, the 3rd Hokage, caught Orochimaru doing his experiments, and Orochimaru fled, aborting all his experiments. All test subjects were expected to die off, but the only survivor was Yamato. In the anime, it shows Yamato trying to escape using Wood Release after seeing many children die from failed experiments.
Yamato's past has very sad, but it resulted in him being very capable.