I love this! Beautifully drawn background with little splashes of detail to keep the eyes rolling over the piece. The trees are interesting and different, and look fairly natural.
There are only a couple of things I can point out that might improve it slightly. One is the horizon looks a little bit flat. A little more deviation would make that more interesting as following the picture with your eyes stops a bit there.
Secondly is the colour choice with the mountains. It feels a little bit like half of another work. Perhaps with the closer mountains, adding a reddish tinge to it might make it feel more natural and tie in with the layers below.