The lone tree on a flat plain is a solid concept, but the scale of that particular tree is small enough that the various small bushes in the foreground take away from the overall effect. I really like the gray sky and the position of the horizon relative to the overall frame, but the foreground detracts from the composition in my opinion.
Gee, I wish I could vote on this one, but I can't. The tree is obviously lonely but enjoying its solitude; the smaller shrubs in the foreground seem to be echoes radiating from it. That buttery smooth tonality of the sky is simply fabulous. The only nitpick I have is the tree/house/pole thing on the horizon - it looks a bit like an emu sticking it's head up.