Most of the trees in this are a photo texture applied over a quickly blocked in area. There is one that is what I would call a 'stamp' brush of a tree silhouette (bottom left on that small hill). You can easily find them for free by typing 'tree brushes' into google. They're not good to paint with all the time and won't teach you anything, but they're nice to quickly suggest forested areas and put down an idea with some speed. You'll see a lot of concept artists using photo textures to quickly suggest what is in an area rather than, for example, laboriously paint a bunch of trees or individual houses. And that was essentially the idea with these two paintings. It's not a method I'd recommend for something more illustrative though.
Anyway, I believe I've started rambling. I hope I've helped you somewhere in there >.<