I like the way the colors blend together. I do think tree brushes would be all wrong for this task. You could grab some satellite tree views and create a forest texture stamp from that, but I actually like the coloration.

I would shrink the parchment (it looks like you stretched it to fit?), use it at a lower resolution and just blend it together with a fuzzy eraser brush to get the full size. The texture looks lower resolution than the lineart you've created for the city. I might add an outline on the coast, not sure about that, I do like the general effect. I think graphic design's going to pull this one together.