Micro poli displacement

i’m trying to create a desert like scene using the micropolygon displacement and i stumble onto a problem where the mesh in the foreground is very poorly subdivided. any ideas how i can work around that problem ? thanks in advance

here is the scene itself

https://drive.google.com/open?id=0Bwj4upeQgykHSkhfT3IwUGlwNk0