@Joga, nice Model so far. But you've wasted too much polygons in my opinion. Ive made a very basic and simple example to show you how to reduce some polygons... Dont get the details like your one but if i add the "details" to this Model, the polycount shouldn't be over 700 - 800 (Just the half of your count).
heres my "example" (fast and ugly / 368 polys so far)
Tutorials: [Blender]Terrain creation ENG/GER [Blender]Low Poly Tree Modeling [GIMP]Create a Texture for Terrains CLICK HERE