Does the poly count get lowered automatically on flat surfaces of a landscape?
LOD settings in the Landscape actor details- experiment with thresholds that work for you.
Landscape(per LOD level) actually use just single flat tesselated grid mesh. Height is then dynamically modified at vertex shader. Vertex shader also handle seamless LOD morphing. This mean that those flat areas can’t be optimized.
They do get reduced when you go far from it if you give it a LOD Distance Factor higher than 1. (on 1 it gets reduced too but the distance is so great that you wouldn’t notice).