ditto on what Gog_CA1 said.

Cutting down on resolution of background terrains cuts down on polygon counts which conserves memory.   Reducing unseen details again conserves memory.  More memory equates to a faster render.  At least that is what I have gathered as a consenus in other threads.  If that is all true, then extra trees and their location means using unnessecary memory that could be available for render tasks.   My 1/2 cent worth.