I\ll second Greason, what is the purpose of the grid? A scale at the bottom would remove the need for it?

The trees and dirt are absolutely fine. And the density is fine too, how about altering the colour of the grass in and around the trees to ever so slightly darker or lighter greens. That will give a stronger outline and more foresty look without adding more trees.

Regardless, I think it's looking great, especially once you add the water again. Maybe avoid too much detail in the water so that the island is the clear focus of the map.