Force displacement takes the RayTracing out, as Bobby wrote, so detail is not as 'fine'. What did you use for displacement, a PF or the bark texture displacement/bump map? Maybe the latter is too refined, and needs some smoothing. I can make a bump map for you with PixPlant if you wish. The white areas are really smooth so should raise smoothly and quite sharply off the darker parts, and the darker areas are rougher.