If you look at old screenshots, you can see that graphic is much better than in actual version. I'm playing on Ultra graphic settings, but still water, trees and other stuff looks worse comparing with old images.
Is there some hidden graphic settings, like nvidia driver settings or something? Or it was on purpose decision during development to lower graphic quality for some reason?
To answer your question about the current graphics succinctly: no, I don't think they can be adjusted any further by the user.
To thoroughly answer your question about the graphics: it's a bit difficult to compare the old versions to the current going by just the screenshots, especially considering that the old screenshot is from one of the earliest builds from more than two years ago. Unfortunately, I purged my older builds of the game, so I'm mostly going by memory here.
The most visible comparison that could be made is that the islands themselves are different. The original island had different assets, resulting in a much different topography. Most of the plant assets on that original island were fairly static with little interaction possible and only some visible animation. Another notable factor being the limited night/day cycle instead of the current dynamic change and usage of shadows (old version used more of the slideshow style lighting with light shadowing, rather than a moving light source). In addition to the lighting and assets issues, the old version suffered from optimization issues with a minor stutter even on higher-end pc's.
This early map design was abandoned early on in favour of the current map design, which I'm fairly sure is available in the unity assets store considering I've seen a couple other games using it as well. But yeah, the changes were most likely made for the sake of performance and stability for a range of computer specifications.