Here my opinions:
first image: about the trees seems like there in not enough random rotation…and the rocks have some scale issue to my eyes; maybe also the sea could be less blue.
Second one: the trees in the forest are ok, what they need is more random height and color. You can improve the vegetation in the rocky mountain.
Third image: don’t know, the dunes profiles are not so easy to “see”, some shadows in the scene will neutralize this problem(just type desert in google to see what i mean)
About the trees: first 2 are ok, last 2 they can be much better. A bark texture will be useful.
The trees are not really modeled. They are just generated. At least put some textures on them.
Your environments look good enough but not realistic. Take a look at some real photos and copy them as much as you can. You`ll notice the difference. Add variations, details, displacements, fog, blending (sky and land), birds (jungle), snakes, skulls (desert), sharks (insland), more more details and a good artistic lighting.
Critiques: Keep in mind these are only my opinion, not meant to offend.
Island image: Reduce the forward DOF effect. It’s hard to judge the size of the island without comparison to waves upclose. People judge distance of something nearby or what is close and take perspective into account.
Forest image: Your cloud sky’s horizon doesn’t match the horizon of your trees. To make your background more believable line up your light source. In this image your clouds are lit from the right while the trees are lit from the left.
Desert image: I agree that contrast is needed on your hills. Also, I assume this is sand. Add an extra color texture and set it to a grainy noise, with a slightly brownish color. Sand is made from grains, so this is one place where a grainy texture is okay to use. Just make sure it doesn’t look like carpet.
Trees: All of your trees look acceptable at the current minute. I recommend however, that if possible you vary the color of your leaves. Trees are in a constant state of growth and change. Never are all the leaves the same color. Don’t go overboard, just a subtle variation. Also, your trunks and branches could use a quick texture if possible.