Check your tree detail settings in-game.
Click on the 3-bars in the upper left corner when you see the Routes and Sessions menu or while in Driver or Surveyor.
Here you can adjust other things too such as texture details and draw distance to suit.
You don't say what your hardware is, but keep in mind that by increasing tree details as well as other things, you are putting more of a strain on your graphics card and also your CPU in addition to increasing the heat inside your computer case.