Well try to deselect "ORBX tree" in FTX Center and see what happens. If you think the default trees are just too bad for your eyes, try Zinertek Natural Tree Environment X.
And if you still have the problem, try to change the texture resolution in the display settings until it loads up the right trees. After that you can change it back to the original resolution and go flying. But remember that you have to do it every time you start it up,