Are these the same trees that were okay before?
Old alpha-channel trees will appear translucent. SpeedTreez and their relations will appear translucent if you are out and away from them. This is the LOD kicking in.
Have you updated a video driver recently or swapped video cards?
Have you tried switching between Open/GL and DirectX? This might help.
I'm sure other guys will jump in here with some suggestions.
I hope this helps...
John