Trees Disappearing

I have a landscape of about 20 X 20 km. And I’m trying to populate it with a single tree mesh. roughly the bounding box size is 2000. Once populated. the tree leaves are disappearing in certain distance. I have already tweaked the Start and End Cull distance values. DIdn;'t work. Added PerInstanceFadeAmount to opacity mask. DIdn’t work either. Is there any way it can be fixed?

- YouTube This is the issue I’m facing. Btw… I’m running on GTX 660 2 GB