Tree Distance - Terrain Settings

I was trying to lessen the FPS Drop / Lagging of my game by decreasing the Tree Distance on my Terrain Settings. At first, I tried testing if my game is lag-free if there are no trees which was right.

But the problem is, the values 1-2000 are the same. It’s like the value 0 will make ALL the trees disappear while any value from 1 to 2000 can make the ALL the trees appear.

I wanted to only show trees that are near because the trees that are far are covered with black fog which can’t be seen either way.

The real objective of this action is to lessen the lag of my game. Thanks in advance guys!

P.S: Image as reference for my Terrain Settings.!

Sorry if this doesn’t help, (I’m very new to programming), but on the bottom of the screenshot you have, it says that “Tree distance, etc… have no effect on SpeedTree Trees”