Hi Boyan,
It is the normal behaviour of the game. You place any object in FMB+ and auto-generated trees will disappear around the object you inserted. You can see the trees in FMB but not when you fly.
Like Kiwi said, if you want to control the trees you have to place them individually in FMB+.
Use only "building.house$Furniture.TreeLine" in FMB+, do not mix and match with other trees. This way you can see the nice animated trees like KiwiBiggles has around his airfield when you run the game with the highest graphical settings.
If you mix and match, the nice trees will become invisible, but will still be present and you will get angry customers with planes exploding with no
apparent reason. This is a well known bug.
You should not get a drop in FPS but the actors.static file will grow and grow and grow ... 95% of the airfields I do for Leningrad-Volkhov are in or near forests (they used to hide the planes below the trees) so I use a lot of building.house$Furniture.TreeLine.
I wish we could have a "building.house$Furniture.10xTreeLine" which could generate a bigger bunch of trees and save on actors.static lines.
Practical example:
If you do not place trees by hand for this one near Staraya Ladoga you will be on the flat empty plains of central Mongolia without any trees around you, not in the deep forests of Karelia..... Very bad when you are in your cockpit.
In other words, when you place the trees by hand, it looks like this in FMB+ (this is
Manushkino AF):
FPS drop will happen if you have too many complex, different objects in one spot. Try to put 2x, 3x, 4x more houses for Pensacola. You should hit your limit ....