Found this post from 2008:
The 'black square' around trees sounds like you have scenery designed
for FS9 that uses a technique that is not supported by FSX.
I've seen that on a few sceneries I've 'imported'. The only 'fix' is
to identify the files that contain the trees and remove those files
or if the trees are embedded in the main file, disassemble the BGL
and see if you can remove the tree statements in the SCASM code.
Not something for a novice or faint of heart....
Wonderful!
Removed KFLL - NO CHANGE.
Jbadger