Found them on disk two and still didn't change anything. Seems most prominent when the parking garage is behind them like the system doesn't see it as solid. Below the horizon the problem goes away. No idea, but here is an illustration.
The issue is created by the fact that, when you have 2 objects (trees and parking) with alpha elements on them, they should be sorted before drawing. Unfortunately, sorting is a *very* expensive process, probably one of the slowest things to do in flight sim, because all objects will have to be sorted and re-sorted, and each frame. Also, to be sorted, each tree should be an independent objects, which will slow down further the sim. Right now, they are grouped, because a group of, let's say, 500 polygons, draws MUCH faster than 50 objects with 10 polygons each. But we should un-group them, in order to sort them...
Sceneries that use those very old design methods (each small object being independent + sorting of all the one having alpha ) were those made for FS2000 that run in FS9 (like Simflyers, that why they have the peformance they have), that would not need these methods, because it uses the much faster z-buffer for perspective sorting so, sorting is not usually needed in FS9, except for alpha, which would be highly inefficent to do, if you have so many objects that needs to be sorted, like trees.
Converting the tree textures in DXT1 fixes somewhat the effect on the tree foliage, but makes for really UGLY tree shadows because, after the conversion in DXT1, the shadow, that is made as part of the texture ( for the obvious reason that a textured shadow is WAY faster than letting FS9 calculate the shadow in realtime ), becomes pitch black, with no smooth grey overlayed on ground, that is only possible with DXT3. We tried this before releasing the upgrade, but it really looked bad, MUCH worse than having the blue halo in certain viewing angles, because it would appear always. So, we decided to leave it like this.
Of course, this only happens in FS9, because FSX has an entirely different handling of transparencies, so there's no problem there.
We *might* be able to fix it, by making the trees with two different materials, one for the foliage in DXT1 and one for the shadow in DXT3, of course getting a performance penality, because of the somewhat costly material switch (every new material in an object, has an added drawing cost)