Invisible trees? I've cycled through dxlevels 70, 80, 90, 95, & 98; the only thing I noticed was that lower levels (80-) utilise force fading models on static props. I'll assume that's what you meant (if it's the whole dilemma of loading a map and the trees fail to load and are replaced with the pulsing ERROR models, then I don't know what to do other than suggest that you clear your cache and re-join the server / restart TF2).
Take note of the textures for the trees in their current state; those are actually coloured renditions of rasterised silhouettes from the 3D trees I made earlier in development (2 months ago).
I planned to have 3D trees, but had to regress back to 2D trees for reasons I've stated earlier. Render distances, Level of Detail, radius culling (env_fog) only have so much use before going to shít and everything resulting in one heap of trash. Large open spaces are already a big no-no for the source engine; therefor, knowingly disregarding this drawback has its deficits. Having 3D trees tanks the overworld framerate; as it is the map averages around 78fps in a full scene on my machine with max settings and 1 user (myself, I have no idea what it's like with 32/32).
I'll look into sorting out the physical properties of the trees, but the 2D properties will stay. I'd rather keep the visible polygon count under control. $alphatest "1" & $nocull "1" is an efficient detail with geometry and lighting for this particular situation.
(I'm exhausted, so if this sounds like rambling, now you know)