Lennart's post is a bit old, so don't expect a reply.
In FSX SP2 and Acceleration, the Alpha channel is broken for FS9-style objects. I think it is the same for FS2002 and earlier.
This means seasonal trees won't look right in FSX, because FS9 and earlier techniques are broken, and they cannot be made seasonal in FSX.
The default Models in FSX only have one season, and the autogen uses different models for different seasons. You cannot place an FSX object conditionally in FSX, as it has a "persistence", and won't disapper as seasons change.
So... FSX materials will display alpha as transparent OK, but you cannot conditionally display them. FS9 and materials cannot show the alpha transparency correctly.
This post discusses using effects to display Library objects seasonally:
http://www.fsdeveloper.com/forum/showthread.php?t=18180
But, the trees sometimes flash off and on, and I suspect a terrible framerate hit for many trees.
For the most part, we can use Autogen to display seasonal vegitation... only in areas where autogen are excluded ( around runways ) is there a problem.
This might be OK for using effects, as the number of trees could be kept small.
If an FS2004-style tree is cut out for leaves and branches ( uses no alpha transparency ), It could be tweaked to rotate to viewer, and then you could seasonally texture it.
Dick