Sorry, my english is at scholastic level
You were and still are, correct.
Great comparison.
Yes, only the foliage. The trunks look 3d as far as I can tell, but the leaves look very 2d in comparison to GTS.
I'll have to give it another look when I launch up the game.
Forza 6 trees in that picture are "2D", or more precisely, Two 2D planes intersected, criss cross. GTS has those as well.
The trees at the front of the GTS picture are the "high 3D" ones, that is, solid (volume) tree trunks and multiple 2D planes arranged to form the canopy.
GTS has the advantage of treating those 2D elements with light/position information, so that makes it look even more "3D".
Similar to Driveclub
But worry not, right behind them you have the billboard 2D plane to compose the BG.
And harder to notice but between the "high 3D" trees, you have the normal 2D plane tree, they just now rotate around their vertical axis following the viewer and have light/position information as well.
Nurburgring is the exception and not the norm right now in terms of quantity of "high 3D" trees.
Brands has considerably less of those with sections filled with rotating 2D's and criss cross ones.
I don't know why that is the case, but my guess would be that at Brands there are more points where you can see the rest of the track so that geometry is less culled. Or maybe Nurb is their prime darling.