Thx for your helping tips chris_oat and sivan!

I thought to sharing textures is just possible with textures in the same model.. .!??
I - for example – have one tree with 1100 faces. The textures are 128x512x32mipm for the needles-(sprites) and 128x512x32mipm for the trunk.
There is no possibility to share textures in this model … or I am blind…

To use a group of models as one model makes the “texture-sharing” possible…but with such a group I cannot align the bottoms to the irregular floor of the terrain…

My attempt was to make LOD-steps like tree_0.mdl (128x512x32mipm), tree_1.mdl (64x256x32mipm), tree_2.mdl (64x128x32mipm)…and so on…
But neither with *.tga Textures nor with *.dds Textures the framerate-results are better!
@sivan: but with *.dds textures it looks much better

..and thx for the gamestudio-hints!
I will try to reduce the polygons now … maybe this will work.