Short answer is 'yes', but it depends on the maximum number of trees accepted by your fms version.
Here more explanation about 'trees', if needed:
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Look at Switzerland.scn file in the landscape directory of FMS.
You will discover code entities like this:
-------------------------------
///////////////
// SwitzTREEs
///////////////
TEXTURE SwitzTREE TRANSPARENT
POSITION -100 -100 2.4
256 256 -3 0 0
0 256 3 0 0
0 0 3 0 6
256 0 -3 0 6
256 0 -3 0 6
0 0 3 0 6
0 256 3 0 0
256 256 -3 0 0
256 256 0 -3 0
0 256 0 3 0
0 0 0 3 6
256 0 0 -3 6
256 0 0 -3 6
0 0 0 3 6
0 256 0 3 0
256 256 0 -3 0
-------------------------------
that define position and bmp image file (in this case switzTree.bmp) to use for a 'tree'.
You will have to understand the coordinate systems of fms as explained in other topics.
Collision detection is active with this object.
You may define other lanscape items than trees with this system, but each item will remain 2D (here the image may be used twice, in a perpendicular way, to give a 'volume').
For 3D you need to build other objects (using metasequoia e.g.)
For a sample of possible usage of the 'tree' item, you may refer to the 'death tower' that is build with a number of small 'trees' that make a hollow tower that can be flown thru by planes and other models:
Download: (Tour de la mort = death tower)
Phil.
PS: be aware that this landscape was developped for V 2.0 beta 7 and that there is a maximum number of 'trees' (we are close to the maximum for that version with the death tower!). More recent versions of FMS may accept less trees (not tested!).