Hello! I don’t know what I’m doing wrong but when I’m using the foliage tool to place trees, half of them are floating.
http://image.noelshack.com/fichiers/2017/03/1484787989-floatingtrees.jpg
How can I make them snap to the terrain?
Check your foliage placement z-offset, make sure it has proper value so the trees does not placed in air. Next, you probably need use reapply tool(the second button in foliage shelf) to fix z-offset but it may not work as you expect, in this case you probably have to paint it over again
Thanks it’s working! I’m surprised how much I had to decrease the z-offset values. Perhaps Epic could add a snap vegetation to terrain option in an future update? That would save people so much time.
Your issue is most likely that the foliages origin is not set correctly.
In fact it is snapped by default, you can see it if you start sculpt terrain. Like said above this is probably because your tree “roots” is not at origin.
There is a keyboard shortcut (if you have the foliage selected) that does that. (Can’t remember off the top of my head, might be Ctrl+Arrow down)
Using the placement z-offset might help but like mentioned above, the initial problem might be the pivot center of your object. I’d recommend you fix that initial problem rather than using the offset. Makes things easy (sometimes you wanna place/rotate a tree by hand or you might want to reuse those trees in another project)
when exporting your mesh make sure that its pivot point is where you want your tree to touch the ground. Usually you want the trunk to go a little bit further under ground to allow for irregularities in your terrain e.g. when you’re placing those tree on a steep slope.
You also want to make sure that you place your mesh to 0,0,0 on the x, y, z axes in your modeling program before exporting. Also apply any rotation, scaling, etc. before exporting. I believe in 3ds max it’s done by resetting xform.
I hope that helps 
let us know if it worked!