The trees are actually touching the ground.
Its not the clipping , i dont know what it can be , any ideas ?
Here’s the blend if you wanna check it out. http://uploader.polorix.net//files/249/arboles1.blend
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I don’t know why it makes a difference, but if you lower ‘sphere’ a bit, it looks better.
It could be the subsurf.
I don’t know blender very well, I can only suggest scale up sphere.001 or use duplifaces.
i got it…
you need to take the sphere…
“tab” to edit it…
w subdivide it 2, 3 times and then render it…
your problem is subsurf is cutting off the sharp bits which your trees are on 
hope that helped
//charlie
Thanx ! i fixed it ;D