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!