Place trees with script on terrain and offset them in y

We have a script to place trees on a Terrain object at a specific location in x and z using terrain..AddTreeInstance(tree); This works fine but we'd like to offset them from the terrain in y with the same script. Is this possible? We could of course parent the tree under a null object, offset it in y, create a prefab and then place those prefabs with the script but because we have a lot of trees each with a different y offset this is not possible. Any ideas?