16k faces per se is basically "nothing" even on mobile devices. But if you use some specific shader for the terrain (i.e. a splatmap shader) that could be part of the performance degradation.
The trees most certainly would profit from LOD. But you'd have to script it yourself. Make sure that the script won't worsen the performance though (i.e. 1400 scripts with a _process handler each is a bad idea).
LOD would mean either just toggling the visibility of the trees or replacing them with lower detailed meshes, maybe even with a simple billboard texture (using alpha or alpha scissors transparency). Also using MultiMesh might simplify the LOD approach as you will have less nodes to process (transition might be more noticable though).
Anyway I'd evaluate each step one by one.