5 million vertices seems a lot though. (But I can't tell if these numbers are right because that display doesn't seem to work properly on GLES2 which I use)
Is your terrain-mesh that fine and/or do the trees have that many vertices?
(The Blender "Decimate" modifier might help.)
Also, many alpha textures put load on some low performance gpus (including mobile). (The more screen area coverage the worse, it seems.)