Okay, Jude.  Your render settings are fine so that eliminates one area to look at.

Now, when you adjust the scale of the trees, are you doing them one at a time or the entire group at once.  Try scaling the trees one at a time if you're trying to do the entire group because I think this is where the meshes are unwelding.  I usually rescale using the side view window because I can also re-ground in the same window and get a good idea where the tree is.

We'll get it figured out.  It may correct with the reinstall, as well.

Hugz from Phoenix, USA

Victoria

Remember, sometimes the dragon wins. Correction: MOST times.