Hello Ian,
just by accident I ran into your thread on this website.....
It is always amazing what you , and of course other members, accomplish in this , very small, scale!
About the scotchpines:
I think it is about finescale...so why not try to imitate trees as best as possible like the "rolling stock"and buildings ?
Just my" 2 cents" : Try , for big trees12 mm first and than 6 and finaly the 2 mm fibres and than add /cover it sparsly with the "coarse" part of sieved fine turf from woodland Scenics. The fine particals I mostly use for shrubs....
I added the fibres with an ordinary ( kitchen ) strainer and use spray glue to attack the fibres and the turf....
and 2 trees in H0(almost 4mm? scale)
Best regards,
Jos