It's taken a while but I have now finished those sprue-based trees. So to recap:
Tree skeleton made from glued and melted lengths of sprue. Stick to bases; I used plastic card glued to artist's mounting board:
Build up the bark with Green Stuff:
Paint dark grey having 'glooped' the base:
Drybrush lighter grey and paint base:
Add foliage by gluing on Woodland Scenics Poly Fiber:
The paint with watered down PVA and dip into Woodland Scenics flock:
I then sprayed them with hairspray to keep the flock in place.
I think they'll do.