Modelling clay. Then paint brown, add additional highlights as desired - for foreground trees, more detail is needed. For ones way at the back, probably not worth the effort, no one will see them.
I’m happier I was right… if it involves any sort of artistic talent, it’s not for me. But I seem to recall reading about using modelling clay in one or more of the books I have.
For pines made from plastic Christmas trees, you’ll want to apply a little green spray paint and foam after trimming the branches.Ideally this will disguise the twisted-wire center sufficiently–and the base will be inserted into a hole in your layout and covered with glue. If a tree is mostly trunk, clay can be used to disguise wire-based trees…in fact one old technique is to twist bits of wire into a treelike shape, cover with clay, scribe in bark, paint it gray (Look outside! Most tree bark is GRAY, not brown!) and add foliage.