Here are some links sent to me by Bob Glorioso of the Stillmeadow Crossing Model Railroad group here in historic York, Pennsylvania. Some of these websites are useful for getting ideas for modeling trees for miniature wargaming layouts.
Wood dowel, wire branches (mass production)
Bottle brush pine
“Wispy” pines (see the second half of the article)
Fern-branch pines from dowels
Fern-branch pines from balsa
Autumn trees (dried flowers and melted plastic straw)
A thread with really inspiring pictures of model trees, but you have to do a lot of clicking and reading to find the secrets (link to middle of thread)