I've used dried grape stems for the trunks of trees and then chopped up various colours of greens, yellows, oranges, etc,of Woodland Scenics Foliage and glued the bits onto the stem with White glue, depending upon the season I was trying simulate. I have also used a bit of gray and/or white paint on the stems to give the impression of poplars or birches. Don't look too bad. The only snag is that the stems become very brittle with age and are susceptible to breakage quite easily. For that reason, I usually put them in the background where they are less likely to be handled.
I have kept some articles from Model Railroader that pertain to tree construction. One of them recommends the use of the tops of a plant called Sedum, which when dried, make good trunks.
Unfortunately, I didn't make a note of which issues these articles are from, but I could E-mail copies (one page at a time)- I haven't yet figured out how to send more than one page at a time via my scanner. I'm 77 so not super computer literate.
I'm still looking for a photo of myself to post but haven't found one yet that wouldn't scare every off this site!