This is a tree I made just like Russ suggested. I don't remember how many strands I used, but it must have been about a dozen. Florist wire is a very soft iron wire. Here you get it 'natural' (bare iron wire) or green (plastic covered). I prefer the bare wire, since after forming the tree you can solder the twisted strands together. This way it won't fall apart later by untwisting itself.
When I had finished the wire skeleton I smeared some spackle compount over the trunk and the thicker branches, then I painted the whole thing a brownisch grey.
For the foliage I worked some dark brown tinted lichen into the tree crown, then added some Woodland Scenic coarse turf.
Ron