His trees will not be as quick and easy to make as the method Andrew suggested, but will look a lot better in close up.
Joah,
The furnace filter and/or pot scrubber approach is good for this. It makes for a much denser tree, and once the ground foam is on, they are not light and airy - they are positively "thick"
Another source for "armatures" is old (or new ) artificial Christmas trees. Their little branches can look quite good once they have been doctored with scissors and covered with ground foam. Although they are more expensive, I have also used Dollar Store conifers dipped in paint and covered with ground foam (they are usually 3 or 4 small, or 2-3 medium or 1-2 large for $1).
Andrew
Josh, I'm having to do this from memory because I haven't been to the East coast since our last trip to Florida 2 years ago and I haven't been to the Northeast in at least 30 years, but if I remember correctly, when you look at a forrest in the Northeast, you can only see the trees in the front of the forrest. The trees tend to be so thick that you can only see 10 feet into the forrest. You could use the expensive trees in the front of the forrest and then put cheaper trees or even just green foam clumps behind. Many modelers of Appalachian roads only put trunks on the very front row of the trees and the rest of their trees are just foam. Of course these are mostly hardwood trees that may be green or they may be in fall colors depending on the season being modelled, but I think the same sort of technique could be used for an evergreen forrest.