Making the tree trunks is fairly straightforward. However, when it comes to foliage, things get more challenging. Woodlands Scenics makes foliage mats and poly fiber that you tease apart and spread over branches. I have tried that many times, and I have never been happy with the results. So, the search for finding something that looks more realistic took many years. Gordon Gravett's book mentions that he uses "postiche", which is British for a wig, i.e. fake hair. I went to a local store that I'd never thought I'd enter, the local beauty supply store, and bought a package of fake hair. This photo, due to the surrounding light, makes it look horrible, but it is black, straight hair. I cut 1/4" to 1/2" pieces off of the hair and catch it in a big bowl (kind of like cutting hair).
I tried using that straight hair on a batch of trees I was building, but I found that it leads to "spiky"-looking branches sometimes. Continuing on the same theme, though, I found braids of fake hair, shown here, on eBay. Just do a search for "theatrical fake hair" (my thanks to Trevor Marshall for that tip). You will find lots of beards, but also these braids. For some reason this hair is much finer. It is also wavy. The other thing I found was that it is so much easier to cut. I cut mine at about 1/4" - fairly short. I highly recommend this. I will be using this type of hair from now on.
The fake hair simulates the smaller branches that grow out of the main branches of the armature. To apply them, I use the cheapest hairspray I can find at the grocery store, preferably one that is labeled as "odorless" (at least it reduces the annoying perfume smell). I spray the hairspray on the outer edges of the trunks over the trashcan. I try not to spray any on the main trunk. Then, holding the tree over a large bowl (I found this blue one at the local grocery store for a whopping $1.99), I loosely sprinkle the hair over the tree as I rotate it with my other hand. I then hold the tree upside down and quickly spin it around using the toothpick in the bottom. This flings off any loose hair, which is fine. But more importantly what I have found is that it forces the hair that does stick, to stick out from its branch rather than to lay flat. At first not much hair will stick to the tree. But be patient. Then, back to the trashcan and spray another coating of hairspray. Then repeat all of the above steps. Keep doing this until the tree has the shape that you want. I usually have to do this many times, especially with a tree this large. Keep an eye on the lower branches, because they tend to not have any hair on them. They will look out-of-place later on, so make sure they get some hair as well. I spray one final coat of hairspray over the whole tree when I am done. Set aside and let dry. I usually build three or more trees at the same time, so this "downtime" can be used to work on the others.
And here is a profile view of the tree with its hundreds of finer branches. As you add more layers of the hair, some of it will start to stick to previously-applied hair/branches. This will start to form clusters of branches near the ends of the main armature branch. That is really what you are looking for in this approach. This example used the straight wig hair, and you can see how some of them are starting to look a bit too "spiky".
And now for the "leaves". I will show two techniques I use. In the past I have used Woodland Scenics' fine blended turf. This works great in a smaller scale like N-scale, but for S-scale, I wound up with very dense looking trees. I didn't like the result. I then thought that maybe larger chunks are needed since this is a larger scale. I made a couple of trees and then used Woodland Scenics' "Underbrush" ground foam. This looked better, but it, too, quickly made the tree look too dense. Trees need to have an airy feel to them. You should be able to look through them. Next, I tried a few trees with Scenic Express' "Flock & Turf", but the result looked odd. They have some color particles in them that look OK for ground cover, but not for trees. Later, I found a leftover container with some foam material in it. I tried that with one of my recent trees and really liked the result. It turns out that it was Woodland Scenics' "Coarse Turf". I ordered containers of three different colors and tried them out on the tree you see here. It is just the perfect size, shape, and texture, I think. Additionally, I tried a new approach here which looks great in person, but is somewhat hard to see in these photographs. I used a lighter color in the upper part of the tree, and a darker one near the bottom. The photo was taken after I applied "Medium Green" coarse turf to the tops of the tree branches, again applied with hairspray. The coarse turf material sticks to the hair, which winds up giving it the airy feeling that I had been looking for, while also appearing to form groups of leaves from a distance. It didn't want this tree to be too "dense". It kind of looks like a very old tree that doesn't have full coverage of leaves anymore.
I then flipped the tree upside-down, and applied hairspray, but this time I used "Conifer" (a darker green) coarse turf. This, then, sticks to the bottom of the branches. It makes the branches look fuller, and it also conveys a shadowy under-branch area. It looks very convincing in person, I think. This photo shows the tree with all of its leaves. Again, I purposely didn't overdo it on this one.
The other method for simulating leaves is to use Selkirks' leaves. The original owner passed away in 2013, but a new owner, Nigel Knight, has taken over the company (his web site is gone, but you can contact him at: email@example.com). He was very easy to work with in me getting an order of the bags of leaves you see in the photo. Based on Trevor Marshall's recommendation, I bought the O-scale ("standard" size) leaves. I like them, but I think for S-scale the HO-scale ("fine" size) leaves would work well, too. A mixture of both might make for a nice variety. In my order of the leaves, Nigel included a package of his "branches". Upon examination, it turned out to be pre-cut, 1-inch long, twine/rope/sisal, similar to the big roll you see in the photo.
A close-up photo of the same area as above, but after the roots are being integrated into the surrounding ground.
I always save the trees when I demolish a layout. A few of my early attempts have been trashed, but I still have all the rest. While I was still active in the local model railroad club, I'd build a set of a new trees (usually about ten of them) to put on the club's layout for the next show. After the show, I'd "plant" them on my home layout. Then I'd do those same steps again before the next show. This motivated me to keep building these trees, and also have some new ones to show the club members and the audience members each time we did a local show.
A couple of things I have learned along the way. These types of trees are not something you can build if you have a "basement empire" to populate. It takes me several weeks to do ten trees! I don't mind, as all of them are "foreground" trees for my home layout(s), but a different methodology has to be adapted if you have many square feet of trees to plant.
The hairspray does not hold "forever". Each time I interact with the tree, leaves fall off. So, sometimes it is necessary to re-apply the foliage to a tree.
For some reason our cats seem to think the Selkirk leaves are real leaves, and so several of them have chewed on my trees if they snuck onto the layout.