I have been scratchbuilding my own trees since 1999, and this article represents my current best-effort. The method I describe here is heavily influenced by Gordon Gravett's first book, Modelling Trees. I am nowhere near where he takes the art of making scale model trees, but I will keep aspiring.
My construction of trees starts with floral stem wire that you can find at nearly any general hobby or crafts store. I get mine at Hobby Lobby which is not too far from the house. There are ones that have paper wrapping and those that don't. The ones without are thinner, but are harder on the fingers to manipulate. The ones with the paper can be glued together (as I'll describe below), which makes the tree's trunk solid very early on in the construction process. However, the thinner wire allows you get the same thickness tree, but you get a lot more branches. Either approach works.
The process starts by simply twisting two of the wires together at about 1/3 from the top of the wires. Just a few twists are needed; enough to keep them from unraveling.
The length of these wires determines the height of the tree. This shows me building a large tree, so I am using the full 18-inch length of these wires. I used the entire package for this one tree. The more groups of twisted wire, the thicker the tree. To continue, I twist more two-wire pairs.
Next, merge two of those twisted pair into one, as shown here. I like to form them together at random locations for variety. I am not making any particular species of tree here (I am not that advanced yet).
Next, I grab two of those sets of twisted-together pair, and merge them together for an even bigger bunch.
I keep doing that with other sets as well. At some point in time the bunches become too tough to attempt to merge them together again, so I just group them together. I put them together in different locations, so that some of the wires become lower branches and others stick out at the top.
I use 26-gauge florist wire (also bought at Hobby Lobby) to hold the whole tree's worth of bunches together. I simply wrap it tightly around the bunch, hiding the ends inside the bunch (which makes it look nice, and also prevents you from getting stung by it). Only a small section of the main trunk needs to be wrapped, but that is your call.
On a large tree like this, I form additional groupings of bunches and wrap those with the florist wire as well. These now start to form the major trunks of the tree. At the bottom, I trim the wires, and leave some wire sticking out. These become the roots of the tree. You can bury the roots into the surrounding scenery of your layout or diorama with some Sculptamold. Once painted, they look very convincing. You will notice the left-over pieces. These can, if so desired, be twisted in with some of the upper branches to form additional branches. You can also twist sections of the floral wire into the upper branches to form additional skinnier branches.
Here I am showing the effect of the wire left at the bottom of the tree when I pressed it down on the workbench. At this point you can easily twist and bend the wires of the tree to form the overall shape of the tree. You can trim the branches to a more appropriate length at this point, or do that later.