My wife's grandma is 94. When she was only a few years younger, her eyesight was better and here hands more steady. She used to pass some of her spare time making miniature trees with copper wire and beads.
The exact same method works excellently to make trees for dioramas. Here's how you can make diorama trees that are larger than the mini-trees you'll find in a hobby store. They look very convincing. It may look hard at first, but it's really easy.
The whole method consists of making a metal wire skeleton, that can be covered by different methods of your choice to simulate the bark.
You need nine or twelve strands of wire. The length of each strand is easy to calculate : first you decide how high the tree needs to be, then you multiply its heighth with 2,5. (two point five). For example : a tree of 30 cm needs strands of wire of 30 x 2,5 = 75 cm. I use the kind of wire that you'll find in a DIY store or garden center, with a thickness of about 0,5 mm.
Part one : making the roots
You cut nine or twelve strands to equal size, and hold them together. You bend the whole bunch in the middle.
Next, you place 3 or 4 fingers (depending on tree size) on the middle section to form a loop around your fingers.
Next, you twist the strands a few times to make this a firm loop.
The loop that you now created, will become the base of the tree's root system. Divide the loop into bunches of one, two, three or four wires each, thus creating separate loops.
Now start twisting each of these loops to form the root system. These roots are placed horizontally: they become the basis of your tree. This basis will be buried in the base of your diorama, or you can even make part of those tree roots visible on top of the grass or soil the tree "grows" in.
Start twisting each of the individual loops a few times, then split them in two or three again and continue twisting till you get a root-like system.
Now the root system starts taking shape. First you spread the roots horizontally in a way that you like, and that seems natural. The roots in this picture show you the result we have reached thus far (branches come in the next chapter).
Next, you take a pair of pliers and snap the individual little loops in half. Thus, the roots don't end in an individual little loop (one strand thick), but in two loose pieces. Don't cut right in the middle (at six O'clock), but somewhere between three and five o'clock, so you end up with two pieces of different length. This is illustrated further down when we discuss the tree's branches. Straighten the pieces out so they look like real pieces of twisted root.
Part two : stem and branches
To create the top half of the tree, gently twist the 18 (in case you use 9 wires) or 24 (12 wires) to make a stem. You do this for a few centimeters, until the height where you want to make the lowest branch. At that place you take four strands and separate those from the rest of the bunch. Those need not be four strands that were already sitting together. You get the best result when you take two strands from each side of the bunch.
Twist these four strands together to make a branch. You make that a few centimeters long.
After twisting a branch for a few centimeters, take two strands apart and twist those again. Twist the other two strands together also. Make sure there's a balanced distance between the heighth of the first branch, the length of the first branch to the first "split" and the rest of the branch.
Now continue twisting the stem unto the stage where you want to make a second branch. There you repeat the process with four strands of wire, etc...
Adding extra branches
When you make a big tree in this way, and you use relatively thin strands, it may look a bit thin. You can make your tree more bushy by adding extra wires. For this, you can use the same thickness of wire or a thinner wire. Don't use very thin wire : it is important that your tree structure remains quite strong and does not bend too much. Too much flexibility will make your tree vulnerable to damage after you started to cover the metal wiring with plaster or other material (see further).
Wind the extra wire up the stem of your tree, up to the place where you want to add and extra branch. You can use one strand or wire for this, or two. To make the branch, make a loop. Make it about twice as long as you want the branch to be. Then keep winding the rest of the wire up the tree, and cut it at a convenient location (the last part where you cut it can serve as a small branch).
Start winding the loop (branch up to a stage where you want a side branch. If you use two strands here, you can split one off to make the branch. If you use only one strand, twist it in half so you end up with two loops. Split one off as a side branch.
Continue the same process till the loop has ended in several side branches and even smaller branches. You will end up with small loops where the branches have to be, like you did when making the roots.
Cut these loops in half (somewhere between three and five O'clock), so you make each loop into two small branch endings.
You can repeat this process with more strands of wire (blue in the figure) until your tree looks good.
When the whole tree is finished, start shaping it up. Twist and turn the branches till they look natural. When branches split in two, this is often horizontally, not so much vertically. This depends on the type of tree you try to make.
Covering up the tree skeleton - make bark
Next step, you need to cover the metal frame. Try to cover ALL metal parts, to avoid your tree oxidizing later. Before you do this, make sure your tree skeleton looks right. Bend and shape the branches and roots till they "feel right".
Several materials can be used to cover the tree. Several brands of plaster can do the trick. I would not use expensive high quality products like "magic sculpt" or milliput. PU resin is not the best choice either, since it is too runny (and too expensive).
After trying several options, my choice goes to a product that is made to glue bathroom tiles. It is a powdery cement-like substance that is mixed with water, so you can make it as thin as you want and apply it with a paintbrush. I prefer it to plaster for two reasons : it contains a sticky substance that is meant to literally "glue" the tiles to the wall, so it sticks well to the tree frame and does not run off. This gluey aspect makes it a bit more flexible than plaster too (which may crack if you're not careful). Furthermore, it has a slightly sandy aspect, that gives your tree a natural look.
(picture of the same metal skeleton covered)
Making trees like this takes some experience. Even if your first tree does not look perfect, it will have taught you all you need to know to make a splendid second tree.
Adding twigs
Too add finer branches, you can also call in the help of Mother Nature. Several "brands" of shrubs and trees - or their roots - offer a nice selection of fine twig-like extensions that do just the trick. You can add these individually to the tree structure by using a drop of CA glue, or tie them to the branch with a stand of sewing thread. Twist the thread around the "branch" and the small twig(s) you want to add, then add a drop of glue. Finally, remove the surplus thread.
Instead of one piece of twig, you can tie a few of them together first. To do this, you place a few twigs next to eachother, as if you would arrange a bouquet of flowers. When you like the result, add a drop of CA glue at the basis where they must be kept together, and wind a string of sewing-thread around that place. The drop of CA glue will make sure the thread stays fixed for the coming 348 years. Next, you cut away everything below the thread with a sharp hobby knife.
The resulting "bouquet" of twigs can now be added to the tree, at the place where you think this will look right. Keep repeating this process till the whole tree is done. This is a painstaking job that can take an hour - or more - but the end result is worth it !
Here's a few pictures of the same tree. I entered it in the dio contest. Regretfully, I did not take any pictures before the snow was added, but I think the pictures give a good idea of the final structure. Since this is a winter tree, no leaves were added : only branches and twigs.
You could also use the commercially marketed "moss" to replace these twigs, but I think they don't look convincing.
Adding leaves
Ah ! A lot has been said and written on this subject. Many products can help you here, and I'm far from the expert.
To make a naked tree (dead trees or trees in winter, autumn), you can leave the tree as it is and just spraypaint it. You can make it even nicer by adding extra branches from nature. Several bushy plants have roots that look fabulous as model tree branches. you can glue these into your tree, and cover with some plaster/tile glue where needed.
When the whole tree is finished, you can add extra drama by spraypainting it. This too is a time-consuming process, but it works wonders. Start by painting the whole tree dark grey (almost black) or very dark brown, depending on the result you want. Then start adding more spraypaint colour as you go along. Look at trees in your garden or in nature to see how nature "paints" its trees in summer, winter, fall,... Not just the leaves, but also the stem and branches. A coating with dark green (the one-celled moss that covers most tree stems) looks convincing, too. Make sure you mix your colours right before you add them to the tree.
Leaves are not my specialty, but I discovered a superb product that makes the most wonderrful leaves : birch tree seeds. When you roll a seed cone in your hands, it breaks down into individual 1/35 or 1/15 (120mm series) scale leaves. All you need to do is spraypaint them.
There are several diorama wizards on Armorama who can advise on leaves. There's plenty of specialized products on the market, not to mention what mother nature and your wives spices and herbs cupboard can offer you !
Snow
There are several ways to add snow, and several threads in the diorama forum deal with these. To add snow on a tree, you need to make sure it looks 'real". That means adding snow to the big branches and the stem, as well as to the small twigs.
In the example tree, I sprayed a mixture of white glue with water on the tree, and "snowed" microballoons over that. It looked splendid. But for two reasons I'll never do this again : the microballoons are a very unhealthy product that irritated my eyes and throat for several days. Enough reason to ban this from my household forever ! Since they are very light, they spread all over the place. You could achieve the same result with other products like talcum powder or dry shampoo powder. Avoid all edible products like flower or starch : they attract unwanted visitors (bugs !) and colour yellow/brown after a while.
The same thing happened to me with the glue. Don't just use any white glue to add your snow. I'd suggest you add white paint to the glue, to avoid discolouring.
Good luck, and have fun with this !
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and I already made the tree for this post/feature, I used it for the dio contest. That's why the snow was added in the end. 
