Essential wire tree techniques that will save you time and ensure realistic trees.

The first step in the process is to decide on what type tree you want to make. Your decision fixes the way you make your armature.
And do some basic research by looking at prototypes and/or gathering pictures. Without this information your end result is likely to be, to make an understatement, less than satisfactory.
Wires To Use
This really depends on the size of your tree.
The size of the tree determines the diameter of the trunk. With due consideration to the fact that you will cover the trunk with glue and sawdust, you can determine the thickness of the core.
The core can consist of a single wire or twisted fine wires. For a tall pine or beech, you may even choose a wood dowel.
How To Make A Wire Tree Trunk
Here we have to refer you over to pages that discuss specific types of trees.
Here is a link to how to make a Scotts Pine
How To make Pine trees.
Here is a link to how to make a Birch.
Here is a link to how to make an Oak , Fruit Tree or other short trunk tree.
Here is a link to how to make a Scotts Pine
How To Add Branches To A Trunk
How To Make Large Branches And Twigs
Make A Simple Practice Trunk (Better Trunks)