It's possible!!
Hi Max,
Hope I can help you with these answers...
1. How and with what you cover the tree trunks?
I cover it with sawDUST ar first. It has to be d u s t!!
first and second picture
I add it( from out a theesieve) to the ( primered!) frame of wires with a mix of white glue, some acrylpaint( it is a paint you can mix with water and the white glue) and some drops of dishdetergent (?)( liquid stuff you have to add when you have/must do the dishes.) and a very little bit of water.
I cannot say/tell you how much water but the mix has to be the way that it can stick to the frame but not too sticky because it has to be soaked up by the very fine sawdust...these drops (dishdet.) are very important otherwise the mix will not glue the fine sawdust.
So the mix has to be a little bit like sirop, not like honey..(

) just experiment!
The mix is added with a brush(long small one). Mostly I "paint"2-4 branches and than I add with the sieve the sawdust by ticking against the sieve and turn the frame of the tree so that the sawdust will be glued all over the painted branches. contignu with the other branches and at last the trunk.
If some branches are not thick enough just paint them , if it has dried for one night,oncemore and add the second lay of sawdust , just repeat if the branch has to be thicker! this , you can do also with the trunk..
sometimes, at the last, I add some rough sawdust on the trunk to give it an "old"look.(third picture)
2. What are those fibres used on the tree ( where you get em from )?
Those fibers are almost the same as they use in aquaria and pondfilters..only here they are already colored green and white and I bought of course the green ones) Overhere they use it too in flowershops etc. as decoration
(fourth picture)
Jos
btw/ps: the sawdust is made of MDF