Yanks spell it "fiber", lots of info to be found if you google using their spelling (try "making poly fiber trees quickly model railroading" https://www.google.com.au/search?q=poly+fiber+trees&source=lnms&sa=X&ei=ZE3BUqTaCsfckgW5oIDwAQ&ved=0CAYQ_AUoAA&biw=1366&bih=528&dpr=1#q=making+poly+fiber+trees+quickly+model+railroading&revid=206607746). The stuff does not like dyes to colour it, cheap spay paint (brown, grey, black) is a good way to achieve all over coverage. Spray adhesives then work well to attach ground foam or other leaf materials.
Many of the links on the search no longer seem to exist. Looking at some layouts, especially the ones that have modelled the tree side scenes like found in some of the U.S mountains as seen here look exceptional, also some good tips included.http://www.railroad-line.com/forum/topic.asp?TOPIC_ID=22650&whichpage=2
The thing is that looking at the treed hills both real & modelled are certainly something not really seen in many, if any areas in Oz, especially with the denseness of the canopy seen up close like on that forum link.
Sure there are some areas such as along mountain ranges, but few seem to have such a heavy treed blanketing, that could really be modelled in the sort of Puff ball method used here, a shame really as the U.S layouts do look nice. What I see of much of the dense treed areas here is of larger trees of varying heights, without much uniformity, & getting into the bush areas, the underbrush is also quite dense, & at least for me not easy to recreate.
Coming back to the various methods used on those links that are highlighted & a couple of others, it is certainly seems a messy job, & having tried the Micro Mark fibres in black, I have to say, that after using around 1/4 a bag, I gave the rest away, I used both hair spray & spray glues which both mediums seemed to just tighten the fibres & they seemed forever sticky, likewise I never really was able to cover the sprues, spray them & have the fibres retain their shape.
So much so, that I am now progressively removing that fibre & redoing using the Bachman product. The other aspect that I did not like so much with the Micro Mark product was the black colour of it, meaning that when the fibres shrank by the aerosol affect, no matter how hard I tried to cover the black fibre it would not completely stick, even though the fibre remained tacky to touch, it also meant you can readily see the black fibre under the foam.
I do know others who swear by it though, while others prefer to swear at it.