Ok so I was looking around in flickr for some examples of a situations where automatrick tree generator is not suficient, and i fund lots of good pictures. Some depict tree holes, while others depict tree roots or other parts. The funny thing is that I contacted flicker users who uploaded pictures, and asked theyr permision to use images in this blog,. … guess what 100% of people who replyed to me sead ok… that was a nice surprice actualy. So here we go: 
please click on it to enelarge… (i increased contrast in moust of images, i hope authors will not get too angry, its all about showing tree details, not art… well who knows about that :) and if there are any erors wiyh titles names or links, write me, it will be corrected.)
credits:
1. Tree with hole through it, Martin LaBar / Martin LaBar, http://www.flickr.com/people/martinlabar/
2. Tree Hole, Cryodigital, http://www.flickr.com/people/cryodigital/
3. Tree Hole Entry, Minnie Wong, http://www.flickr.com/people/26133907@N00/
4. Tree and hole ,Adventurer Dustin Holmes / Dustin Holmes, http://www.flickr.com/people/dustinholmes/
5. Tree holes, Opa Schoch / Shok the German, http://www.flickr.com/people/opaschoch/
6. Hole in the Tree, Kent Sandvik, http://www.flickr.com/people/kentsandvik/
7. Tree with Hole, brents pix’s, http://www.flickr.com/people/brentspix/
8. Baobab tree with a hole through its trunk, tommyimages_com’s, http://www.flickr.com/people/cuba_photos/
9. Hole-y Tree, Kuzeyli48’s, http://www.flickr.com/people/21915247@N07/
and, not saying that its best or anything like that, my manualy hand modeled tree…
and a wiki entry on a matter here :
http://en.wikipedia.org/wiki/Tree_hollow
and some copy paste from this article: (aka interesting fackts (if we trust wiki :) ) )
A tree hollow or tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree.
Hollows may form as the result of physiological stress from natural forces causing the excavating and exposure of the heartwood. Forces including wind, fire, heat, lightning, rain, attack from insects (such as termites or beetles), bacteria, or fungi. Also, trees may self-prune, dropping lower branches as they reach maturity, exposing the area where the branch was attached. Many animals further develop the hollows using using instruments such as their beak, teeth or claws.
In Australia, 304 vertebrate species are known to use tree hollows in Australia: 29 amphibians, 78 reptiles, 111 birds, 86 mammals.[2] Approximately 100 of these are now rare, threatened or near-threatened on Australian State or Commonwealth legislation, in part because of the removal of hollow-bearing trees[8][1].
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