Tree ratings in kN

I just came across this article describing research that estimates the kN of force a tree can withstand. According to the article, I am sometimes using trees that are too small for anchor building. What are your criteria for using a single tree as an anchor?

Article: http://blog.alpineinstitute.com/2019/09/tree-ratings-in-kn.html

Very interesting topic. 

Ideally, of course, you want to build your anchor on a very strong looking tree.
However, in reality, it's not always possible.
Last winter, while ice climbing I found myself a the top of a pitch (snow slop) and all that was available was a medium size (still standing) broken dead tree. No other choice but to use that.
Worst than that, also ice climbing, I once build an anchor by joining many branches and linking them with a massive sling, most of it were not much more than bushes of twigs...

Bottom line, go for a tree that is:
- Alive
- Seems to have strong and deep roots
- Big enough - I would say 20 cms of diametre is a minimum 
- If you can't do any of that try to have a a second or even a third point (other tree, cams, nuts, screws, rocks, bolts, pitons...) and just do your best

Those numbers are circumference. It seems that a 7" diameter tree is solid. That's been my experience, although I've rapped off thinner than that.

As was briefly mentioned in that linked article, it will also depend very much on the root system of said tree and the soil depth/condition around it. There are fairly small diameter trees that have very deep root systems whereas some larger trees have only superficial roots. Perhaps there is a climber-tree expert who can expand on this. 

Daniel Joder wrote:

As was briefly mentioned in that linked article, it will also depend very much on the root system of said tree and the soil depth/condition around it. There are fairly small diameter trees that have very deep root systems whereas some larger trees have only superficial roots. Perhaps there is a climber-tree expert who can expand on this. 

This is way more important then diameter. I climb trees professionally on top of rock climbing, but I trust trees on a cliff way less then those in someone's yard. 

In a yard on a nice big tree with an expansive root system in good soil I will travel out on to branches that are only 2 or 3 inches in diameter which also may be leveraged out away from the base. I'll even use dead branches that are 1inch in diameter given that I'm as close to their attachment point so as to not leverage them. Tree species will also play a role in my decision making here, but to a lesser extent. 

One of the main differences is that I'm not exerting much dynamic force in these situations.

The other biggest factor is the root system. A large tree on a cliff may have a small root system which would be it's failure point. The best rule of thumb is as above, so below - but shouldn't be trusted 100%. Bonsai trees stay small in part because they have a small root system, which can also be said for cliff trees. A big cliff tree likely has a larger root system drop into the cracks.

 If you aren't putting much force on a small cliff tree for a rappel, it may be perfect and the root system in the crack may act as a stopper. The same tree may snap at the junction point between roots and stem if you took a lead or even top rope fall on it. A deep rooted tree on top of a cliff of only 3-5 inches diameter I would feel more then comfortable rappelling of off and would certainly add it to an anchor set up if nothing else existed (I don't like using trees for anchors if I have gear because slinging trees, even the proper way, damages them a bit). The same tree size with a shallow root system, because the top of the cliff is mostly rock, I would be very worried it would tip over or  even slide because the root system has grow to stabilize the tree to vertical forces but doesn't have much to grip in the soil for horizontal forces. 

no i Hard and fast size rules are hard for biological anchors. In general, arborists consider wrist size to be the smallest diameter advisable to tie in to. That's species dependent however. Consider also that most rappels in rock climbing are from the lower third of the trunk, and exert much smaller forces than tying into the top. 

My time in tree care has made me more trusting of trees as anchors, you can't believe what I've seen them support in rigging operations. 

Might be able to find some interesting bits and bobs here: https://m.facebook.com/pages/category/Community/Biomechanics-Research-with-Trees-189328794443827/

I was supposed to go last year but