Understanding forks a bit more

[Island Lescure]

my experience of working in trees with co dom included bark unions tells me that once the weight is removed above the union you can often see the two stems sitting closer together, in fact I've seen a climber get his foot jammed as the space between the stems closed after he removed a head above the union.

 

Often see the saw jamming in the cut if it's started from the outside of the union as the compression from the other side forces the other stem down on the bar of the saw.

 

The way round that is to bore into the centre of the union and cut outwards.

 

Ya, totally agree! I have gotten some saws and feet jammed. But, and let me try to get this right, the closing of the unions after the removal of a load would seem to happen because the forces used to support the weight are suddenly not having to support so much weight and therefore the forces, compressive or tensile depending on the load above, but in the case of the union closing, compressive on the outside and tensile within the fork, push or pull in their respective direction with more ease than if they were loaded.

 

And then the saw gets trapped because the two stems/branches, no longer loaded, are pushed against each other, but I would think that that is the reason, indirectly because of the forces within the stems themselves.

 

That makes sense to me, please correct me if I am wrong.

[scotspine1]

If you look at the lower right drawing of the longitudinal crack, assume you've removed both heads of the stems and cut below the union so all you have left is what is shown in the diagram, in my experience the saw will often jam if you try to cut through parallel to what Mattheck is calling the lower crack front - this happens because of the compression, have seen it in both conifer and deciduous species, Lawson Cypress and Beech for example. Will try to get a vid and post on this thread.

Edited September 3, 2016 by scotspine1

[Island Lescure]

Yes there is some compression there. I have definitely gotten a saw trapped that way before and indeed boring through the middle prevents that from happening. But I think the idea is that this compressive strength is not strong enough to cause the splitting of the forks and when it is, the cambium layers, from each stem, will crush/kill each other resulting in no further, significant, compression.