Interesting, it might be related to the performance of the cambium, a non-functioning cambuim isn't the right terminlogy here, but a cambium of let's say limited capacity might be a factor in limb drop.
Larry
Cambium is a thin layer of active cells that devide to produce xylem and phloem.
One cause is that there is a temporary loss of turgidity in the cells, especially the rays, which causes weakness.
Another, or related is that there is translocation occuring then there is a calm period on a hot humid time of year. Suddenly transpiration at the leaf stops, while transport of fluids continues, then the limb is overloaded with fluids. A gust of wind causes movement that cannot be supported at the moment of bend.
With the breaking of a limb under the climber, often that is caused by some dry wood on the tension side of the area of moment. The wood is not rotten, but altered. In wood with karge ray cells this is more common because the turgor pressure helps with the cohesion of the xylem.
These first few rings shearm then the rest peel out. About 8 years ago I had an ash go like this with SLD at night. I posted pics on Russ Carlsons KnotHole and he and Scott sent me copies of the pics with red arrows showing the initial points of failure and described the probable modality of the failure.
Since then I've ben able to examine quite a few limb failures and see the pattern. Often there is a distinct reaction zone in a small part of the limb, and a wound down-stem.
If you do not ahe one, keep a lop or magnifying glass on the truck. Look at the break, and then the rings just beyond the torn fibers. Silky saws are perfect for this because the cuts are so clean.
Have fun, be curiouse!
