Why Branch Fell off of Mesquite Tree

 Q. This large branch suddenly fell off our 22 yr. old Mesquite tree. We have no idea why. Any thoughts?

When a stem becomes a branch, there is just a thin cylinder that is living inside this branch. The rest is dead wood.

A. When stems are first growing and just young small suckers the entire inside is living and pumping water up and down to the leaves and roots. If this upright sucker is getting plenty of water and lots of light, the growth of this sucker will be fast. As this young stem becomes older and larger, the inside of it dies and becomes dead wood. But inside every large branch there is a cylinder of life protected by the bark that is alive. Instead of having small suckers, stems, that move the water up and down these larger branches don’t need that much living parts of the stem and so this internal "vascular tissue" dies and becomes the internal but dead wood. This internal wood strengthens the branches and keeps them attached to the tree. 

Branches are dead on the inside. This cylinder of living wood inside the branch is "rolling over" the dead wood inside and will eventually fully transport water back and forth between leaves and roots.

Internal Wood is Dead but Strong

This internal wood, since it is dead, can very easily decompose if infected. The outer bark of a tree protects the wood from rotting but if it is damaged by improper pruning or a knife or other accident, then this dead wood on the inside can become infected and die.

 Your Mesquite Branches

The internal wood of this stem on your Mesquite tree has rotted. The stem has weakened because the internal dead wood has rotted or is rotting. This is why I encourage people to sanitize the saw, lopper or pruner before cutting into the bark. Sanitizing sharp pruning instruments helps to protect the internal wood from getting infected, rotting and weakening the limbs. Dead or dying limbs are blown off the tree more easily than strong healthy limbs.

 

This mesquite tree is infected with slime flux or wetwood disease which is common on mesquite. This bacterium will rot the interior of the branch and weaken it.

If you look closely at the internal damage of this branch, most likely you will see where the bark has been injured and disease rotting organisms have gained a foothold inside the branch.