Tree Root Failure: A Discussion

Earlier this week an arborist friend and I were discussing root failure.  The discussion revolved around, if roots were severed, which direction would the tree fall.  His argument was the tree would fall towards the severed roots, while my argument was the tree would most likely fall away from the severed roots.  I believe my argument, the tree would most likely fall away from the severed roots is supported by a publication from the University of Georgia Root Strength  & Tree  Anchorage.

Root failure occurs in association with wind load and gravity.  'Beyond the root plate area, root tensile strength becomes more critical to anchorage.'  That is, as the wind blows roots on the wind ward side of the tree, the tension roots, are doing the most work.  Relatively speaking of course.  If these roots should be severed or damaged the tree would fail in on the opposite side of the force, or away from the root damage. Mattheck describes the roots on the tension side of the tree as forming holding knots in the soil.  Again, considering damage to the tension side of the tree will give a hint to where the tree may fall.

In addition to these references, I have also seen trees that are root decayed fall opposite the decay.  Now I could be wrong in my conclusion, but I think I make a pretty good case.  Some other interesting information from the UGA publication:

-'To summarize, a few large diameter and long roots can not provide effective resistance to failure.  It is in the proliferation of smaller roots in consolidation of the root plate which provides anchorage success.  (Stufka & Kodrik 2008)'

- 'Compression strength increases for a short distance from the stem base before declining with length.  Root compressive strength was found to be roughly the same for angiosperms and gymnosperms, but bending strength was found to be much greater in angiosperms. (Stokes & Mattheck 1996)'

-'Anchorage is concentrated in two general locations around a tree base: 1)  close to the stem base on the
leeward side and focused on several large diameter roots; and,  2) farther away from the stem base on the windward side in many, smaller, large surface area, near-surface roots.  (Danjon et.al. 2005)'

-'Windward roots have forces applied which are concentrated approximately 1.5X (one and one-half times) farther away from the stem base than leeward roots.  (Stokes 1999)'

Lions Tailing: How not to prune

Lions tailing is when interior branches and sprouts are removed along a stem or trunk.  This is an incorrect pruning practice because these interior branches are functional parts of the tree.  These branches produce carbohydrates that help with branch/stem taper.  Branches/stems with good taper are less prune to failure during storm events vs. branches/stems that have little variance in stem diameter throughout their length.    

During hot days leaves on the exterior of the crown close their stomates to prevent water loss.  While stomates are closed photosynthesis stops, and these leaves no longer produce food.  However, these exterior leaves shade the interior of the canopy, keeping it cool. The interior leaves will continue photosynthesis.  

There are few reasons to prune branches on the interior of the canopy outside of pruning crossing and rubbing branches.  If crown thinning is required, the cuts should be made closer to the branch ends to help alleviate weight on the branch (in my opinion).

While the picture below isn't great, it is a good example of what not to do when pruning a branch.  Wedding decorations are being hung from this branch, so we may be lenient on the arborist that performed the pruning.

Armillaria: Tree Failure

Armillaria, or honey fungus, is a genus that covers several species of soil born root decay fungi.  The common name is derived from the yellow-orangish mushrooms depicted in the pictures below.  Armillaria causes a white rot, which like Inonotus (as described in an earlier post) begins by breaking down lignin and in advanced stages breaks down cellulose and hemicellulose.   

While white rots are usually described as slow moving decay organisms, Armillaria is noted as being more aggressive in its attack on tree structure.  Armillaria gets its other common name 'black shoe string fungus' from the black mycelial fans it puts out to devour wood.  Unlike other structural root decay fungi, Armillaria also can affect the vascular system of the tree, so early signs of infection may be noticed by a decrease in tree vigor and poor tree health.

The pictures below show what happens to trees in advanced stages of Armillaria infection.

The yellowish orange mushrooms of Armillaria protruding from the bark on  the lower stem of the tree.

Notice the tell tale black mycellium penetrating the wood fibers of it's victim.

Reduction of a Silver Maple

I am a firm believer that we can save more mature trees with defects.  Large mature trees provide us with a higher quality of ecosystem services than small trees.  I am not saying we shouldn't be planting trees, but what I am saying, is there are options for retaining our larger trees that some would consider a hazard.

The pictures below are case in point.  This silver maple, thought of as a weak wooded species, has a large cavity in the lower stem, and several decayed root flares.  Three years ago, instead of removing the tree, a drastic crown reduction was performed.  Over all crown weight and height was reduced, not topped out.  The tree still stands today, now shorter, but better protected by wind and weather by its neighboring mature trees.  The concept is basic, shorter and lighter objects are less likely to fall over.

I tell everyone the same thing, "trees don't want to fall over." But, as with everything tree species, age, size, and vitality play into whether or not this is an appropriate treatment based upon the perceived defects.

The arrows point out specific large pruning cuts 

Topped Beech

Simply stated, it should be a crime to top a European beech.

I still love the irony that people think they are making the tree 'safer,' even though the sprouts that form as a panic response are more prone to failure. Boo to this homeowner and what ever hack did the job.

Fall Fungi

As the cool nights and mornings of fall slowly begin creeping in, and we once again are blessed by regular precipitation, start looking at the base if your trees. 

Last week Inonotus dryadeus conks began appearing along the base of willow oaks here in Charlotte.  These conks are the tell tail sign of root decay.  Inonotus causes a white rot which breaks down lignin, a structural component of wood, in the tree.

Many people believe that the amount and placement of decay conks is representative of the amount of decay found in a tree.  While that is usually false, it does hold true with Inonotus.  This is definitely something that needs to be considered when assessing trees with these conks present.

The Case for Proper Pruning

This tree was pruned in Feb 2012, and these cuts were made at the same time.  Notice how the bottom 2 were made correctly just outside the branch collar and are healing nicely.  The top cut was left as a stub, a short stub, but a stub all the same.  This cut shows no sign of callus tissue.

This is a great example of why proper pruning is essential.  This stub is now an entranceway for decay fungi for years to come.  Keep an eye on your cuts everyone.