Is it over reacting?

Trees respond to damage or decay by forming reaction wood.  The placement and extent of reaction wood can be telling of where and how long the tree has been affected by decay.  When we think about a tree responding to decay organisms,  most often our first thought is the compartmentalization of decay in trees (CODIT) model.  However, the CODIT model is based upon a tree's reaction to damage to the bark and cambium.  When decay enters from the tree's structural root system, or perhaps an old tap root that has died off, trees react a little differently since the decay organism is growing in duramen (heartwood) tissue, which is effectively dead structural tissue.

The root flare of the willow oak pictured below has been quite disfigured by years of attempting to out grow Inonotus dryadeus and Ganoderma lucidum.  When sampled with an IML-Resistograph, what appears to be a substantial amount of wood reveals areas and pockets of substantial decay.

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)'

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.

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.

To Stub or not to Stub?

For decades we were taught to prune all limbs, big or small, at the branch collar.  However, over the past few years a debate has been growing about what to do when pruning large limbs.  Some would argue that when removing large branches or stems it is better practice to leave a stub.  The thought is by leaving a stub, decay organisms would take more time to enter the parent stem, thus offsetting internal decay and structural weakness for some time.

I have debated this in my head for years, but recently while reading Fungal Strategies of Wood Decay in Trees by Schwartze, Engels, and Mattheck, I came across these lines.  "After removal of a branch, this cut surface is like a chemical battlefield, as not only spores of wood decay fungi germinate there but also the spores of many other fungi, e.g. the mold fungi.  Because of the competitive pressure and interplay of different fungi, it is much more difficult for wood decay fungi to become established on such a substraight..."  It goes on to say that leaving a  large stub allows more potential for wood decay to become established, and once established, wood decay is difficult to slow down.

Now of course, tree species and specific wood decay organisms all play a part. From the conservation biology point of view, large declining stubs are also home for many species of arthropods and fungi that would otherwise not have a home, so there may be some intrinsic value.  With that in mind, site use and targets may come in to play.