Structural Root Decay: Another Reason to not be a Fan of High Nitrogen Fertilization

While browsing through Fungal Strategies of Wood Decay in Trees by Schwartze, Engels, and Mattheck, I came across an interesting piece of information.  The authors state decay progresses faster in wood with higher concentrations of nitrogen (N).  They then imply a correlation between high N fertilization and increased amount N in structural roots.  If these hold true applying fertilizers high in N, like many commercial tree and lawn companies, will actually speed up wood degradation by fungal decay agents and increase the risk of tree failure.

Now in cases where known root decay is present but not at the point where removal is considered necessary, some tree managers will recommend tree fertilization in an attempt to out grow, or at least stay pace with, the decay.  If the proper fertilizer analysis is not recommended we may actually be speeding up the decay process.  Custom fertilizing based upon the results of a soil sample is really the best practice when specific tree health goals are priority.

Mmmm nitrogen.

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)'

Compaction for the Sake of Grass?

One of my favorite calls is from a contractor, in this case a very large landscape company, that has been busted by the city for failing to do any tree protection.

So our assignment is to decompact areas leveled for the sake of sod installation.  Our medium is compacted red clay.  If you have used an AirKife or Airspade in this scenario then you know attacking glazed clay is futile.  I go about it by finding an area where I can penetrate the soil as close to the severe compaction as possible.  Once deep in the soil,  I angle the air tool under the glazed area.of compaction.  By repeating these steps around the area, it eventually begins to break up in to large peds.

In the past I would try to break up peds into finer soil, but after having gone to Jim Urban's 'Up by Roots,' I now try to preserve peds.  These peds are an essential part of soil structure, especially in heavy clay soils.  Of course, I then mix in composted organic material to help maintain soil structure and improve nutrient holding and water holding capacity.

An argument may by made that August is not the best time to be using an AirKnife in such a large area, as it may dessicate fine roots.  In the face of progress, there are only so many options.

Roots and Backhoes

We were called out because the developer wanted to save this tree... after,of course, they already prepped the area for the new retaining wall.

The minimum safe distance for trenching on one side of this 40 inch plus Southern red oak according to industry standards should be around 12ft.  This excavation is about 7ft away from the trunk.

Tree health is no longer a concern, instead this tree has been made a potential hazard.  This picture is a good view of the amount of root mass under the soil.

Vertical Root

We have been working with a local builder on some tree preservation.  This picture is of a vertical sinker root and parent root about 15 feet from the base a 40 inch+ willow oak.  The top of the horizontal root is about 2 feet below grade.
Maybe not the greatest picture, but a rare glimpse of tree structure within the soil.