That's going to be trouble...

When assessing a tree's risk of failure we must sometimes look for the finer signs and symptoms that may indicate a tree's potential to fail.  The convenience of giant decay conks, broken roots, and huge decayed cavities may not always be evident on a tree that presents a high risk of failure.

Let's look at some pictures of a tree that from a distance may seem fine, but upon closer inspection proved to be cause for alarm.

Notice the soil heaving on the tension side of the lean.

Saw dust or frass present on several areas around on the lower trunk.  When sounded with a hammer decay was detected.

Small armillaria fruiting bodies protruding from the root flare of the tree on the tension side of the lean.  Armillaria root rot is a serious structural root decaying fungus which results in a white rot.

From a distance this willow oak has a perfectly green and balanced canopy, and is in a beautifully maintained yard.  Upon closer inspection the tree described above displays some serious evidence of strength loss.  Further proof that when walking a property, we can take nothing for granted.

'Crispy black stuff'

Wood decay fungi come in many shapes and sizes.  Their fruiting bodies are generally pretty easy to identify.  Mushrooms and conks along the base of a tree or attached to the trunk can be eye catching.  One commonly over looked and miss identified structural root/basal decay fungi is Brittle Cinder Fungus (Kretzchmaria deusta  formerly Ustulina deusta).  Brittle Cinder causes a soft rot that breaks down cellulose and hemi-cellulose followed by lignin.  This creates a decay that leaves wood feeling brittle.  Early stages of this decay can be hard to detect with a traditional 1/8th-inch bit and drill.

Perhaps the hardest part of identifying this decay fungi is simply noticing it.  Brittle Cinder fruiting bodies first appear as grey-white masses growing only slightly raised from the bark of the tree.  At first sight, they may be mistaken for dead lichens.  As the fruiting bodies mature, they become black and appear as burned bark.  Deusta means 'burned up.'  Unlike most common wood decay fungi, Brittle Cinder is an Ascomycota versus a Basidiomycota.

Brittle Cinder affects a vast array of tree species,including; beech, oak, maple, and linden.  Infection usually occurs through wounds in the bark.  Brittle Cinder can result in significant strength loss, so careful consideration should be taken if this fungus is located on a tree.

Here, Brittle Cinder is growing on the root flare of a red maple.  There is only 1.5-inches to 0-inches of sound wood around the affected area.

Anatomy of a Tree Failure

All trees will eventually fail.  By observing tree structure, site conditions, work history, signs of decay, etc. we may glean some insight as to how likely failure will occur in a given time period.  Let's use the following example as a case study for likelihood of tree failure.

The specimen is a mature willow oak with a 30%-40% lean towards the North East.  Prevailing winds usually blow in from the South West.  The site is several yards from a creek that is known to flood, and the area received several inches of rain over the past few weeks.  Primary power lines are close to the tree. Utility pruning has been performed on the subject for decades, leaving an uneven crown with weight distributed on the leaning portion of the stem.

Upon closer examination Inonotus dryadeus conks are present on the tension side of the lean on the root flare.  Inonotus causes a white rot in the lower stem and structural roots of trees, and is common on willow oaks in this geographic area.  Resistance drilling tells us that up to 50% of the root flare is compromised by some form of decay/damage.
 

Finally, a live structural root, also located on the tension side of the lean, is cracked all the way through.

The culmination of these defects resulted in whole tree failure with property damage (thankfully, no one was injured).  The tree failed in the direction of the lean.

Notice the extensive root decay.

Here we can see partial root place failure due to wet soils.

This tree is an almost text book example of a high risk of failure tree, with final results as such.  As defects compound so does likelihood of failure.  That being said, I have participated in workshops where the same high risk of failure tree has been used for years, and still stands to this day.  Assigning risk and predicting failure is one of the hardest things we do as arborists.  Documentation and communication is key.  

The Root Collar, Oh No

By now we all know trees in the landscape, big or small, should have an exposed root collar. The root collar is the area of the lower trunk that flares out at ground level.  It is the transition area between the stem and the structural root system of a tree.

Tree root collars have evolved over eons to be exposed to air.  Buried root collars are detrimental for several reasons.  Cell respiration is disrupted, adding just one more stress to the tree.  This area of the tree can be intolerant to prolonged soil moisture, which can decompose tree bark and give way to stem pathogens.  Finally, buried root collars can lead to and disguise stem girdling roots.

Stem girdling roots arise in a few ways. When root collars are buried the tree reacts as if part of the root system has been damaged, and the tree forms a secondary root system.  These secondary roots can grow parallel to the stem, and begin girdling.  The other danger in this scenario, the secondary root system may take over water and nutrient uptake for the tree while the primary/structural roots slowly rot away.  Thus, a tree that looks perfectly healthy is standing with little to no structural root system.

Another way girdling roots develop on our landscape trees is through container production.  Many of the trees we plant are started in pots. When trees are allowed to stay in these pots for too long roots interact with the sides and begin circling the container. In many cases this is allowed to happen through every change in pot size, creating an almost hopeless structural root system.

With all that said, here are some pictures of messed up root collars and girdling roots:

Girdling roots around the circumference of the lower stem.

Large impacted girdling root, notice the diameter of the root compared to the diameter of the stem.

Severed girdling root, notice the large amount of stem damage that has occured.

Conservation Arboriculture

Conservation arboriculture is neat.  It seems their are two main ideas behind conservation arboriculture.  The first, trees (especially urban trees) are of themselves complex ecosystems.  There are obvious and not so obvious examples of this.  Obvious examples are the vertebrates (ie. squirrels and birds) that use trees as their home and food source.  Then there are the not so obvious examples, like the invertebrates and fungi species that only a few may appreciate.  The bark alone can be home to arthropods, lichen, and fungi.   Taking an ecosystem view of a tree forces us as arborists to think of tree management differently.  For example, there are dozens of insect and fungi species that rely upon the dead wood in trees for survival, dead wood that an arborist would traditionally recommend for removal.  Conservation arboricultures teaches us their is an intrinsic value to these forms of life living in the tree.

The other idea behind conservation arboriculture, there is intrinsic value to very old trees.  Trees that we consider, due to structure or health issues, candidates for removal.  Some would call these ancient trees, and in places like the United Kingdom there are organizations dedicated to identifying and preserving these trees.  In my opinion the term 'ancient' is relative.  An ancient white oak may be 600 years old, while an ancient aspen may be 70 years old, but to me the idea is the same.  Older trees have a story to tell us, and the older they are, and the more perceived defects they have, the more likely they are to be the home of a variety of life.

As mentioned earlier conservation arboriculture challenges our tree management strategies.  Our first priority is always safety, but our question becomes how can we manage the perceived risk of our ancient urban/suburban trees while maintaining there vitality as a thriving multifaceted ecosystem?  In some cases it may be easy.  Moving a target or erecting a fence to keep people away from the fall zone.  In other cases it may be making the call on questionable reduction cuts, and leaving pieces of dead or dying branches in the tree that would normally be removed.  Would it be crazy to make reduction cuts on an already dead limb to reduce its chance of failure?  And still on our not so ancient trees, maybe we step back and let some deadwood build up or leave some smaller pieces in there.

This approach may not be for everybody, or every tree, but is an important idea to consider as we learn more about the trees we cherish.  Below is a link to an article written by Neville Fay which was run in Arborist News.  It is a real interesting read.
http://www.treeworks.co.uk/downloads/CONSERVATION_ARBORICULTURELearning_Review4-16-06-2011.pdf

The Angel Oak of Charleston, SC

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.

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 Structural Pruning

Here we can see the results of a tree allowed to mature with no previous thoughts of crown structure.  You can see how one side of the tree was completely ripped of exposing internal decay.

An over-extended co-dominate stem combined with some internal decay resulted in this trees demise.  If care had been taken earlier in the life of this tree to prune for one dominate leader, and well placed scaffold limbs, this tree would still be intact.

Structural Pruning

I went to the NC Urban Forestry Council's annual conference yesterday, and had the opportunity to here Dr. Ed Gilman speak about one of my favorite topics... Structural pruning.  Now I must admit, while I enjoy listening to Dr. Gilman and I love the topic, I felt like sitting through another lecture on structural pruning was a bit below me. I was pleasantly proven wrong.  Here are some of the highlights:

- Pictures presented displayed a tree just before being thinned, just after being thinned, and the same tree 12 months later.  Twelve months after thinning the tree looked almost identical to just before it was thinned.  The conclusion is, thinning trees may not be the best practice when attempting to abate the risk of branch/whole tree failure.

- Don't be afraid to remove over 50% of a tree's canopy when structural pruning at planting to promote a "hyper" central lead.  Young trees recover fast from pruning.  Removing or subordinating all competing branches at planting will guide good tree structure for some time.  Competing branches can actually shade out the central leader, weakening it, and this may contribute to branch failure in the future.

- Don't be afraid to make big cuts to remove or subordinate competing branches on medium size trees.  Again, Dr. Gilman presented pictures of aggressive cuts, and how the tree recovered with much improved structure a few years down the road.

- Dr. Gilman introduced the idea of aspect ratio when it comes to branch size.  Lateral/secondary branches should be smaller in diameter (smaller in aspect ratio) then their parent stem, and the smaller the better.  Branches with larger aspect ratios are at greater chance of failure.  Ideal aspect ratio was not specified, but smaller the better was the conclusion.

- Finally, we where shown video of a tree before canopy reduction being blown by a dynamic wind load machine.  There was about 12 inches of play at about 1/2 way up the stem.  The same tree was then pruned to reduce about 30% of the branches from the crown.  The tree then had about 3-4 inches of play at about 1/2 way up the stem.  We may relate this data to individual large branches in trees with stem and root defects.  If we reduced all large branches by 30% on tall trees with defects that may predispose the tree to failure, then we may be able to retain more trees vs. removing them.  Awesome!

As with everything, we must take in to consideration the tree's species, condition, and the site when applying this data in the field.  Never the less, a great presentation, and I can't wait to get out there to get my structural prune on. 

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.