Tippy trees.

Some trees lean.  Leaning trees are a natural phenomenon in nature.  While there are many reasons why trees lean, most of the time the reason involves the search for light.  This is called a phototropic growth response.  Trees growing in this manor put on adaptive growth and reaction wood to maintain stability.

Once a tree has grown to a point where there is a clear view of the sky the lean will 'self correct,' and new growth will begin to grow with a more vertical orientation.  At this stage the stem lean should be at its maximum, and if the tree remains structurally sound the angle of the lean should remain constant.

Look carefully at the pictures below.  The picture on the left is a screen shot from Google Street View of a post oak taken in 2011.  The 2nd picture is of the same tree taken in December of 2013.

2011                                                                                       December 2013

Notice the difference in orientation of the lean.  The root collar of this tree is not well exposed.  The homeowner indicated she noticed a change in the lean of the tree in July of 2013, and was aware of at least one 3- to 4-inch diameter root that was severed to install the brick walkway earlier that same year.  After some peripheral probing with a soil knife to about 6-inches deep, no structural roots were detected on the tension side of the lean. 

Could this tree be actively failing, albeit slowly?

I'm fallin'

When discussing tree risk assessments we talk about looking for signs a tree is likely to fail.  The following pictures are of a white oak that some may argue is in the process of failure.  Let's take a look....

As we approach the tree, notice first the lean. What this picture doesn't show is the residence within 50ft of the tree's base.

Soil heaving, especially on the side of the tree opposite the lean, is pronounced.

Here we can see where large roots are separating from the soil.

Another image of large roots separating from the soil, and some resulting voids in the soil.

After discussing these issues with the client I discovered that 5 years ago this white oak was on the edge of a wooded area that had been cleared to make room for a new home lot.  This tree had spent most of its existence to this point with mature neighboring trees protecting it from wind and storm events on the side opposite of where it is now leaning.

By altering this tree's environment, a tree that may have lasted for years to come was turned in to a tree that is now at an extremely high risk for failure.

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.