Sunday, March 31, 2013

Getting to the pH of the problem

When we fertilize, our goal is to make nutrients available in the soil.  Plants may then absorb and use these nutrients in biological processes.  By taking soil samples we may determine what nutrients may be lacking, or what nutrients may be in such abundance they are creating an antagonistic environment for the plant.  Most soil samples will also return pH.

Soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+ or, more precisely, H3O+ aq) in a solution. It ranges from 0 to 14, with 7 being neutral (I copied all of that from Wikipedia).

Soil pH is important. Nutrients can be made unavailable in soil at different pH ranges. Classic examples are iron (Fe) will get tied up in soils at higher pH, while phosphorous (P) will get tied up at lower pH. The pH scale is logarithmic, so there can be a big difference in availability between 5.5 & 6.

Chart of soil pH and Nutrient Availability

When managing trees in a man-made landscape, pH ranges can vary within a few feet of each other. Removal of soil layers, exposure of soil layers, and type of building material can all effect a 'micro pH environment.' Most trees enjoy a pH of around 6 (give or take). Again, depending upon multiple factors, adjusting pH to an ideal range for trees may be difficult. In addition, depending on which way you are trying to tip the scale, and which product you are using, results may take months and may be fleeting.

Take soil samples and pay attention to pH.  All of the nutrients required for growth may already be in the soil and just tied up.  Knowing this information will help arborists choose the correct soil amendments and reduce the chances of creating an antagonistic environment from over applying fertilizers.

Sunday, March 24, 2013

Conservation Arboriculture: It's About to get Weird

Conservation arboriculture sees trees as more than just landscape fixtures, but as whole ecosystems, and argues for the intrinsic value of the macro and microscopic organisms living within mature and veteran trees.   Last week a buddy of mine forwarded me a whitesheet by Neville Fay on retrenchment pruning.  This form of management is a concept practiced in conservation arboriculture.

When trees are damaged in nature it`s usually due to some catastrophic event (i.e. severe wind storm). Branches damaged in this way have wood fibers break and tear, while bark is pulled away from limbs and jagged stubs are left behind. In the aftermath all manner of fungi and arthropods make a home in the tattered remains.  Fungi feed on the newly exposed wood, insects eat the fungi, birds eat the insects, and so on.

In landscapes, trees eventually mature to where their risk of failure reaches a threshold that some mitigating action must take place.  For conservation arborists, this is where retrenchment pruning comes in.  By using coronet cuts to mimic naturally damaged limbs after reduction pruning, conservation arborists invite the natural order of things to take place.  Another technique, natural fracture pruning, is simply tying rope to branches and applying force until the branch breaks. This seems to be most popular in the UK, but I've seen coronet cuts used at The Capilano Suspension Bridge in North Vancouver, British Columbia.

Coronet cut in action. Pic from David Humphries
Final coronet. Pic from David Humphries
Obviously this isn't for everyone, or every tree.  In fact, retrenchment pruning seems to go against all traditional pruning techniques and goals.  This concept isn't meant for the feature tree in the average front yard.  But, this may be appropriate for that mature tree in the wood-line at the back of a property which poses a hazard to the swing set.  This idea may also work for any veteran tree that has fallen into the spiral of decline.  In the Capilano Suspension Bridge example, entrenchment pruning was used on declining trees along forested walkways before they became hazards.

Another thing to keep in mind, retrenchment pruning in its idealistic form can take decades of management.  Vigorous sprout growth can result from damaged limbs, and so with this form of tree management. Talk about commitment from both arborists and tree owners.  The long-term goals of all invested parties need to be discussed before this type of work is performed.

To learn more about entrenchment pruning and conservation arboriculture check out Tree Works Environmental's website:

Sunday, March 17, 2013

Not to Kick You when You're Down, but another Incredible Tree Failure

I always hate seeing a tree fall, but sometimes there is no better way to learn about the way trees grow than from a post failure forensic investigation.  And while I know our last post was about a fantastic large tree failure, this was too good to pass up.

Our subject is a large mature willow oak that suffered a tremendous root plate failure.  This tree was growing in a known flood plain, and was one of the tallest trees on the block.

Notice the amount of standing water in hole left behind when the roots pulled from the soil.  Keeping the size of the root plate in mind, and the proximity to the camellia in the picture below, notice how the wet conditions allowed the roots to be pulled from underneath soil like a magician pulling the table cloth from a fully set table.

The only indication of decay at time of failure was a cavity along the lower trunk/root flare of the tree.

Now, here is a part of the tree we don't get to see often.  With the root flare up ended we can see the structure and arrangement of the structural root system, and associated decay.

The decay column moving upward in to the stem is a common occurrence in many tree species.  It is a result of the original tap root dying off and allowing a path for decay to move into the stem.  This is a process that can take years, if not decades.  In species that compartmentalize well it is just a peripheral event, but for poor compartmentalizers, it may the defining defect in their demise.

Good thing the Jeep was insured.

Sunday, March 10, 2013

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.