tree

An aristocrat pear has separating foliage masses, a sign of overloading.

Bill Pramuk photo

During previous years of drought, we saw many trees growing more slowly and foliage thinning out. At the same time, invisibly, trees were rendered more susceptible to certain diseases, decay, and structural problems.

In trees’ inherent priorities for energy allocation, production of self-defense compounds is the first thing to go when reserves run low.

Top priorities for survival are:

1. Respiration — using oxygen to release stored energy

2. Production of fine roots and leaves

So, when trees received generous rainfall last winter, many grew heavy loads of foliage this spring and summer.

Coincidentally, because oaks tend to have alternate years of heavy and light crops of acorns, third on the order of energy allocation, this year we are seeing an unusually heavy production of acorns, at least in the local coast live oaks after last year’s very light production.

This all adds up to an increased risk of structural failures. As the truism states: Failure occurs when load exceeds strength.

A perfect example occurred a couple weeks ago. A property manager called requesting I examine a tree that dropped a limb. A person who happened to be nearby narrowly escaped getting bashed.

On inspection, I saw that the tree is a very healthy looking coast live oak. The fallen branch was thick with healthy green leaves and heavy with young acorns.

At the point where it broke, the limb, still lying near the tree, showed brown discoloration in a portion of the cross section, not the normal almost white inner wood.

What does that say? The limb must have cracked some time ago and oxidized in the exposed portion in the crack.

Then, the weight of new foliage and acorns exceeded the strength of the compromised branch. It did not require weather like a windstorm or rain to overload the limb.

What caused the crack? Perhaps an inherent weakness in a tight V-shaped branch attachment; perhaps changes in the wood.

Some of the literature discusses drought-stress induced microscopic changes in wood structure, and “delamination.”

A more crude and light-hearted expression says: “Wet spaghetti bends. Dry spaghetti breaks.”

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Visiting properties and just observing trees around town, I have noticed some species more than others tending to have heavy loads of new growth: some elms, Bradford and Aristocrat pears, Liquidambars, Southern Magnolias and black walnuts, to name a few.

One good way to judge whether a tree, or portion of a tree, is overloaded is to look for gaps, open sky, in the canopy. A well-structured tree typically has fairly contiguous foliage masses on adjacent limbs, but overloaded and overextended limbs often have their foliage mass separating from those of nearby limbs.

If you watch these on a breezy day, sometimes you will see them wallowing in the wind; moving as a single mass as the limb bends and sways. In contrast, branches in a well-structured tree tend to move chaotically in the wind.

That is the load aspect. Now consider strength loss. Does the limb show signs of cracks, decay, sharp bends, or a deep crease in the attachment where it joins to trunk? If so, its strength might be compromised.

Pruning is the most common first step in reducing the likelihood of limb structural failure. In many cases, an overextended limb can be pruned to reduce the load without spoiling its appearance. This kind of pruning may take more time and money, as it requires thoughtful removal of crowded outer branches by a patient and skilled tree worker.

In some instances, when limbs are long and have few or no inner lateral branches, or decay, or weak attachments, pruning is not enough. In those cases cabling, bracing, or propping might suffice. If not, then the limb or the whole tree might need to be removed.

Cabling requires a commitment to check the cables and hardware periodically, and adjust or revise the system as needed. Three years is a good interval. Braces and props usually have a much longer interval for maintenance.

Trees change over time and respond to changes in their environment. Keep an eye on them.

Bill Pramuk is an ASCA registered consulting arborist. Visit his website, www.billpramuk.com. Email questions to info@billpramuk.com or call him at 707-226-2884.

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