Autumn tree risk issues: A look at wood

By Bill Pramuk
Trees & People

In my previous column, we looked at the subject of autumn tree health issues. Now, let’s look at the other side of the coin:  risk and the properties of wood.

The two issues may go hand in hand, but it is crucial to know that that they are not necessarily related. As it says in the “vigor” footnote I put in most of my reports: “ Vigor is a measure of a tree’s current health and ability to withstand pests and diseases. It is not a measure of structural stability.”

A perfectly healthy tree can be an imminent hazard. How can that be? The structural element that keeps a tree standing upright — wood — is, for the most part, not alive. Think about tree anatomy, working from the outside inward:  bark, phloem, cambial zone, xylem and rays. The first three elements contain most of the living portion of the tree. The fourth, xylem, begins life as water and mineral conducting tissue. As it ages, at different rates for different tree species, it loses its water-conducting ability and becomes wood. 

The fifth, rays, begin life as living tissue and remain living in deep layers of wood in some trees. Coast redwood and coast live oak are good examples. Take a close look a an oak table or chair. Those prominent, dark flecks, evenly distributed through the wood, are rays.  Although they do not contain cellulose and lignin, the tissues that make wood strong, they are an integral part of it. At a lecture in San Francisco, I recall tree research engineer Claus Matteck saying, in his thick German accent: “Big rays means strong wood!” The meaning I took, is that big rays are present in strong wood, but not strong of themselves.

The living tissues in trees are intimately related to the wood, but as trees age the inner wood dies, dries, hardens and has less capacity to respond or react to stress and wounds. In effect, a large old tree is a thin shell of living tissues  surrounding and protecting a core of inanimate wood.

And what happens to wood when it is exposed to the elements, insects, bacteria and fungi?  It breaks down. Is it any surprise that large wounds spell trouble for trees?

With that in mind, take a stroll through your property or your neighborhood and take the time to look closely at the larger, older trees. You can  see the history of wounding. A practiced eye will detect warning signs of decay and structural weakness. These include fungal fruiting bodies growing out of a wound or the bark, cavities, bulges, cracks, splits —  anything that doesn’t look like the normal, efficient, graceful body of a sound tree — and wounds on the top side of limbs.

The last one, top-side wounds, may be difficult to spot from ground level, but it is critical.  The wood in tree limbs gets strength in two ways: 

1) The solid compressive strength of lignin, which is like the basalt blocks in a building;

2) The flexible, tensile strength of cellulose, which is like the steel re-bar embedded in the walls.

Trees react to the stress of windsail and wood-weight by growing reaction wood. One form of it is tension wood on the top-side of the limb. It is made up largely of cellulose, which also accounts for the flexible strength of the newspaper your are holding in your hands right now (assuming you are not reading this on-line!). Tensile strength, on the top of a limb, is disrupted by top-side wounds. Think before you cut!

 Sharp eyes are just as important as sharp tools.

All of that being said, it should be kept in mind that even the most perfect looking tree can fail unexpectedly. I’ll continue on the subject of risk next time.

Bill Pramuk is a registered consulting arborist. Visit his Web site www.billpramuk.com , send questions to info@bill pra muk.com or call him at 226-2884.

Napa Valley Register Copyright © 2009