2017-10-04 / From the Forest

The Spiral of Decline

By Ryan Trapani, Director of Forest Services, CFA

People are concerned about their trees. On most days, the phone rings at the Catskill Forest Association’s office and on the other end of the line is a concerned member about an ailing tree. Sometimes it’s a sycamore in early summer that’s dropping its leaves. Sometimes it’s caterpillars munching away on an apple’s leaves. Maybe a Norway spruce is loaded with tiny galls; or a Norway maple that contains ugly tar-spotted leaves is the problem. None of these factors normally lead to a tree’s death.

Often we’re asked to visit a property when a tree is on the verge of mortality. Large fruiting bodies of mushrooms (decay fungi) or sawdust from abundant wood boring insects have already made their way throughout the tree. It’s easy at this stage to blame boring insects or fruiting mushrooms for the death of the tree, but normally they’re not the cause of the tree’s death, but simply the last ones to arrive on the scene. This leads to a commonly asked question: “What caused the death of the tree?”

The Spiral of Decline

Last week, I attended a conference in Syracuse hosted by the New York State Chapter of the International Society of Arboriculture’s (ISA) Fall Conference. One lecture discussed the Spiral of Decline. The Spiral of Decline refers to factors causing or leading to the death of a tree. The three factors include: (1) Predisposing; (2) Inciting; (3) Contributing. Contributing factors seem to be most noticed since they arrive closest to mortality. Contributing factors include: boring insects, armillaria root rot, nematodes, and decay fungi.

Predisposing Factors

However, it’s often (1) Predisposing and (2) Inciting factors which often go unnoticed and which can sometimes be mitigated before a tree dies. Predisposing factors include: tree species, compacted soil, salt exposure, low soil moisture, too much soil moisture, too little sunlight. Many of these predisposing factors are manageable. For instance, perhaps the wrong tree was planted in the wrong spot. Or perhaps that white pine is too close to where road salt is spread in the winter. Or maybe that apple tree just needs more sunlight or some pruning.

Inciting Factors

Inciting factors just persuade the big bad tree reaper to make his way a little closer. They include: defoliating insects, excavation (too close to roots), drought, excessive salt, and frost. If your trees are healthy, they can take a year or two (or three) of defoliation from caterpillars. However, if they were planted in a wet area, and were defoliated, and experienced too much competition for sunlight, then that brings us back to those (3) Contributing factors.

Contributing Factors

Contributing factors usually finish off a tree. They include those wood boring insects and mushrooms mentioned earlier. They often are confused as the main culprit or cause for the tree’s demise. But, as you can see, there are normally other factors involved. All too often, a tree is dying from inciting factors like excavation that severed or compacted a tree’s roots 10 or so years ago. Before making a judgement or decision about what killed a tree, many questions should be asked about the site and its history. As the presenter said, “don’t make assumptions. Don’t fixate on what you know; you’ll miss out on what you don’t know.”

Exceptions to the rule

It’s easy to blame the mushrooms and boring insects, but often it’s what we don’t know about the tree that really added to its demise. Of course, there are exceptions to this rule. The emerald ash borer for instance, is one boring insect that ignores these rules by bypassing (1) Predisposing, (2) Inciting; and (3) Contributing factors, causing death of an otherwise healthy ash tree within three years. Emerald ash borer is a serious pest for this reason, but is not “normal.” In the mean-time, we can try to pay more attention to some of those predisposing factors before they become factors contributing to mortality. www.catskillforest.org

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