CONTRIBUTED CONTENT: Potential failure

“It wasn’t even windy!”

This is the exclamation of a customer I visited last week while looking at some branches that had fallen out of a couple of hybrid poplars. As I scratched my head trying to figure out why these 3-inch-diameter branches were laying on the ground and not in their proper place among the canopy, the conversation turned to what can cause seemingly healthy branches and trees as a whole to fail.

A healthy tree should be able to withstand ordinary environmental stresses, so when we have tree or branch failures during calm or typical weather, it’s worth asking what has happened prior to the event and what was happening inside the tree long before the failure occurred. Structural defects, decay, root systems, and even the overall size of trees and branches are all factors that can influence tree failure. Sometimes even the slightest breeze, or the added weight from rain on leaves and needles, is enough to be the tipping point for failure.

There are many different structural defects we look for when examining a tree for potential failure. The first thing I always look for is weak attachment points on larger branches, where the size and mass of the stem or branch is too large and heavy for the attachment point to adequately support it. More obvious defects would include decay, cracks, cavities, or pests, fungi, and diseases that can weaken the tree.

Overextended limbs are also a significant failure point in trees. When a limb or a tree puts on more growth in length and height than the trunk or branch wood can structurally support, you can end up with a potential failure point due to the excessive leverage placed lower on that specific stem.

Seeing how the roots are the foundation of the tree, this is another critical component when talking about structural stability. What is commonly overlooked, however, is the soil and growing conditions that the tree is in. Overly saturated soils, restricted root zones, and compacted soils can all play a factor in the roots’ ability to maintain a healthy and stable structure. Damage from construction, severed roots, and trenching along the root system are more obvious examples.

Of all these factors, overly saturated soils can be particularly concerning. When roots are constantly growing in soil that retains too much water, they can become more susceptible to root decay and disease, while saturated soil can also reduce the soil’s ability to provide adequate anchorage. When the soil becomes saturated, especially when combined with other factors, it can reduce a tree’s ability to properly anchor itself under loads such as wind, rain, and ice.

These are just a few of the things that we, as arborists and tree risk assessors, look at when determining the potential for tree and branch failure. My point in all this is that regular tree inspections from someone qualified to perform them are key to catching potential problems before an event happens. Those 3-inch branches that fell from the hybrid poplars weren’t necessarily healthy simply because they were green and covered in leaves. Their failure was a reminder that what we see on the outside of a tree doesn’t always tell the whole story. Sometimes, the factors that contribute to failure have been developing quietly for years before the day that a branch finally ends up on the ground.

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