Tips for Tree Risk Assessment and Limb Failure.
While not a perfect science — given the unpredictability of nature — there are many signs to look for that could indicate the potential for tree failure. Some caused by improper pruning practices, and others occur naturally. In this article we will discuss signs and growth patterns that may indicate a trees potential likelihood for failure, along with how you — as the manager of your own little slice of our urban forest — can be educated when hiring a tree service contractor in hopes of mitigating these potential risks.
It’s easy and understandable to be concerned about large trees near or over your home with the annual threat of hurricanes. As long as your house is within striking distance of a tree, there is the potential for impact if such a storm does occur; or, if structural problems with the tree go unnoticed or untreated. The goal of this article is to familiarize you with some common defects so you can be informed and are able to address potential issues. Also knowing how to identify some of these problems and understanding mitigation options will help you weed out potential “tree choppers.” These individuals may feed off of your fear of property damage and make unnecessary recommendations that can cost you significant money and oftentimes be damaging to your trees.
Tree Risk Assessment
First off, check to see if your tree is over or within striking distance to any targets. These include structures, high traffic areas, playgrounds, etc. If it isn’t, move on to others. If it is, let’s take a closer look:
Does the tree have any large, dead limbs? These are most easily spotted in summer when healthy limbs generally have leaves.
Do you notice a lean in the tree? Has the lean noticeably worsened over time? You could see heaving of the ground/roots on the opposing side of the lean. Are the majority of the limbs (weight) on the side of the lean, or does the canopy balance out the lean?
Now let’s look at the larger branches, leads (large tree parts that grow from the truck off of which branches are attached), and trunk. Do you see any decay on these? Binoculars may be helpful. Keep an eye out for cavity dwellers: bees, raccoons, etc.. These are an obvious sign of hollow areas. Look at the “forks” of the large limbs. This is the area where the trunk branches out to smaller sized leads and limbs and often times where limbs break in high wind. The optimal fork will have somewhat of a “U” shape on the top side. Spread your fingers wide, and look at the base where they meet your hand. Notice the small webbing like appearance — that is what we are looking for in an optimal tree fork. This is the strongest growth form. If you instead see a harder, more “V” like connection, often times with swollen or discolored bark — this could be included bark.
Included bark will cause a weakened attachment of the leads predisposing them to possible failure. A supplemental support system (Cables and/or braces) may be necessary to prevent failure.
Moving on, let’s look at the trunk and base of the trunk, “root flare” (Root flare is the base of the trunk where the trunk meets the roots)- do you see any cracks, cavities, loose or missing bark, conks or water Stains? Do you see flat areas as opposed to the mostly uniform roundness? Do you see any deep recesses or ridges at the lower portion of the trunk near the root flare? These are often times an indicator that the trunk may be hollow. If you don’t spot any of these, let’s move on to the roots.
Do you see any mushrooms or other such fruiting bodies? These could indicate root rot issues. Has there been a driveway recently installed, or other such disturbances that may have damaged the anchoring system of the tree? Do you see any lifted areas of soil or roots? Is the area a low spot? Oversaturated soil due to poor drainage could cause a tree to uproot easier in high winds and is also a breeding ground for root rot causing pathogens further weakening roots.
Risk Assessing Multiple Trees on Your Property
Now let’s take a broader look at all of the trees on your property that could be a risk. Here is a list of potential hazardous trees and the risks they pose.
- Trees left after development. Often densely wooded lots are cleared with the exception of just a few trees. These trees are more likely to fail in high winds, given they previously shared wind load with other trees. They are usually tall with very thin trunks, low live-crown ratio, and lack a root flare. Root flare is swelling at the base of the trunk caused by added structural wood as a result of wind loading and is a desirable structural feature.
- Water Oaks and Laurel Oaks. These trees have a short life span and often develop decay and included bark with age. Keep in mind that just because the leaves are green and appear healthy, the trunk may still be largely hollow. Further inspection may be necessary by an ISA Certified Arborist if the tree is in a critical location.
- Trees that have previously been topped or lions tailed. We will dive further into this later in this article.
- Large limbs that extend well past the crown of the tree.
- Trees in low lying areas. Soil may provide poor anchoring, upping the potential for uprooting in high winds.
- Trees with excessive leans.
- Trees with low live-crown ratio – usually Pines. Live-crown is measured from the lowest limb to the top of the tree. Optimally this just be about ⅔ of the total height of the tree. The more limbs are concentrated above the halfway mark of the tree, a lever arm action is created in high winds.
- Bradford Pears. These are the perfect candidate for limb failure given all of the limbs generally grow from the same area on the trunk, resulting in weak attachment.
How Improper Pruning Practices May Result in Tree Hazards
Topping is the indiscriminate cutting of tree limbs to stubs or to lateral limbs that are not large enough to assume the terminal role. Often times people assume that by shortening a tree they are ensuring it is receiving less wind load or is less of a hazard because it now won’t reach the target if it falls. While the immediate result of topping is just that, many more problems arise down the road. Depending on the severity of the hack job, the tree may die shortly after. Remember — trees need leaves to supply food to sustain. If the tree does survive this travesty, it will sprout out numerous shoots all from one area at the end of the nubbed off limbs. These shoots grow to large limbs over a few years and are weakly attached and prone to break much more than before. Compounding this issue is the trees reaction to topping. It will allocate all of its energy to putting out new shoots/leaves and very little energy to the sealing process, leaving the end of these limbs open for decay. The newly grown limbs will be even more prone to failure. Avoid topping trees.
Lions Tailing is when all of the inner limbs of a tree are removed, leaving only the foliage near the ends of the large limbs – resembling a lion tail. The misunderstanding is that by doing this the wind will more easily flow through the tree reducing its wind resistance; however there is actually the adverse effect. Now that all of the wind resistance is at the tip of the limb, a lever arm is created and forces are amplified. Instead of removing this interior growth, weight/limbs should be shortened. As an example, try holding a 5-pound weight with your arm fully extended. Now rest the 5-pound weight directly on your bicep. The latter is much easier. Furthermore, the tree will react to lions tailing by exponential sprouting where the limbs were just removed in an effort to replace lost leaf area. You will hire someone to do it all over again and again. Avoid lions tailing limbs.
Making Large Pruning Cuts on Mature Trees
Large limbs/leads of trees over your house or valuables can be nerve wracking. The first thought is often to remove the entire tree part (i.e limb or lead) all the way back to the trunk. This may not be the best choice. It’s important to know that every cut is a wound. The larger the wound, the longer it takes to seal – if ever. Open wounds from large cuts are the perfect opportunity for decay causing pathogens to enter the tree. Instead of having large limbs removed, consider removing weight/limbs from the tip to lighten the load.
Hiring a Tree Professional
Now that you are informed, this should be much easier. Avoid “point and cut” companies. These are the ones who know little about arboricultural practices but have a chainsaw and are willing to cut wherever you point. Find an ISA Certified Arborist — preferably with a Tree Risk Assessment Qualification. They have the knowledge to guide you on what is best for both you and your trees. Yes the initial cost may be a little more, but the results will save you far more down the road. My hope is that this brief “do it yourself” tree risk assessment is helpful. Importantly, this is a very watered down version as a guide to help you spot defects and hazards and should not be a substitute for a tree professionals thorough inspection and advice.
By Shem Kendrick
ISA Certified Arborist SO-7151A
Tree Risk Assessment Qualified
TCIA Tree Care Specialist
Coastal Arbor Care