Post Storm Tree Assessment - Guide to evaluating trees

PostStorm Assessment:

To assess trees in the aftermath of storms it is important to understand the types of damage that are

likely to occur. Trees are made up of four primary components; roots, trunk, limbs, and crown. Each of these components are subject to damage that have varying impacts on tree’s health, structural integrity, and the eventual decision to keep or remove the tree. This document will review common post storm damage and help guide the decision of whether to remove or prune storm damaged trees. This document denotes four stages to help tree owners better understand the assessment process. This document provides some recommendations, however, it must be noted that every tree and situation is different, if any defects are noted appearing similar to any of the aforementioned tree defects, it is recommended you then seek the expert guidance from a suitably qualified arborist.

Step 1: Targets:

Trees in forest settings have a low risk factor as there are no structures and few people. Trees in gardens, play areas, adjacent road or footpaths, need to be evaluated more closely. Homes and other urban structures, power lines, roadways and most importantly people are potential targets if a tree fails or falls. Another factor is frequency of use of the area under the tree in question. Areas with less human activity present less of a risk as opposed to trees over frequently used patios or playgrounds for example. The intensity and frequency with which trees are assessed and evaluated is largely based on threat to human safety and welfare.

Step 2: Tree Species:

First, identify the tree species. Some species of trees are more susceptible to disease, decay, or structural problems than others. This becomes important to future health of certain trees when large broken branches are pruned. For some species that are mature or over‐mature large pruning wounds are not recommended or considered professional as this may lead to future safety concern issues as a result of decay developed following pruning.

Step 3: Consider Tree Age

The younger the tree the easier it will be to prune damage branches and expect full recovery. Like people young trees are more resilient and are far easier to correctively prune, removing damaged branches. Small broken branches pruned from trees will seal quickly with few lasting signs of damage. However, as trees mature the loss of large branches and the corresponding canopy increase incidents of decay and future structural problems as well as tree decline from loss of leaves. Additionally, root problems such as leaning trees and broken roots are more common in larger trees. These problems in smaller trees can be easily corrected by staking with little risk to safety and homes. Large leaning trees, however, are potentially concerning and cannot simply be propped up. Unfortunately there is no clear delineation that separates young trees from mature trees. However the following rules of thumb can be applied to trees needing storm pruning or lean correction.

Trees less that 5 years – High success

Trees between ages of 615 years – Moderate success

Trees between 1530 – Moderate to poor

Trees between >30 – limited chance of success 

Step 4: Assess Root damage

Some of the most concerning structural damage can occur below ground in the root system. These problems are often hidden and careful examination is needed to identify problems. High winds often cause trees to rock causing the severing of roots and in extreme cases whole tree failure. The most prominent sign of root failure are leaning trees and root plate mounding, see picture 1&2 below:

A good indicator of a problematic leaning tree is a soil mound at the base of the tree on the opposite side of the lean (Pictures 1). On the inside of the lean there may be an indentation in the soil. Cracks in the soil on the opposite side of the lean especially when combined with mounding are a serious hazard. Leaning trees with mounding often indicates broken and damaged roots, and should be examined by a professional immediately and usually require removal.

However, trees may also have a lean due to the trees natural growth patterns. Examples include trees leaning out towards sources of light or leaning as a result of prevailing winds. Naturally leaning trees will often have branches or tree tops that have adjusted to the lean (Pictures 3, 4). In such cases the branches will be growing towards the light even if the tree is leaning away. Call a Certified Arborist if you see any of:

Step 5: Assess the Trunk

The trunk of the tree is primary stem between the roots and the limbs and foliage above. The trunk

serves as the conductor of water and nutrients and supports the total above ground weight of the tree making it an important component in any tree. Damage occurring on the main stem or trunk is often structural in nature and can be exceedingly damaging and dangerous. High winds will often push, bend, and twist stems causing whole trunk failures, cracks, splits, or bark ripping. Step 5 will outline these damages in more detail in the sections below:

a. Cracked Trunk or Branches:

During storm events trees are twisted and bent and in some cases can develop cracks. These cracks may be in the crotch of a co‐dominant stem or in extreme cases in the middle of the trunk. All cracks are a major problem and will require immediate attention. Trees with cracks can fail at any time and will develop decay compounding the problem. In many cases these trees will need to be removed. Contact a Certified arborist if you observe any of the following:

b. Broken or Split Trunk:

Start by carefully walking around the tree looking for cracks (see previous section), twisted wood spiralling around the tree, torn or ripped bark, or breaks. Note the severity of the damage to provide the proper management technique. Below is a chart to help quantify damage and receive corresponding management recommendations.

• Splits occur as a result of large branches breaking away from the tree. These usually cause large wounds that will allow decay to enter the tree and cause structural problems later (Pictures 1 & 2)

Large splits or broken stubs as in Pictures 1 & 2 will require removal because they exceed 50% of the diameter.

• Smaller splits or broken limbs from the trunk as in picture 3 below, are easier for the tree to seal.

These woods are still risky and will require monitoring.

Broken or stubbed trunks even in hardwoods will require that the tree be removed. Even if lower limbs remain the loss of the top of the tree is a major concern (Picture 5). If the loss occurs in the

lower 2/3rds of a mature tree removal will be required. Loss in the upper will need further assessment.

c. Twisted or Bent Trees:

When high winds twist or bend trees it is important to look for any cracks or broken tops. Mature trees will seldom twist or bend without either cracking or breaking. Also trees that have bent or twisted near the ground should be examined for broken roots or any mounding that will indicate a lean. Young trees however are still flexible and can sometimes be bent without breaking.

• In the case of young trees that are twisted and bent stake tree trees to their upright position and monitor their progress (Pictures 1 & 2)

d. Bark Ripping on the Trunk:

Bark ripping often results from a limb partially breaking. The partially broken limb as if falls will rip the bark down the side of the tree exposing the wood beneath. This damage is not initially structural in nature but has the potential to promote disease and decay by exposing live tissue to the elements/diseases. Below are recommendations based on the percentage of the trunk diameter that is affected.

Step 6: Assessing the Crown:

Following severe storm events it is not unusual to see broken branches and trees stripped of their leaves. The loss of leaves is usually not a problem as a healthy tree will usually generate new leaves in the weeks following the storm.

Broken limbs, however, are more serious because they require pruning to reduce the risk of decay forming as a result of parasitic/disease infection. Trees with minimal broken limbs are seldom a problem. However, trees that have lost large limbs or several limbs need to be assessed taking into consideration future structural issues as a result of decay. Decay may weaken trees in the future and potentially represent an unreasonable risk to members of the public given onset of decay and position on the landscape (high risk areas). Below are several diagrams to help gauge whether pruning or whole tree removal is necessary:


After walking the survey area, a typical laymen with a keen eye is likely to spot potential defects and can take pictures to further illustrate the issues found.

In the first instance and when genuine concerns with regards to tree safety are exposed, it is recommended the following is adopted:

  • Make a note of the tree location, for example “to the rear of” or “in front of” a particular object, fence, house or other landscape features is very useful for locating tree later on

  • Record the type of damage and location, for example if a branch is damaged, estimate how high the damaged branch is in the tree

  • Take a picture of the damage close up and also to scale, stand back and get a picture of the whole tree, using background features to give scale can be very useful later on

Following collection of data and pictures it is recommended in the first instance that you contact a professional arborist and seek advice, also sending the description of damage, tree/s location and pictures for consideration.

If the arborist is concerned after considering the evidence presented he or she can then make a suitable date and time to visit site and provide recommendations.

Mark Hines ND ARB

Director - DGS Trees

February 2014