Chapter 11: Structural Defects, Tree Failure, and Risk

If a tree falls in a forest and no one is around, does it pose a risk?


The trees that grace our homes and fill our parks and forests provide many benefits: relaxation and recreation, pleasing surroundings, shade and protection for our buildings and activities, wildlife habitat, fruits and nuts for our consumption, and more, all in addition to their critical ecological functions. However, counterweight to their benefits, trees also present us with the potential for damage or harm. Like us, they may fall prey to natural events or human intervention, or finally to old age and death. A small tree that falls in a corner of a backyard or even a large tree in the midst of a forest may not have much impact of note, but the larger the tree and the closer its proximity to us, the more risk it poses to life and property.

Tree failures cause a significant number of fatalities and a substantial amount of property damage every year. In the United States between 1995 and 2007 there were 407 deaths caused by wind-related tree failure. These were spread across varied weather events: 41% were caused by thunderstorms, 35% by non-convective high winds, 14% from hurricanes, 7% from tornadoes, and 3% from snow and ice. Wind-related tree failures were responsible for approximately half of the deaths from thunderstorms and high winds and a third of the deaths from hurricanes.[1] These figures do not include additional damage such as nonfatal injuries, property damage, indirect deaths (e.g. lack of access to medical services due to downed trees), or trees and limbs damaged by the storm that do not fall until later. Property value losses in the state of Georgia from wind-related tree failure alone are estimated to be over $10,000,000 per year.[2]

The definition of tree failure given by the International Tree Failure Database User Manual is “A structural failure or physical breakage of the tree trunk, one or more branches, or one or more tree roots.”[3] The vulnerability of a tree to failure is dependent on three factors: the structure and condition of the tree, the particulars of the site, and the force to which it is subjected. In this chapter we will examine each of these factors and hopefully begin to gain an understanding of the complex interactions that affect whether a tree stands or falls. All the information covered in previous chapters has described many of these various elements; in this chapter we will focus on their integration into the relationship between cause and effect.

As Tree Stewards, we may be called upon to help evaluate the safety of a tree. It is important for us to understand the structure of trees; how their stability and strength is impacted by various stressors such as poor condition, environment, weather events and other factors which can cause or contribute to their failure; how to look critically for the presence of these conditions or circumstances; and to have an informed concept of what defines the extent to which a tree poses a “risk”. In addition, it is critical to understand the Tree Steward’s specific role in observing, evaluating, and explaining the condition of trees, including the limitations of that role, and to be cognizant of the importance and proper role of credentialed tree care professionals.

Learning Objectives

  1. Understand the concept of tree failure and how trees fail.
  2. Understand how tree and site conditions contribute to a tree’s vulnerability to failure.
  3. Understand how weather events affect trees.
  4. Understand the concept of “risk” and how it is evaluated by Tree Stewards and by professionals.
  5. Understand the limitations of Tree Stewards in advising clients.
  6. Understand what Certified Arborists are, what their function is, and when their services should be recommended.

Tree Conditions Contributing to Failure

There are a multitude of factors that influence the structural integrity and strength of a tree.  It begins with the genetic characteristics of its species which lay the foundation for its innate structure, strengths and weaknesses.  As the tree grows, these are integrated into its individual dynamics such as growth pattern which are then influenced by climate and site conditions, its “life experiences”.  The tree’s health and vitality then help determine its susceptibility to pests, disease, and decay which are again, circularly, influenced by its genetic resistance to these.

In addition to its biological functions in support of growth, each component of a tree constitutes a piece of the equation that defines the tree’s structural integrity and mechanical strength.  As each particular component individually loses health or becomes injured, it also contributes to a synergistic effect which degrades the strength of the organism as a whole and affects its ability to deal with load stress as a unit.

Scientists have used several theoretical models in an attempt to understand how trees as physical structures respond to wind load.  Some models view the tree as a static (rigid, nonporous) object subjected to a constant force.  Others conceptualize it as a dynamic interaction which considers variable factors such as wind gusts.  A third view is based in fluid mechanics, where trees are seen as “flexible porous structures that change their shape as the wind blows.”[4]  A 2006 study found that trees do not simply sway back and forth under dynamic loads, but instead move in a complex looping pattern.  As the branches move around in the wind, they dissipate wind energy, which reduces the load transferred to the trunk and increases the mechanical stability of the tree.[5]

Thus, the vulnerability of a particular tree or part of a tree to failure is a summation of a number of complex and interrelated factors.  In this section we will attempt to describe some of these structural factors, how they may be caused, what they look like, how they may interrelate, and finally how to evaluate their potential for harm within the limitations of our knowledge.


Roots are literally the foundation of the entire tree; they are the most critical component for its stability and literally hold the tree in the ground. Ideal root development would be an evenly distributed circle of major roots which grow fairly straight away from the tree with unimpeded horizontal space to allow them to stretch to their fullest. Approximately 90-95% of roots grow within the top three feet of the soil, more than half of which are in the top foot.[6] Optimum anchorage depends on the ability of the tree to grow its roots to a length proportionate to its crown (approximately 2 to 4 times the width of the canopy or dripline), at their natural depth, and in a place where a stable soil structure and adequate nutrients, water, and oxygen are available.

Trees with shallow root systems are more prone to uprooting than deeper rooted types.[7] Those growing in compacted clay soil, in a site with a high water table, or over hardpan will usually have root systems which do not grow to the optimum depth that they might elsewhere[8].

Trees with limited root space are not allowed to grow to anywhere near their appropriate breadth, which limits the size and also threatens the health of the tree. Confined root systems typical of urban plantings in areas such as sidewalk cutouts, street side planting strips, and parking lot islands result in limitation not only of total root mass but, more important for stability, the horizontal root growth necessary for anchoring. Trees which have roots covered by an impervious paving material will likely decline or die because of their inability to access the water and oxygen they need.

Root girdling prevents the outward growth necessary for good anchoring ability, in addition to strangling the tree and possibly causing its death. Those growing too close to the surface can be exposed to man-made injury such as mower damage.

Damage to the roots, especially major roots, will not only impact their ability to carry out their part in maintaining the health and vitality of the tree as a whole, but also the health and strength of the roots themselves, and thus reduces their mechanical strength. Shortening them, by cutting through them or by damage that weakens them, will reduce their ability to hold on to the ground. Wind load is shared equally by roots on the windward and leeward side of the tree, so structural roots on all sides of the tree are critical to stability.[9] If there are only two or three main roots, or if roots are missing or rotted on one side, then the tree is more likely to blow over. It is considered at high risk if more than a third of the main structural roots are missing.[10]

Finally, decayed root systems may have very little to provide in the way of anchoring ability if the roots are weakened to the point of easy breakage.Root damage to a tree can kill it suddenly, within a year or two, or the tree may appear healthy and show no sign of decline for twenty years and then begin to decline gradually or die quickly due to long-term problems which had not been visible.


Trunk (“stem”) structure is a major factor in defining the mechanical strength and load distribution in the tree. Ideally a tree would have a single trunk which is fairly straight, the root flare visible at the bottom, and tapering somewhat with height. The strength of the trunk structure (and each major branch) depends on its ability to maximize its elasticity and its resistance to breaking when bent; it is most effective at doing this when the tension from load stress is evenly distributed. Any irregularity, i.e. defect, interrupts this flow and creates a point of vulnerability and weakness.

Splits and cracks create weak areas that are more vulnerable to load stress. Cracks can be lengthwise or crosswise depending on the cause, and can occur due to previous wind stress (bending or twisting), drought, growth expansion (usually horizontal across trunk or large branches), frost cracking (from temperature change between a very cold night followed by a warm sunny day), incomplete graft unions, bark inclusion, or a stem girdling root. When trees are bent past their ability to absorb the stress, they begin to crack horizontally as the wood collapses inward, often leaving external wood or bark at the top of the crack projecting outward.[11] This is an incomplete failure which should be evaluated immediately by a certified arborist.

Codominant joined trunks are, in fact, not connected; they are separated by included bark. The apparently joined length over the indented area indicates a defect that extends the length and through the width of the join.[12] Over time, the trunks grow in length and subsequently further away from each other, increasing the likelihood of failure.Sections or long strips of bark loss can be indicative of a previous lightning strike which has gone down one side of or through the tree. The internal effects of lightning damage are unpredictable. Loss of bark may include loss of the cambium.

Cavities and large decayed areas create points of weakness. Cavities from large branches that have been pruned or broken near the trunk may have decay that extends far down into the trunk. Branches of more than approximately half the diameter of the trunk are more likely to decay, especially if flush cut, resulting in cavities, cracks, and hollow areas. Wood that forms around cankers grows quickly in order to support the weak area. A larger canker has a significant impact on the overall trunk strength; even when callousing occurs to strengthen the area, it also creates an irregularity in relation to the rest of the wood.The extent of internal damage, whether from lightning, rot, decay, or any other means, cannot be accurately assessed by external evaluation; even professional arborists have only a few tools at their disposal.

The root flare is a critical juncture which provides a mechanical transition point between the vertical trunk and horizontal roots, where the effect of load forces are concentrated. Roots grow fairly close to the surface near the trunk to help distribute the force more smoothly at the point where this transition occurs. The area between the root flare and the lowest main branch attachment is the most vulnerable area for failure. Any cavities, decay, or damage at or near this area has a significant impact on susceptibility to failure. Note that cracks or inclusions in the root flare are normal for some trees (e.g. laurel oak, red cedar, arborvitae) but they may still be cause for concern.[13]

Damage to the lower trunk area including root girdling, cambium loss, rot due to too-high mulch or soil level, or lawn implement damage (such as mowers or string trimmers) will greatly impact the tree’s health and stability. Cambium loss will kill the tree if it girdles it, so sections with bark loss are suspect because they often include cambium loss. Loss of a substantial section of cambium may kill a portion of the canopy (for many trees, that portion directly above it).The degree to which a tree’s mass is directly centered over the trunk base greatly influences its ability to resist mechanical stress by distributing the load most evenly. When the tree’s mass moves off center then gravity compounds the effect of the load and increases the chance of failure by uprooting. A change of center of gravity during a storm can occur due to root or soil failure, stem (trunk) bending, or branch loss; if the trunk or crown are off center to begin with, then it will exacerbate the effect.[14]

A leaning tree may or may not indicate instability; if it has been leaning for quite some time (as evidenced by corrective trunk and branch growth patterns) it has grown to adjust to its situation and may be relatively stable. However, if the lean is recent or getting worse it is a matter of concern. If the root plate is heaving (the roots and/or soil surface on the side away from the lean is raised, and/or there is a corresponding indentation on the side it is leaning toward) then it is an indication that the roots are pulling out of the ground in response to a previous wind load, soil grade change, or damage to the roots themselves. This indicates a failure of the roots’ ability to anchor the tree. It may or may not take a significant wind force (or any force) for the tree to fall; it should be assumed that failure may be imminent.

Trees respond to mechanical load (i.e. wind or snow and ice) by adding reaction wood to reinforce areas under stress so that the force can be distributed more evenly. This is called “wind firmness”, which is directional. Hardwoods add reaction wood toward the wind load on branches and trunk; conifers add it on the side away from the wind.[15] This may cause trunks or major branches to be oval-shaped in cross section instead of round. The tree has made an adaptive response, but the presence of this response indicates exposure to a long term load stress which may be ongoing.

Unusual bulges, accented ridges, spiraling deformations, or indented areas indicate an abnormality that affects the trunk’s structural integrity. The presence of any of these likely indicates an area that has been previously exposed to external mechanical stress (bending or twisting) that has left an internal irregularity (fault) which the tree has tried to strengthen by adding wood. This fault may increase the chance of storm damage and make the trunk more prone to failure.

Crown and Branches

Crown and branching structure and size are the major determinants of load (wind or weight) distribution. Optimally the crown and individual branches are well balanced with a good live crown ratio (LCR) where the crown takes up at least two thirds the height of the tree; where major branch circumference is half or less of the circumference of the trunk at the point of attachment; and with a U-shaped rather than a sharp V-shaped (almost vertical branch) or very wide (almost horizontal branch) attachment angle between trunk and branch.

Reaction wood is a strengthening adjustment formed by the tree as a response to frequent exposure to a load. It is either compression wood, which resists a force pushing against it, or tension wood, which resists a force pulling away from it. Conifers form compression wood on the underside of the branch and tension wood on the top. Hardwoods build tension wood on top of branches and, when that no longer holds the branch up adequately, it reinforces itself by adding additional normal wood on the underside.[16] Branches may break in storms in response to a strong downward, upward, or twisting load, but are most vulnerable to an unusual upward or twisting force. When subjected to an unusual upward force, conifers tend to break because the compression wood under the branch is not strong enough in tension to withstand being pulled upward. Hardwood branches fail because the tension wood on top of the branch isn’t strong enough to withstand the compression of being pushed upward.[17]

Numerous studies on ice storms have cited “poor form” as leading to damage.[18] Poor form was generally defined as poor branch architecture including large branch angles, forks, greater branch and twig density, weak branch connections, and branches growing opposite or whorled at nodes.[19] Specific branch structural problems included large decay columns, large and/or open cracks, old injuries to wood and bark, unsound branch connections, and decay pockets (visible or not) as having created faults that then failed under ice loads.[20] Increased crown size was reported as more important to risk of failure than density or surface area; trees with larger canopies failed in greater proportion than those with smaller.[21] Tree total height increased the risk of uprooting, and larger height-to-diameter ratios increased the risk of damage from high-wind events.[22]

Forks and codominant branches have been cited as one of the most significant causes of branch failure.[23] In addition, opposite branches growing from the trunk are problematic; they can grow aggressively, pulling resources from and weakening the leader. The height at the place of attachment on the trunk of two large opposite branches is a weak point where the trunk may break.

Narrow angles of branch attachment increase the possibility of included bark. This is where the inside of the joint of the trunk and branch has been covered with bark at earlier stages of growth. Branches with joints that have included bark are more likely to fail since there is a layer of bark in the inside of the crotch which has kept the supportive wood layers from forming to adhere the branch securely to the trunk. On the contrary, the included bark keeps the two physically separated. Proper branch attachment allows space for successive layers of wood to wrap around and correctly strengthen the joint as the branch and trunk increase their respective diameters.

Branches which are distributed along the height of the trunk (leader), or foliage and small branches spread evenly along the length of the main branches, will distribute a load more equally. When most of the crown is at the top of the tree or most of the foliage is at the end of a branch (“lion-tailed”), the trunk or branch is more prone to failure. If foliage or crown is disproportionately located at the ends of branches or the top of the tree, wind or snow load will be focused on those areas with little force on the central portion. This acts as a lever arm and the stem or branch is then more easily broken. Similarly, when trees develop large low branches that grow until overextended to the point that the outer part is disproportionate to what can be maintained by branch taper and diameter, the branch is likely to break under its own weight. The wider the branch angle and the longer the branch, the more damage is incurred during ice storms compared to more upright branches.[24]

In an ice storm the surface area on a larger crown will increase the amount of overall ice accumulation, but the resulting damage will be less for an equivalently-sized crown if it is symmetrical since the load would then be evenly distributed.[25] Trees with excurrent branching patterns (i.e., conifers) are generally more resistant to ice storm damage, especially if there is strong branch attachment and less surface area of lateral branches.[26]

A type of damage known as “summer branch drop” may occur when wet weather follows a period of drought. Wood may crack internally from lack of water during the dry weather, but when it becomes re-saturated after a significant amount of rain the cracked wood is then not strong enough to support the normal weight of the branch.[27]

Note that an overall sparse or poor-looking crown may indicate a generally poor condition of the tree as a whole, but unless it is indicative of general decline or a specific problem it usually reflects a source problem at the roots. The transfer of nutrients happens just under the bark and in most trees these conduits run vertically, so an isolated section of dead or unhealthy canopy on only one side of the tree may often be traced to problems with the major root(s) directly underneath. (This depends on the species and circumstance; there are exceptions.)

Insects and Disease

Insects and pathogens are often a secondary problem that arises in a tree that is already under stress from environmental conditions or root problems. When a tree is stressed, it attracts pests and is more vulnerable to the effects of both pests and diseases, which then compound the original problem. Thus, the overall environment and condition of the entire tree must also be examined to look for underlying problems.

The actions of pests and diseases can have a substantial impact on the vitality and structural integrity of the tree. Pathogens can cause weakening of portions of the tree and may spread throughout it. Depending on the specific pathogen and how much damage it does, it may destroy the tree. Borer tunneling may be extensive enough to jeopardize branches or trunks, or the borers may spread a disease which is a considerable threat on its own. Presence of cankers, mushrooms, or fungal growths at the base of or on the trunk can be signs of a disease which may weaken or kill the tree. It is important to diagnose what has infected or infested the tree in order to gauge the extent of the damage that it may already have caused or potentially could cause. Again, the extent of internal damage to the tree that has already occurred is often not directly visible and can only be conjectured without further investigation by a certified arborist.


Wood decay has been described as “the biological process by which cellulose and lignin, the two most abundant organic compounds on earth, are converted to carbon dioxide and water with a release of energy to maintain forest processes.” [28] Decay is a necessary part of the cycle that moves nutrients, water, and energy to other uses, from dying trees that cannot use it to living plants that can. It is obviously destructive when it affects a tree, dead or living. Decay is an efficient process, but a living tree’s ability to resist it can provide a fairly effective defense.

Decay starts in either living or dying sapwood and spreads into dead heartwood. The sapwood compartmentalizes the decay organism, with greater or lesser effectiveness depending greatly on the species of tree. The decay organism may continue to move through the heartwood but the tree continues to add new wood on the outside; this is how trees become hollow. If the tree maintains at least a two inch to four inch depth of crack-free wood in the outer trunk, it may well have sufficient strength for it to survive.[29] Survival depends greatly on the tree’s ability to compartmentalize well in order to isolate and close the wound, and whether it has a rapid growth rate and can quickly add reinforcement to the outer supporting wood.

The decay process begins with wood discoloration which does not initially cause a loss of strength. It is a response to the invasion of pathogens and slows the process of decay so that the cambium can continue to grow. Subsequent decay begins a breakdown of tissue which results in loss of strength. Decay progresses quickly in exposed sapwood, more slowly in heartwood underneath it, and is walled off by the barrier zone formed after the infection. Oxygen is needed for active decay; if compartmentalization completely isolates the wound then decay stops. Bacteria thrive in low-oxygen environments where “bacterial wetwood” may develop.[30]

Different types of pathogens attack either only living wood, either living or dead wood, or only dead tissue. Heartwood-decaying fungi are often seen attacking dead wood on the forest floor. Fungi that attack both living and dead wood are the most common decay fungi seen on live trees. Some are weaker pathogens that attack sapwood exposed by wounding, but others are much stronger and attack, and can kill, the wood or bark of roots, root flare, trunk, and branch stubs. Advanced decay may cause breakage of the stem under wind force.[31]

Root-rot fungi attack roots below or above ground, particularly through cracks at and below the root flare. The fungi spread into the roots and then move into the stem through the less resistant area between the buttress roots. Compartmentalization is less effective in containing these fungi because they kill bark and cambium and thus block the tree’s attempts at barrier zone formation and wound closure. [32] Root rot is difficult to detect since the roots are largely not visible unless uncovered for examination; canopy dieback or decline may be the only evidence.

Canker-rot fungi infect branch stubs, usually those between ¾ inch to one inch in diameter, at a height of six to fifteen feet. They are effective at removing these stubs which aids the tree in compartmentalizing the site. However they also kill live wood and bark, so the fungi does not become completely compartmentalized. In pines, pitch at the base of the branch stubs may be seen early in the infection process indicating the presence of the fungus.[33]

Cracks which may form along compartmentalization boundaries, (i.e. ring cracks, closure cracks, or callousing) all create weak points which will be more vulnerable under stress. Cracks may actually be of more concern as weak points than the actual decayed area.[34]

Evaluation of decay can only be done by estimation since the internal decay cannot be seen. Questions to ask to help gauge the current strength and viability of the tree include: what specific decay organism is present and what its ability is to damage the tree; whether sufficient new wood is being added to counterbalance the wood lost to decay; whether trunk growth and wound closure is adequate to maintain the tree’s mechanical strength; and whether there are cracks in the main trunk or between the buttress roots. Root and stem rots, cankers, insect infestation, or any previous damage increases vulnerability to wind damage.

Stress and Decline

Stress and decline are often the underlying cause precipitating the presence of pests or disease, resulting in an unhealthy tree. Young trees are much more adaptable to various conditions and are generally more resistant to stressors; mature trees are less so and are more likely to be affected by them. Stressors can be many: insufficient or too much water, too much or too little sun, inappropriate temperature range, limited or damaged roots, chemical or mechanical injury, and more. Each of these compounds the effects of the others. Examination of the tree can help to identify it as being under stress or in decline, and observation of the environmental conditions may reveal possible causes. Note that there is often a substantial lag time between cause and effect. The effect of root damage may become apparent within a year or so, or impacts of damage may not become visible for twenty years or more. Unfortunately, most trees affected by decline will usually die even with supportive treatment.[35]

Symptoms of decline are often seen in the crown where the foliage may be sparse, or the leaves may be smaller than normal or may turn color and drop earlier in the fall than is the norm for that species in the same location. Epicormic growth may be seen as thin vertical sprouts along the upper surface of branches where the tree has rapidly sent up shoots to increase photosynthesis. Fruits or nuts may fail to mature and drop off the tree early. Trees may flower out of season or produce an abnormally large quantity of seed. Dieback of twigs and of progressively larger branches may be seen beginning in the upper crown and moving downward. Leaf or needle drop may move from nearer the trunk outward.

When evaluating a tree for decline, some techniques when making observations include: viewing all sides of the tree for damage; walking both clockwise and counterclockwise; examining the root flare for rot including probing at and below the soil line and between the buttress roots; and comparing new twig growth against prior years’ (length between nodes) for an estimate of annual growth and how long the tree has been declining.[36]

Inherent Species Characteristics

Many characteristics related to vulnerability to failure are greatly determined by what is typical for a particular species, and it can be very helpful to have that information available. The tree form (excurrent or decurrent) and the general tendency toward a particular growth pattern can either help or hinder a tree’s symmetry and load pattern, which is an important factor in resistance to storm damage, as is wood strength (although see the last bullet below). Good or poor compartmentalization is a critical factor in resistance to decay. Knowledge of the average life span of a species may provide a helpful frame of reference to evaluate decline since its genetics alone will set a general limit to its viability.

Below are a few examples of some types of species-specific differences. Always be aware that some studies may not agree; as in any research, the complexities and specificities of what questions are being asked, how the data is being collected, and what other variables are at play all define the challenge of finding valid results.

  • Hurricane damage: Conifers tend to be more susceptible than hardwoods.[37]
  • Wind damage: Conifers’ susceptibility to wind damage increases rapidly after about age fifteen, but the amount of hardwood trees’ damage increases more gradually with age.[38]
  • Ice and Snow: Trees with excurrent form are better able to shed ice and snow than decurrent forms. Vase-shaped trees are especially vulnerable.[39]
  • Ice Storms: Species is cited as a moderately important factor in the amount of damage sustained during ice storms.[40] Roughly two thirds of ice storm studies designate evergreens as more susceptible to damage.[41]
  • Exotics vs. natives: A number of non-native trees in urban forests tend to experience more damage than native trees.[42]
  • Native range: Species planted outside their native range may be more susceptible to damage.[43]
  • Roots: Some trees can survive the loss of half of their roots, but other species are extremely sensitive, even to root cutting outside of the dripline.[44]
  • Strength: The significance of tree species’ strength values in resistance to ice storms continues to be debated.[45]

There are many places to look for lists of species characteristics; below are a few. Again, keep in mind that, as is true of most research, it is extremely difficult to separate the effect of the various factors and so all conclusions may not agree. In addition, the research is constantly being refined and modified.

Site Conditions Contributing to Failure

The complex micro- and macro-environment of a site surrounding a tree, including topography, wind direction, exposure, size, location of an individual tree within a group, soil content and depth, and any changes to the site, all affect the tree’s response to load stresses from weather events. Research has only begun to tease out some of these interrelationships and attempt to unravel how a tree is actually affected by a given set of circumstances in order to understand and quantify the impact of major storm events: “Landscapes provide complex and chaotic variability in ice loading and tree resistance.”[46]

Hurricanes and tornados (wind and water) and winter storms (wind and ice or snow) are the two weather events with the most severe impact on trees. Much of the research and general information we will discuss here is primarily related to ice storms but, especially since the effect of wind load is a critical common factor in both, much of the theory can be extrapolated to hurricanes as well. Even in the absence of an ice load (accumulated weight), additional wind will cause an increased load which may well result in an equivalent or greater total load. The total load experienced during any event is a sum of all mechanical stressors.


Exposure describes how the vulnerability of trees in a particular site is affected by topographical elements including elevation, slope, and slope aspect (the direction it faces). These factors influence the wind load and ice accumulation, which also have an interactive effect on each other. The formation of a storm event itself and the duration of the resulting ice load are also both associated with the topography.

Note that trees in cities are exposed to very different wind patterns than in forests or even in suburban areas. They experience greater turbulence from wind movement around or between buildings (funneling), with consequently greater bending force than those in forests at the same wind speed.[47]

Elevation: The higher the elevation in mountainous terrain, the worse tree damage is seen in ice storms. Elevation increases exposure to ice and wind and is one of the most influential factors associated with a greater amount of ice storm damage, but it is difficult to isolate its specific effects (type or amount of damage that can be attributed solely to it) because it is so interdependent with other variables.

Slope: The degree of the slope is related to the tendency of trees to uproot. Steeper slopes have thinner soil, and the root impervious zone (depth at which the substrate does not allow roots to penetrate, e.g. rock) is closer to the surface. Less soil volume is available for secure rooting, so it will take less total ice and wind load for them to fall. An increase in the degree of slope is correlated with a decrease in crown damage but an increase in severe trunk damage.[48]

Aspect: The aspect of the slope is an important determinant of wind loading and of the duration of the ice load. Windward positions (facing the direction of the storm) and exposed slopes and ridges tend to have greater total loading (wind plus ice) and subsequent damage. The additional wind load on an exposed location can counter the effect of a smaller ice accumulation and can result in an even greater net amount of damage. On a slope with less sun exposure it will take longer for the ice accumulation to melt; the sustained load compounds the effect. Northeastern and eastern exposures are generally associated with more damage.[49]

When a given ice load accumulates, an additional mechanical load is produced by wind velocity in the form of either average wind speed or peak gusts. Storm loads can vary from heavy rain with no wind (vertical downward load of lesser amount) to heavy ice with strong winds (substantial lateral load in addition to downward). The direction of the load force, vertical or lateral, will modify the effect on an individual tree.

Stand Characteristics

Stand characteristics describe the environment of a group of trees as a whole and how these characteristics form an effect profile that, depending on where an individual tree is located in relation to the surrounding stand (group) of trees, is very different from a solitary tree.

Stand Density: The Basal Area (roughly the density of tree footprint in the ground space) within a stand greatly affects how much exposure to wind and ice load an individual tree is subjected to. A lesser density allows for freer air movement through the stand and allows the wind force to dissipate thus reducing wind exposure. Conversely, greater density allows a tree to be supported by its neighbors. Greater density is associated with more bending but less breakage, and with greater ice storm damage because of increased surface area for ice accumulation. The presence of vines has the same effect of increasing surface area and is associated with greater ice accumulation and increasing wind drag.

Relative Tree Height: Greater height increases the exposure of trees which are taller than the rest, putting them at increased risk for damage. The taller the tree is relative to surrounding trees, the more it is exposed to potential wind and ice load. This relative height is the strongest factor determining susceptibility to wind damage in a stand.[50] Generally, smaller trees are more likely to bend severely in response to ice loading, mid-sized trees show both bending and trunk breakage, and larger trees experience less bending or breaking, but show increased crown damage and branch loss. Immature trees that have not developed a taper (especially “pole” trees, often found in commercial forest stands) are more vulnerable to damage because their trunk structure is not as strong.

Location within the Stand: Location within or on the edge of the stand changes the amount of wind force that a tree is exposed to. Those on the edge of a stand are more exposed to wind but have developed reaction wood over time which provides additional strength; however, they are still more susceptible to damage than interior or protected trees. Edge trees which have been recently exposed are more prone to damage until they have had time to form reaction wood. When stands are thinned, storm damage to interior trees is increased until they can adjust to their new loading exposure by adding new wood. Long-established exterior trees and interior trees can have a similar damage potential unless the edge tree has an unbalanced crown.

Edge trees tend to have larger, more unbalanced crowns, with more lower branches on the open side where they receive more sun. They accumulate more ice on the open side due to the increased exposure, resulting in more branch failure, crown breakage, and uprooting.[51] Interior trees generally have smaller crowns and fewer lower branches due to less available light, and typically show less storm damage than edge trees. However, the greater the amount of ice accumulation during a particular storm, the less difference location within the stand makes. The tendency for damage also varies depending on species.

When in close proximity to larger buildings such as a home, a tree on the leeward side of the building has a more protected location. Microclimates, which are small areas with differing characteristics than the surrounding larger site, create a unique site environment that must be viewed as an independent factor in addition to the tree’s position in the larger area.

Soil characteristics

Soil characteristics largely determine the health of the roots and their ability to anchor the tree in the ground.

Volume: An appropriate volume of soil allows optimal root growth and strength; reducing the available soil increases the chance of uprooting. The presence of erosion, boulders, bedrock, and physical barriers to root growth limit available soil volume. Heavy clay soils are more difficult for roots to penetrate deeply, and loose soils such as sand do not have the holding ability of denser soils.

Saturation: Soil saturation increases the possibility of a tree uprooting. It is one of the most prevalent factors involved in windthrow due to the instability of the soil.[52] In addition, periodic flooding or increased moisture levels over time, if the tree is not adaptable to wet soils, will reduce the roots’ access to oxygen and suffocate them.

Compaction: Compaction due to frequent walking, vehicles being driven, construction activity, or any weight-bearing impact, leaves a dense soil which is less able to move water and oxygen to the roots resulting in limited root growth. Clay soil has a particular tendency toward compaction.

Salinity: Salinity is seen in soils near some coastal areas (including well water) or very low-rainfall regions. Most trees are not tolerant of high salt levels which dry out the roots. Trees near roads which are treated with deicing salts, especially if they are on the downhill side, are also vulnerable to salt damage.

Site modifications

Site modifications that affect trees often occur during construction, utility work, landscape planting, or grounds maintenance, and especially include any activity that modifies or comes in contact with the roots. Depending on the extent of the activity, it can be extremely harmful to or kill an existing tree.

Raising the grade: Raising the grade (increasing the soil depth), even temporarily, will suffocate roots due to lack of oxygen; the greater the added depth, the more severe the impact. This includes planting too deep, adding soil to the root ball, or adding mulch to a depth of more than three to four inches. If the trunk flare is not visible, then the tree is too deep and the roots are suffering from insufficient oxygen. Adding heavier soils, such as clay, has a greater impact than lighter soils. Problems can occur with only an inch of clay soil added.

Toxin-producing bacteria can thrive in the resulting anaerobic environment. Adding soils different from the base soil may cause differences in drainage and moisture level, ground temperature, and oxygen availability. Covering roots with an area of impermeable pavement such as asphalt or concrete will likely cause them to suffocate and die.

Lowering the grade: Lowering the grade likewise causes serious problems. Roots may be severed or exposed to sun and air, become desiccated, and die. Even removal of a small amount of soil exposes roots close to the surface to dry out and die. Removal of protective leaf litter or mulch causes ground temperature and moisture changes.

Cutting Roots: Cutting large roots jeopardizes the health of the roots and the tree itself, and leaves the tree structurally unstable and subject to falling even without wind. Cutting roots on one side of the tree may cause decline or death on that side. Decline due to root loss may be immediate or progressive depending on the size of the roots cut. Tilling or extensive working of the soil under the tree will damage medium and smaller roots that are critical for tree health.

Root cutting impacts the tree in two ways. In terms of the effect on its general health and vitality, any root cutting should optimally be limited to outside of the Critical Root Zone (CRZ), which is a circle with radius extending one foot from the trunk for each inch of the tree’s Diameter at Breast Height (DBH; the trunk’s diameter at 4½ feet from the ground). For example if the DBH is 40 inches (the tree measures 40 inches in diameter at a point 4½ feet high), then the CRZ extends radially from the trunk 40 feet in every direction. Any root impact including cutting, damage, grade change, and compaction, should be avoided within this area. Trees that are old, unhealthy, or more sensitive to root cutting should be given a CRZ of 1½ feet (instead of 1 foot) per inch of DBH.[53] 

Roots greater than 1½ inches in diameter should not be cut, but instead tunneled under.[54]

However, when the intent is only to prevent destabilization (that is, considering only the effect on the tree’s stability) a more commonly used guideline is to restrict root cutting to three to five times the DBH.

Trees in urban areas located next to sidewalks, parking lots, and roads are in particularly poor environments for root growth and tree health. Soil volume is limited by the closely abutting impermeable paving materials. Compaction is invariably present, caused by the previous construction, and these trees seldom receive sufficient water. Trees generally struggle to grow in these situations, but when the roots are interfered with or damaged then the tree’s ability to survive becomes even more tenuous.

A study on urban trees adjacent to sidewalks found that after thunderstorms and windstorms, trees which had experienced root or trunk collar impact or damage during a previous sidewalk repair were more than twice as likely to pull out of the ground (resulting in at least 50% of roots protruding) as those which underwent sidewalk repairs where the roots and trunk flare were not impacted, or those where there were no repairs performed.[55] When resources for root growth are severely limited and the roots are then damaged, their ability to anchor the tree is significantly impacted.

Chemicals – Construction chemicals, some types of treated wood, mechanical fluids (e.g., oil or gasoline), or other household or yard substances including herbicides are toxins that can leach into the soil and be taken up by the roots.

Changing the pH – Changing the pH of the soil may have an adverse effect on the tree depending on the amount of the change and the tree’s sensitivity to pH level. Changes in acidity level can occur from the use of alkaline clays or limestone used for fill or paving, or by concrete coming in direct contact with the ground when being mixed or washed out.


Recent weather events or historical weather tendencies also play a part in creating the characteristics of the site, either over time or at a particular point in time. Repeated stressors often have a cumulative effect, exacerbating suboptimal site conditions and aggravating previous damage to the tree, increasing its vulnerability to failure. The closer together the weather events occur, the more stress the tree is under and the more opportunities there are for new damage and faults to occur.

Non-critical weather factors can have a significant impact on a tree, even if they are indirect. Following an ice storm the duration of the ice accumulation affects its cumulative load. The length of time that goes by until the ice melts is affected not only by sun exposure (versus valleys or shaded hills), but also by how quickly air temperature increases after the storm and the warm or cold air flow at the site.

Ground saturation or erosion from recent storms can cause trees to uproot more easily by modifying the soil characteristics. The ability of roots to maintain their anchoring ability is greatly decreased by very wet soil or shallow soil, especially when roots are newly uncovered by flooding or erosion. Frequent or prolonged saturation can drown or damage roots and encourage the growth of rot organisms.

A normal amount of rain after a year or more of drought can cause tree death. Roots often die back during a drought and the tree is weakened, then renewed moisture encourages the growth of fungal soil pathogens which may attack the weakened tree.

Often it is an abrupt change in conditions which can cause problems. Frost cracks on trunks are caused by the bark’s inability to adjust quickly enough between a very cold nighttime temperature to a warm sunny day immediately following. These cracks are generally seen on the southwest side of the tree since that side usually faces the winter sun.

Even time of year can be a factor. Hurricanes usually occur while deciduous trees are still in leaf which adds to their wind resistance.

Tree Failure and Precipitating Events

We have looked at various factors describing the tree and site conditions which may increase the tree’s susceptibility to failure. The most determinative factor, however, is the force to which the tree is subjected. Trees of various types have evolved to be able to successfully resist the various forces that constitute their “normal load,” that is, the most common weather they are exposed to in a particular location. However, the strongest tree in the most advantageous location may be maximally optimized for strength and survival, but there is always a point beyond which it will fail when it has been exposed to a load more severe than it is capable of handling. It is not an absolute limit- it will vary because of the interaction of factors we have discussed previously- but a limit does exist. There is a specific load which, when applied to a specific tree in a specific place at a specific point in time, will cause it to fail. The description and quantification of load mechanics, measurements, prediction, and other specifics is far beyond the scope of this manual. In this section we will describe the short-term and long-term effects of the damage incurred when a tree is impacted by a force beyond its ability to withstand.

Types of Tree Failures

The concept of tree “failure” implies a level of structural destruction that is severe, but we can introduce the topic a little more precisely by further defining where and how the damage occurs and what physical change occurs in the tree. A description of these are given below.[56]

Blowover- Blowover occurs when the tree is physically pushed over by a constant wind or gust (hurricanes, downdraft, or tornados). This occurs when the wind force is too great for the wood structure.

Stem Failure – Stem failure occurs when old or new wound create a spot that is weak or is enough of an interruption in the tension distribution that it is more susceptible to giving way. When a wind force strikes a tree, the tree is pushed to its further limit away from the wind. When the force is removed the tree snaps quickly back to its upright position or, depending on the amount of the initial force, swings back toward the source of the wind. Trees with heavy crowns can be overloaded by snap-back during intermittent wind gusts and calm, causing stem breakage.

Crown Twist – Asymmetrical crowns are particularly susceptible to crown twist. When the crown is lopsided, there is more wind load on the larger or heavier side causing a twisting action on the major branches and stem. The tree can adjust internally to the effects of twisting, but this exacerbates old injuries and can cause the wood in the stem to split or branches to collapse.

Root Failure – Both woody structural roots and fine absorbing roots help to provide anchorage for the tree, one by means of strength, the other through quantity. If the root collar is damaged in some way, or if the volume of the trunk and crown incurs greater stress than the root collar can handle, the roots’ mechanical strength is overwhelmed and the roots can pull out of the ground or snap, resulting in the tree falling or leaning.

Branch Failure – The attachment of branches to the trunk can be good in relative terms, but in an absolute sense it is not a strong attachment. It has evolved to be that way for a reason: it allows the branch to be flexible and to be easily shed if needed. Unusually strong storms may cause a branch to tear downward or snap from an ice or snow load or the force of a downburst.

Lightning – Lightning is an immediately life-threatening event for the tree. It destroys tissue by electrical disruption and heat. It either moves down the tree from the branches through the stem to the roots, or along a path that encompasses the entire tree. There can be massive unseen root damage.

Storm Effects

Trees may fail at any time. If a tree’s structural strength has degraded enough or its stability is jeopardized sufficiently, it may reach a point where it will simply break or fall at the behest of gravity. However, more often than not, failure will be precipitated by an event that subjects it to forces beyond its everyday equilibrium.

Wind Events (Hurricanes and Tornados) – Wind causes perhaps the most obvious and most immediately destructive force commonly occurring during a storm event. “’Wind loading’ [is defined as] a straight wind from one direction applied evenly over the stem, branches, and leaves… ‘Wind release’ [refers to] the removal of load causing the crown and stem to snap back. Gusts and calm alternately load and release the tree.”[57]

The Enhanced Fujita (EF) Scale, which is used for ranking tornado intensity, provides a frame of reference for the range of destruction that may occur at various wind speeds. At speeds of 74 to 75 mph, hardwood and softwood trees may incur breakage of branches greater than 1 inch in diameter; at 91 and 87 mph (respectively) hardware and software trees may uproot; and at 110 and 104 mph hardwood and softwood trees may snap.[58]

The Saffir-Simpson Hurricane Wind Scale states that Category One hurricane winds (74 to 95 mph) are able to snap large branches and may topple shallowly rooted trees; Category Two (96 to 110 mph) will see “many” shallowly rooted trees uprooted; Category Three (111 to 129 mph) will snap or uproot numerous trees of any root depth; and Category Four (130 to 156 mph) will snap or uproot “most” trees. As tree uprooting increases, there is an increased amount of damage to houses and other buildings, and increasingly greater impact on community access due to multiple road blockages.[59]

Ninety-three percent (93%) of tornado-related deaths occur at EF scales F2 and above (over 113 mph winds), but of the seven percent (7%) of tornado-related deaths that occur at levels F0 or F1 (under 112 mph), 38% are due to wind-related tree failure. In other words, a greater proportion of tornado-associated deaths due to wind-related tree failure begin to occur at lower wind speeds than deaths due to building or vehicle damage.[60]

Hardwood trees usually survive breakage. When tops are lost, new branches will sprout, but loss of large branches also allows entry of decay fungi. Most species of pine will die if the tops are completely broken and no live limbs remain. However for loblolly or slash pines, if at least three or more live limbs are left at the top, there is about a 75% chance of survival. One of the remaining branches will become the new terminal branch and the tree will continue to grow, eventually showing only a sharp crook where that occurred. Young trees may suffer bark damage from extreme bending, which makes them more vulnerable to disease. Small trees less than fifteen feet tall usually straighten even after severe bending. Pine trees that bend to the extent that they crack and the resin flows may be invaded by bark beetles and disease-causing organisms.[61]

Cyclonic winds from tornados and some hurricanes cause twisting and separation of wood fibers in the main stem. The trees may still appear normal but internal damage of some type has occurred. Pines may show some pitch flow along the trunk.[62]

Uprooting of trees causes soil disruption and erosion. After storms which result in extensive tree deaths there is an increased risk of wildfires, pest and disease attacks, and infiltration of newly opened areas by aggressive invasive plants.[63]

Indirect damage may be caused by flooding or saturated soil. In standing water, available oxygen is quickly used; loss of soil oxygen leads to root mortality and tree death. Most trees are injured by flooding, especially during the active growing season. Weakened trees are then often attacked by insects and disease.[64]

A Florida study which compiled data from several hurricanes attempted to quantify tree survival by various factors, as follows below.[65] (Note that the study included some tree species that are not common in Virginia and that the soil of coastal Florida is dissimilar to most Virginia soils; however most observations should still be generally applicable.)

  • Wind speed:
    • Urban forest tree loss increased with greater wind speed.
  • Foliage and Crown:
    • More leaf loss was equated with better survival. Losing leaves and small twigs during the hurricane increased survival and resulted in less crown damage.
    • Trees with dense crowns lost more branches but survived more often (84%) than moderately dense (74%) or open crowns (67%).
  • Form and Wood:
    • Species with shorter, thicker stems and denser wood tended to uproot instead of break. Species with lower density wood had greater mortality.
    • Generally, pines tended to snap (stem breakage) versus the tendency of broadleaf trees to uproot.
    • Trees with higher wood density had better survival rates and were less likely to fail by uprooting or breakage.
    • Decurrent trees had better survival (80%) than excurrent (69%), but experienced more branch damage (22% versus 17%).
  • Size:
    • Larger trees lost more branches (30%) than medium (25%), smaller (20%), or the smallest (12%) trees.
    • In forest stand environments, trees with larger stem diameters were more likely to be damaged by wind as opposed to smaller trees which were more likely to be indirectly damaged by other falling trees.
  • Stand:
    • Trees in groups had proportionally better survival (80%) than those that were more isolated (70%), although branch loss was the same.
  • Species:
    • Species responded differently for uprooting, stem breakage, and crown damage.
    • Most data showed that native trees survived better than non-natives.
  • Soil:
    • Increased rainfall from the hurricane and saturated soil resulted in more tree mortality, especially due to uprooting.

Ice Storms – Ice storms affect trees through the synergistic effects of wind load and ice or snow weight; the ability of the tree to resist these combined forces is key. The accumulation of ice can increase the weight of a branch by thirty times or more. At an accumulation of ¼ inch to ½ inch, small branches and weak limbs break; at ½ inch to 1 inch larger branches may fail.[66]

Changes in the position of the center of mass over the root plate as the tree moves in response to wind magnifies any existing asymmetry in the crown and, with the addition of ice and wind loads, will compromise the stem’s resistance to loading.[67] As the weight load increases, the load resistance points in the tree will shift, concentrating on previous faults or weak areas in the wood. The tree becomes stiffer and less able to move in response to the wind, which produces more drag (resistance) initiating more damage and new faults which may then lead to failure.[68] However, the strength of sound branches may not be as consequential as the ability of the tree to withstand breakage at branch junctures, or the density of fine branches or a broad crown that can add to the total amount of ice accumulation.[69]

A compilation of 45 studies on ice storms summarized 56 factors associated with tree damage. The importance of each was scored between 0.0 and 1.0 (low to high importance).[70] Ratings were as follows:

  • Ice thickness (weight) and increased wind loads were four to five times more important in causing damage (1.0 and 0.8, respectively) than the next highest factors.
  • Ice load duration, leaf type (needle or broadleaf), and asymmetrical crown were rated as 0.25.
  • Tree size, degree of slope, amount of canopy surface area, and branch and twig density were rated 0.21.
  • Topography, stem diameter, poor branch architecture, poor tree form, tree age, and branch structural problems ratings ranged from 0.21 to 0.16 (respectively).

The primary causes of ice storm damage, then, are outside of the ability to be modified through human intervention, but changes made by correct pruning may help somewhat in reducing the likelihood of damage.

Post-Storm Outcomes – The aftermath of a storm or other damaging event is only the beginning, at that point in time, of a sequence of events that may unfold over ten to twenty years or more. Within this time frame all the previous vulnerabilities that we have described earlier in this chapter will again come into play: immediate mechanical damage; new faults in the tree leading to reduced structural integrity; openings for insects, disease and decay to invade; root damage and death; and even site changes. In this post-event period after a storm of significant intensity, the changes that occur may be immediate or long-term; they can simply add a few more factors that affect the tree’s vitality and stability or they can be the final stressor that causes tree failure or death.

After a weather event it is helpful to have an idea of what the tree’s prognosis might be and what steps might be taken to care for or determine whether to remove injured trees. Some of the information below is based on ice storms but should also be generally applicable to other weather damage and can also be cautiously extrapolated to tree survival after damage in general.

The compilation of 45 studies on ice storms cited previously found that estimation of the amount of canopy loss from storm damage generally predicted mortality of damaged trees at three to five years:[71]

  • Less than 25% canopy loss was “insignificant”.
  • Less than 50% was “usually survivable”.
  • More than 75% loss was “usually fatal”.

The USDA Forest Service similarly states that “potential for survival is related to the extent of loss of the crown” and provides further details on these severity categories:[72]

  • If less than 50% of the live crown is damaged there is a high chance of survival. Growth may slow due to canopy loss.
  • If 50-75% of the live crown is damaged, trees may survive but with reduced growth and increased chance of infections, especially where damage includes larger branches or tops, shattered branch bases, or torn bark. Trees should be monitored.
  • If more than 75% of the live crown is damaged, there is a low chance of survival. Those trees that do survive will probably become infected.

The University of Georgia gives the below guidelines in “Community Tree Damage Control Based on Future Tree Health Expectation” to assist in making decisions on disposition of damaged trees.[73]

  • For any of the following, the tree should be removed to eliminate liability, additional costs, and further tree problems:
    • Dead tree.
    • Stem (trunk) broken from snapping or twisting.
    • Major branch collapse (greater than 50% of live crown affected).
    • Roots broken- tree pushed over or leaning.
    • Leaning or bent pine.
    • Lightning strike- hardwoods if more than 30% of bark circumference affected; any pines should be removed.
    • Mechanical damage to main stem if more than 30% of bark circumference affected.
    • Branch damage leaving severely lopsided crown (70% or more of crown on one side of tree).
    • Large stress or twist cracks in main stem.
    • Interferes with utility right-of-way safety and maintenance.
    • Split tree (remaining stem may fall).
    • Live branches broken or damaged are more than 50% for hardwoods, 30% for pines.
    • Top broken, if more than 50% of the live crown was lost for hardwoods, 30% for pines.
  • For any of the following, minimize stress and water the tree; wait for one growing season to fertilize and prune:
    • Lightning strike- hardwoods if less than 30% of bark circumference affected (pines should be removed).
    • Mechanical damage to main stem if less than 30% of bark circumference affected.
    • Twigs and small branches blown off or broken.
    • Foliage destroyed.
  • For any of the following, prune dead and dying branches, cut back to next major living branch (drop crotch) and water the tree; wait for one growing season to fertilize and prune to shape:
    • Top broken, if less than 50% of the live crown was lost for hardwoods, 30% for pines.
    • Live branches broken or damaged less than 50% for hardwoods, 30% for pines.
    • Stagheaded (dead lateral branches at the top of the tree).

When roots have been damaged, the crown size should be reduced somewhat to temporarily lessen the demand on the roots. Thinning or reducing the crown will also help with wind resistance in future storms. This type of pruning has been shown to reduce movement in any wind speed, but raising the crown (limbing up) did not.[74] Raising the crown too much or stripping out the interior of the canopy shifts wind force to the edges, which can reduce the tree’s ability to dampen movement and result in limb breakage.[75] If a tree has developed a recent lean due to a storm or other known factor, it should be evaluated by a certified arborist before making a decision as to whether to leave it in place. Trees which have been placed into a lean of no more than 20° may survive but must be assessed by a professional as to whether it is an acceptable risk.[76]


Lightning strike is a devastating event. The extent and types of injury it causes are unlike any other type of damage:

“Each strike of lightning can reach more than five miles in length, and produce temperatures greater than 50,000 degrees Fahrenheit and an electrical charge of 100 million volts… Along the path of the strike, sap boils, steam is generated and cells explode in the wood, leading to strips of wood and bark peeling or being blown off the tree.”  [77]

“The most serious tree injuries caused by lightning are from the acoustic wave (explosive shock wave) radiating from the lightning path… The explosive shock can also cause the tree to flex and energetically rebound, causing bark and wood loosening or expulsion. The shock wave shears-off cellular connections, pulls wood fibers apart, and loosens bark, phloem, cambium, and xylem. Multiple strokes in a single lightning strike can generate multiple shock waves. The shock waves bounce off the inside of the tree stem and cause tree tissue shifts along the stem’s circumference.”[78]

The most visible injury from a lightning strike is bark splitting and radial cracking. Leaves will wilt from disrupted water transport; permanent wilting on a major area is also an initial symptom identifying a lightning strike. If all leaves on the tree wilt immediately it will likely die within a few days, but if it does survive and leafs out the next spring it has a good long-term prognosis.[79] Some trees may have intermittent foliage (coming and going) over several months, leading to twig death.[80]

Twenty percent or more of trees will have no visible injuries, but there is likely to be significant damage internally or in the roots (lightning dissipates into the ground). The tree will still be prone to increased stress, lowered defenses, and be more susceptible to pests. It may decline and die while still having little or no visible damage.[81]

A tree may take anywhere from days to several years to decline and die from a lightning strike. It may recover, but recovery usually takes several years. If the strike was only on one side there is a good chance of wound closure and survival. If the strike passed through the tree (bark and wood damage is seen on both sides) it usually will not survive.[82]

If the tree appears that it may survive, dead and hazardous branches should be pruned immediately but other corrective pruning should be put off for two to six months and then the tree should be evaluated by an arborist. Lightning-struck trees, especially pines, are very susceptible to pests.[83] Survival is dependent on good site conditions, particularly water, to support new growth.[84]


The extent of damage caused by fire depends on the part of the tree affected and the profile of the fire. The tree’s survival greatly depends on its ability to resist pest infestation that may follow.

Trees are usually killed outright by crown fires or high intensity fires. Severe crown scorch is caused by hot gases from ground fires which “bake” foliage and twigs into a set position from which they cannot recover. Low intensity fires may cause partial tree kill.[85]

The recovery and growth of a fire-damaged tree depends on its ability to continue with its basic processes, especially photosynthesis. The degree of crown scorch, foliage loss, bud death, and damage to trunk bark and cambium are the key factors determining the likelihood of survival. Conifer survival depends mostly on the viability of their buds. Fire-related mortality is greater the second year than the first; ninety percent of mortality occurs within three years.[86]

Pest infestation is a major problem after fire damage. Newly killed and fire-damaged trees are very attractive to bark beetles and wood borers. Borers introduce decay fungi and bacteria which aid decomposition of dead and dying trees but may also infect and kill surviving trees. Most mortality from bark borers occurs within the two years following the fire.

Any surviving trees should be evaluated the following spring. If there is 70% healthy-appearing crown the next spring, the tree has a reasonable chance of survival.[87] Conifers may be assessed by the amount of needle scorch in the crown and the health of buds and twigs. If needles are still green, the buds may still be alive: check by slicing one open.

The best means of minimizing future tree failure is attention to good tree care throughout its life. Selecting an appropriate species suited to the site, following proper planting and watering guidelines to aid in strong root growth, avoiding any impacts to roots, and proper pruning when needed by a certified arborist, are the best insurance for the health and safety of a tree. A certified arborist should always be consulted in any case where there are questions related to safety or hazard.

Tree Risk Assessment

Now that we have discussed how to look at a tree and discern possible problems and issues of concern, how should we respond to a homeowner who asks us how dangerous their tree is and whether they should have it removed? Can we as Tree Stewards evaluate the risk and provide an answer?

What is “risk” and how can it be assessed? The process of evaluating the risk posed by a tree is part science, part experience, and part uncertainty. It is a challenge to tree care professionals who have much more training, experience, and diagnostic tools than any Tree Steward will have. The probability of any particular event occurring can never be absolutely predicted, so how do we respond? The homeowner poses a question that does not have a simple answer, and that Tree Stewards must undertake to answer with all due care.

Understanding Risk

To return to the adapted quote at the beginning of the chapter: if a tree falls in a forest and no one is around, does it pose a risk? If it is large, old, rotted, badly leaning, with its roots lifting, is it a risk? What if it’s in a playground? What if the playground is closed and secured and no one ever goes there?

Risk occurs at the intersection of the likelihood of failure, the severity of the possible outcome, and the chance of impact. The more likely a failure is to occur, the more severe the consequences might be, and the greater the chance of it occurring while someone or something is present to be impacted, then the greater the “risk” posed by a given situation. Like the assessment of a tree’s structural stability, an assessment of risk is also multi-faceted and is arrived at after the examination of a number of factors (of which tree condition is only one). Risk exists at a point on a spectrum; its evaluation, and the evaluation of each of its contributing factors, encompasses a range of possibilities and degrees.

The presence of a target is the first requirement for a situation to be considered a possible risk. Without a target that could be impacted if a tree fails, there is no risk involved. There may be multiple potential targets; they may be human, animal, property, or use of the space.

Associated with the target is the severity of possible consequences. This can be personal or monetary, and can range from a minor impact such as a piece of broken furniture, to very severe such as injury, death, or loss of use of a critical facility like a hospital. Consequences can also include disrupted activities such as a loss of business.

The next factor is the likelihood of tree failure (failure of the entire tree or significant tree part). This consists of evaluation of all the observations that can be made of the tree and its environment to determine items of concern and the degree of hazard that they pose. These components are what we have covered in the previous sections of this chapter.

The final factor that brings together target, failure, and consequence is the chance of impact. This introduces the elements of time and distance by examining the usage pattern (frequency and timing) of access to the target zone, the proximity to the target, and the mobility of the target. The target zone could be a busy city street which is constantly filled with passersby; a school yard which is only occupied on weekdays during the day; a back yard which is only used occasionally, or a remote hiking trail that is rarely used. The more frequently the space is occupied, the greater the probability that the tree will fail at the same time that there is a target present. If the target is not moveable (e.g., a house) then impact would inevitably happen if a tree failure were to occur.

Risk may be mitigated (reduced) by making a change to one of these factors. The target can be moved, rerouted, or access to the space can be restricted; or the tree can be pruned, supported, or removed. These efforts can serve to reduce the risk posed, but it is never completely eliminated unless the tree is removed. Risk cannot be definitively calculated; it represents a spectrum of possible outcomes and likelihoods, under a “normal” (or commonly expected) load, in a given situation which changes at different points in time. What residual risk remains after any mitigating actions should hopefully be at an acceptable level. Each tree-owner will have a level of risk which is tolerable to them, where they are prepared to accept the level of consequences that may occur.

Certified Arborists and Professional Risk Assessment

This subsection is included to provide EMG Tree Stewards with an understanding of how certified arborists assess trees in order to expand your comprehension of the assessment process. Note that it is NOT an attempt to teach any professional method or process to Tree Stewards. Tree Stewards are neither encouraged, nor authorized, to follow specific industry assessment protocols or to represent themselves as qualified to assess risk or safety.

The basics of proper tree care are enumerated in the American National Standards Institute (ANSI) A300 Standards, interpreted and explained by Best Management Practices (BMPs), which are voluntary industry guidelines that describe optimal tree care practices. Tree Risk Assessment is included in the A300 standard (Part 9).

An assessment will usually include the following steps: visually examine the structure of the tree; describe any defects; evaluate the likelihood of failure; and note the possible damage if failure were to occur. A certified arborist assessment may consist of one of three levels. Level 1, a limited visual assessment, covers only what may be seen from one side, and is used for quickly scanning a number of trees (often by vehicle or utility company flyover) to pick out the most obvious problems. Level 2, a basic assessment, is what is usually performed to examine a tree individually with attention paid to all sides and components of the tree. Level 3, an advanced assessment, includes more extensive means of examination to provide additional information (including attempts to determine internal decay by means of various equipment) such as climbing, drilling, tomography, etc.

The evaluation of something so subjective is difficult, and when performed on different trees in different situations by different individuals there is likely to be a wide divergence in the resulting assessments. The International Society of Arboriculture (ISA) has developed a protocol to create a single standardized system of evaluation for use throughout the industry by arborists trained and qualified in its use. This is the Tree Risk Assessment Qualification (TRAQ) program. It is based on a systematic process of reviewing each of the components of risk, and then using the weighted results to arrive at a final quantified assessment of the level of risk. The process is supported by the ISA Basic Tree Risk Assessment Form[88] (along with its instructions)[89] using risk categorization methodologies from ISA’s Tree Risk Assessment BMPs. The information contained on the form is described below. The first sections describe the targets and areas of concern observed on the tree or in its environment; the remainder assesses the level of risk.

Many of the components will be familiar to you from the previous sections of this chapter, but it is informative to see how the targets are evaluated, how the factors are quantified, and how both are then combined to reach an overall assessment. In addition, becoming exposed to the descriptive terminology, which is designed to be in line with legal and insurance nomenclature, is in itself helpful.


Target Assessment for each potential target within striking distance:

  • Description of the target.
  • Factors which might serve to protect it or to minimize impact or damage.
  • Target Zone: Whether the target is within the drip line; within striking distance if the tree falls (1x height of the tree); within range of large flying debris if the tree falls (1.5x height of the tree).
  • Occupancy rate: Rare; occasional; frequent; constant.
  • Whether target could be moved and/or access can be restricted.

Site Factors that may influence tree failure:

  • History of failure.
  • Topography.
  • Site changes: None; grade change; site clearing; changed soil hydrology; roots cut.
  • Soil conditions: Limited volume; saturated; shallow; compacted; pavement over roots.
  • Prevailing wind direction.
  • Common weather.

Tree Health and Species Profile:

  • Vigor: Low; normal; high.
  • Foliage: None (seasonal); none (dead); % normal; % chlorotic; % necrotic.
  • Pests; biotic or abiotic problems.
  • Species failure profile, to list any relevant species-specific tendencies (typical problems).

Load Factors including dynamic (variable) and static (constant) loads and related factors:

  • Wind exposure: Protected; partial; full; funneled.
  • Relative crown size.
  • Crown density: Sparse; normal; dense.
  • Interior branches: Few; normal; dense.
  • Presence of vines, mistletoe or moss which would increase weight or wind resistance.
  • Changes in load factors.

Tree Defects and Conditions Affecting Likelihood of Failure for trunk, crown, branches, roots, and root collar:

  • Crown unbalanced.
  • Live crown ratio (LCR; ratio of crown height to tree height).
  • Dead twigs and branches.
  • Broken, hanging, or over-extended branches.
  • Pruning history: Cleaned; thinned; raised; reduced; topped; lion-tailed; flush cuts.
  • Trunk or major branch cracks.
  • Lightning damage.
  • Codominant branches or trunks.
  • Included bark.
  • Weak attachments.
  • Cavity or nest holes.
  • Previous failures.
  • Dead or missing bark or bark with abnormal texture or color.
  • Cankers, galls, or burls.
  • Damage or decay to sapwood or heartwood.
  • Conks or mushrooms.
  • Response (reaction) growth.
  • Poor trunk taper.
  • Trunk lean and whether it shows corrected growth.
  • Sap ooze on trunk or root collar.
  • Root collar buried or not visible.
  • Stem girdling.
  • Cut or damaged roots.
  • Root plate lifting.
  • Soil weakness affecting anchorage.
  • List of conditions of concern, including size of tree part and fall distance.
  • Load expected on tree defect: Minor; moderate; significant.
  • Likelihood of Failure: Improbable; possible; probable; imminent.

Risk Categorization for each target:

  • Description of the target; tree part; conditions of concern.
  • Likelihood of Failure (from above): Improbable; possible; probable; imminent.
  • Likelihood of Impact for that target: Very low; low; medium; high.
  • Combined Likelihood of Failure and Impact for that target within a given time frame, e.g. 2-3 years (based on cross reference of Likelihood of Failure with Likelihood of Impact): Unlikely; somewhat likely; likely; very likely.
  • Consequences of Failure are an estimated amount of possible harm or damage to that target: Negligible; minor; significant; severe.
  • Risk Rating for that target (based on cross reference of Combined Likelihood of Failure and Impact with Consequences of Failure): Low; moderate; high; extreme.

Mitigation options for each risk.

Residual risk remaining after recommended mitigation: Low, moderate, high, extreme.

Overall Tree Risk Rating is the highest risk for the tree and highest rated target.

Overall Residual Risk is the remaining risk after the highest-risk tree part is mitigated.

The Role of the Tree Steward in Risk Assessment

As a Tree Steward or Tree Steward intern you will have learned a great deal more about trees and tree hazards than most people will know. With this knowledge and your position as a Virginia Cooperative Extension Master Gardener volunteer comes significant responsibility. We must always present ourselves clearly as non-professionals who have some advanced training, and make sure the homeowner knows that we do not have the training, professional certification, or experience to provide an expert opinion. Homeowners may well make decisions based on what we tell them. Knowing this, we must be sure to disavow them of any impression that we are more highly qualified than we actually are, and to be cautious in the opinions we express. What we communicate can affect people’s lives and property; we do not want our words to have the unintended result of injury or destruction. In addition, since we represent VCE there is likely to be an automatic assumption of us as an “authority”, which will not only give our words an added weight but may also place a legal responsibility on us to make only recommendations and evaluations which are appropriate to our position.

When examining a homeowner’s tree we should be as thorough as possible. There is a 2021 VCE Pub SPES-313P, How to Evaluate a Tree, which can help both the Tree Steward and the homeowner. Review mentally or make a list of the elements to observe related to the tree and its environment that may be relevant to its health and structural stability. It is helpful to document what we observe. Each item of concern should be discussed with the homeowner, and an explanation provided for the impact that it may have on the tree. The mission of VCE is, first and foremost, education, and this is what we provide to the homeowner. We may examine their trees, but our appropriate function is to teach them some basics of tree biology and structure which will help them to understand the implication of the concerns we point out to them. By communicating scientific observations and providing unbiased, research-based explanations, we are helping them to become informed consumers and more knowledgeable tree owners.

We may make statements to the effect that defect x may be due to or associated with reason y, which would then make it a concern because of possibility z. We may NOT make statements of an absolute nature such as that defect x is due to reason y, or that defect x or reason y will cause outcome z. Most important, we can NEVER state that defect x or reason y will NOT cause outcome z, i.e., that a tree or part of a tree is SAFE or is NOT a danger, because there is no way that that can be determined. We do not have the ability nor the authority to make such a statement.

We can say, if we are certain, that a specific thing is normal and is therefore not usually a concern. We can say that a condition (or the tree generally) could be a concern, discuss the relevant observations, and recommend that they consult a certified arborist for a professional evaluation. We can (and should) say if we have concerns that it may be a serious hazard or pose an imminent danger, and strongly recommend that they have a certified arborist assess it as soon as possible.

In summary, our role as VCE MG Tree Stewards in tree risk assessment is to observe, educate, and provide a frame of reference as to whether the tree is probably not a concern, may be a concern, or is an urgent concern that should be professionally evaluated immediately.

Consulting an Arborist

When advising a homeowner to consult a certified arborist, they will often ask for a recommendation. VCE is not allowed to provide recommendations or give out names of any businesses or individuals. However, we can give the homeowner some guidance on how to find one by providing them with the following information.

VCE Publication ANR-131, “Hiring an Arborist to Care for Your Landscape Trees”, contains helpful information when searching for a qualified (certified) arborist. It describes what an arborist does, when to call one, how to evaluate them and more. It also refers the reader to the following two organizations’ websites that can be used to search for a certified arborist:

The International Society of Arboriculture (ISA) is the organization responsible for verifying that an arborist has met certain criteria to then be designated as “Certified”. As the name implies, this is an internationally-recognized designation and it is the industry standard. There are several specializations as well as a Master Board-Certified Arborist, but what is needed is the Certified Arborist designation. The query asks for the country (USA), and then the best results seem to be found by searching within 100 miles of a zip code (instead of a city name). It will give the names of individual certified arborists and what company they work for (which will likely also include arborists employed by noncommercial entities such as a city, utility, government agency, etc). 

The Tree Care Industry Association (TCIA) is a trade association which developed the ANSI A300 standards. It accredits tree service companies which have met its criteria for operation and have undergone an audit. Its query will return a list of tree service companies which are TCIA members or, if selected, only the member companies that have also been accredited by TCIA. Membership in or accreditation by TCIA is not an industry-wide standard or requirement, so there may be other good companies besides those listed. The best results seem to be found by searching with the first 2 or 3 numbers of a zip code, or for the entire state and then looking for nearby companies.

Any companies to be considered should be able to answer these questions in the affirmative:

  • Do they provide a current certificate of insurance?
  • Do they provide a written contract?
  • Do they have a certified arborist who will do the evaluation and estimate?
  • Do they follow the ANSI (American National Standards Institute) standards or BMPs (Best Management Practices)?

In other words, the Certified Arborist title shows the individual’s qualifications; following the ANSI standards and/or BMPs shows that the company uses approved methods in their work.

Review Questions

  1. What are the three factors that contribute to tree failure?
  2. What are the parts of a tree that need to be evaluated when looking for problems?
  3. For each of these parts, what are some important symptoms or problems that may be of concern?
  4. What are some general guidelines on the extent of root or trunk damage which may affect the tree’s stability or likelihood of failure?
  5. What are the factors at a given location that affect the health and structural stability of a tree?
  6. Describe how the effect of wind or ice/snow is different for a tree located in a group as opposed to an isolated location. Describe the difference for a tree located on the edge of a group as opposed to within the group.
  7. How does the tree protect itself against collapsing under a frequent or continual load? How might that load change and what might happen if it does?
  8. Describe how the soil at the location can affect the tree’s risk of damage.
  9. What kind of changes at a location could be damaging to the tree?
  10. What are the six ways that a tree can fail?
  11. What are some of the general guidelines on the extent of crown or trunk damage to consider in whether a tree is likely to survive extensive damage?
  12. What one element of supplemental care is the most important to a tree’s recovery from damage?
  13. When planning a new landscape or planting a tree, what are the most important things to consider to prevent future damage?
  14. When (under what circumstances) should a Tree Steward suggest a consultation with a certified Arborist?
  15. What are the limitations of a Tree Steward when making recommendations to a client about a tree? What kind of statements should we be cautious about or avoid making?
  16. Questions to consider as you move into working as a Tree Steward: Think about the trees in your area. What different types of locations are they growing in? What trees do well or poorly in your area? What are the most common problems with them? What are the most common site and weather stresses for them?

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