What is tree response? Essentially, it’s the way that trees interact with their environment; the life of a tree is made up of one response after another, along with basic biological functions. Tree response is an action or reaction to a stimulus; it involves tissue growth in response to the stimulus or stimuli influencing it.

Influences of tree response

There are many forms of stimuli that affect trees and how they respond. Sunlight, moisture, essential soil elements, soil biology, gravity and even wind will help develop useful growth and structure. A deficiency of any of these stimuli will also elicit a tree response. In some areas there are seasonal changes that develop cold temperatures, limit sunlight and aren’t favorable or efficient for continual tree growth. In these areas, trees respond by going through a process of storing carbohydrates, followed by a period of limited activity. Then, as seasonal changes become favorable again, the tree resumes normal growth and activity.

Branch failure after the top of the branch has been wounded. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Response to loading

The normal loading of trees or tree parts by wind, gravity, and various forms of moisture are important stimuli for tree strength and structure.

Consistent exposure to moderate wind as a young tree grows encourages the tree to respond by developing a strong trunk with taper and a greater canopy to height ratio. Consistent exposure to sunlight also adds to this growth, helping encourage a greater canopy to height ratio. Trees protected from wind — and those that must also compete for sunlight in the middle of a stand of trees — respond by growing more slender, flexible trunks and a taller profile with a reduced canopy to height ratio. Being somewhat protected from the wind and receiving support from nearby trees, individual trees within the stand don’t need to have strong, well-tapered trunks. They act as individual branches within a group, doing their part to dampen the wind for the whole stand. The competition for sunlight also encourages more rapid elongating growth and less trunk taper growth — again acting as an individual part of the whole stand.

Tree branches loaded by moisture in the form of rain, snow or ice as a part of a normal seasonal cycle respond to this kind of loading by developing stronger branches with greater branch taper. Gravity provides consistent loading and has an important role in developing branch strength.

By observing, analyzing and understanding these response traits to loading, over time we can become more aware of tree conditions that are either well-adapted to normal loading or have a greater potential for failure. When failures do occur, there is a lot we can learn from an investigation and analysis of the failure and by continued observation of how the tree responds to the failure in the long term.

Response growth is strengthening and supporting this overstressed branch that has started to fail. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Causes of tree injury or wounding

Some types of stimuli inflict injury on trees. Weather conditions can cause wounding to trees. Strong winds or excessive moisture loading can result in tree and branch failures. Hailstorms can scar young, thin-barked branches with hundreds or even thousands of small wounds that can require years of recovery. Sudden, extreme temperature changes from warm to very cold in a matter of hours can damage living cells in the tree’s cambium or cause cracking and disruption in the xylem. Boring insects, animal activity and pathogens can all inflict damage on trees.

Human activity also causes a great deal of wounding to trees. The careless use of grass-cutting tools, for instance, as well as the effects of things wrapped around, attached to and bored into trees are some common examples. Pruning of trees also causes wounding. The type of pruning, the use of proper tools and techniques, as well as the size and placement of cuts influences the magnitude of injury. Pruning should be looked at in terms of its necessity and benefits to a tree with a good reason supporting the decision to make each pruning cut, because every wound is an injury that the tree must then expend energy to respond to.

Response to wounding

When tree tissues are wounded, the wound is permanent. The tissues don’t get better or heal. The wound becomes a lasting record of the event that caused the wounding. Years later, a person can observe the wound and often determine its cause and see how the tree responded. Exposed wood tissues almost always decay over time; the larger the wound, the greater the extent of decay will be.

Trees respond to wounding by moving chemicals and carbohydrates to the site of the wound to form barriers that slow down or stop decay within xylem tissues. The tree also initiates new specialized cell growth to begin closing over the wound. This new cell growth forms a very effective barrier to prevent decay from infecting consecutive years of growth following the wound. The tree attempts to encapsulate the wound and any decay as rapidly as possible. Decaying fungi require oxygen to function; if the tree successfully encapsulates and closes off or severely limits the oxygen supply to the decaying fungi, the decay will slow or even stop.

Opening and lower trunk vertical response growth columns connected to exceptionally large buttress roots for stability. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Response to major defects

All tree wounds can be regarded as defects. However, some defects can develop in areas of critical structural importance, which may lead to an increased likelihood of failure over time. Wounds can provide an opportunity for an infection to enter the tree and cause decay. Sometimes, large pockets of decay, or hollows, can develop, especially in rapidly growing trees with soft wood. A major defect, such as an area of advanced decay that weakens a tree’s structure, can be readily apparent or it can be subtler and difficult to observe. When there is an area of weakness in a tree’s structure, there will also be response growth, which not only forms barriers and closes wounds, but also shores up the weakened area, strengthening the tree’s structure. Here, the term “response growth” is used to indicate adaptive growth of wood produced by the tree that has stronger structural properties, thus strengthening a weakened area of the tree.

A tree with an open hollow will often have response growth that enlarges and strengthens the wood around the opening. We might also observe vertical, raised areas of growth on the tree trunk with sunken areas in between. These raised areas in the lower trunk of a tree may have a muscular appearance and sometimes form in conjunction with the development of buttress roots. Sometimes, a trunk area or large branch area will increase in size briefly and then reduce again, giving it a swollen look. These swollen or raised areas are often evidence of the tree’s attempt to strengthen a hollow within the tree. If no openings are visible to identify large pockets of decay or hollows within a tree, the outward signs on the trunk may alert us to what lies within.

Another defect commonly observed is a codominant stem. The union of codominant stems is structurally weaker than other branch unions. Often, codominant stem unions form with a seam exhibiting included bark. Response growth will sometimes cause the area of included bark to flare out to strengthen each side of each stem where they come together.

Tree defects that could result in a major failure of a whole tree, or large parts of the tree, in an area where the consequences could be severe, deserve to be examined and analyzed very carefully and an appropriate tree risk assessment should be performed.

This overstressed branch has sheared at the interface between tension wood and compression wood. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Response to branch loading

Branches of deciduous trees are formed with tension wood, made up primarily of tightly-packed cellulose strands, on the top of the branch. These act like ropes that flex and have some stretch, but also possess strong holding power. The bottom of the branch is formed with compression wood that contains a higher content of lignin that adds resistance strength when branches are bending downward.

In general, branches carry a downward load due to the loading pressure of gravity. As branches increase in weight with leaf development or moisture, the additional loading can cause the tree to respond by strengthening the branch (developing response growth and adding taper). Poor pruning techniques that remove small branches from larger branches, with leaves primarily left at the end of a branch, interfere with and lessen the tree’s ability to strengthen that branch and increase taper. This is why trees that have had the small inside branches removed generally elongate branches at a much faster rate than they produce taper. The result is longer, more slender, and weaker branches that break under wind and moisture loading. Tree branches are relatively weak when they receive upward loading because they aren’t designed for this type of loading. Often in wind events, branches are broken because of upward loading by the wind.

Trees that have been excessively thinned through pruning have an increased likelihood of upward loading by wind, and they have decreased dampening properties that can help resist breakage. The tension wood on top of the branch meets up with the compression wood on the bottom of the branch to form a shear plane. This interface is where branches tend to split or separate under stress. There are many examples that can be observed of tree branches that have been overstressed and started to fail and the tree has responded by adding wood to strengthen the weakened area.

Proportionally, tension wood is the most important factor in a deciduous tree’s branch strength. If the top of a branch is wounded by squirrel activity, a climber’s footwear or sunscald, the tree has a hard time responding with new wood in this area. The wound usually decays, and the branch’s strength is compromised. Branch flexibility is lessened, and the loading capacity before failure is greatly reduced. It’s useful to recognize this and take it into account when inspecting and assessing a tree’s canopy for defects and weaknesses.

Trees in the middle of a stand grow taller, more slender and flexible with less trunk tapper and less canopy to trunk ratio. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Learning from observation

As previously mentioned, every wound becomes a permanent record held in the tree. Some wounds are buried in the heartwood of a tree, but many wounds remain visible. There is much we can learn from careful examination. Even closed wounds often yield information.

Pruning is the practice of removing branches (or occasionally roots) from a tree or other plant, using approved practices, to achieve a specified objective. To truly understand pruning and tree response to pruning, it’s important to take every opportunity to observe past pruning practices and analyze whether the tree has adequately responded to this type of wounding. It’s also educational and beneficial to observe the overall response of a tree to various pruning practices. By repeating this routine of observation with hundreds of trees over time, arborists can discern patterns of success or failure of trees in response to certain types and sizes of wounds and overall pruning practices. The same holds true for patterns of branch failure in trees — the history of trees and the clues they provide can help arborists determine how and why trees fail.

Trees can provide arborists with extremely valuable information that can be used as a learning tool to help conduct optimal tree care. Arborists who climb trees have an especially great opportunity to practice making observations every time they climb a tree. This wealth of observational time will result in learning about how trees respond to the stimuli that affect them. An understanding of tree biology and physiology are critical when trying to understand what you are observing.

Trees that grow in the open, exposed to wind and sun, tend to be shorter with more trunk tapper and canopies that are well proportioned. PHOTO: INTERNATIONAL SOCIETY OF ARBORICULTURE

Tree response an indicator of tree health

Observations of tree responses can provide us with information regarding a tree’s health and energy production. Healthy trees photosynthesize and produce sugar for the tree’s energy expenditures. Trees that are struggling to photosynthesize, for whatever reason, have less energy reserves and are less healthy. Thus, tree responses can often be viewed as indicators of tree health.

When working with mature trees, an assessment of a tree’s level of health is important. Anything we do or recommend be done to a mature tree should only be initiated if it makes sense from the standpoint of the tree’s health. Trees struggling with health issues tend to respond more slowly and less vigorously, or they may respond with dieback. Tree health, as well as tree response, is tied to and driven by sugar production and stored carbohydrates. The annual growth of trees in girth and in length can be referred to as growth increments. Tree growth, especially the elongation of branch twigs, can be a good indication of tree health. If, over several years, the annual growth of twigs is getting smaller throughout the tree, this may be an indicator that a tree’s health is in decline. An increasing volume of twig and branch dieback throughout the tree may be an indication of ongoing health problems. Additionally, if we observe more rapid growth than is normal for a healthy tree, it’s important to discover what may be pushing this growth, as it may increase loading the tree is unable to support. An overuse of nitrogen fertilizer can result in this condition.

When pruning wounds or other wounds are slow to close, or are showing no signs of closing, this may be due to limited carbohydrate reserves and poor health. Increased sprout production in the middle to lower parts of the canopy, combined with upper canopy dieback, is a fairly common occurrence of older trees responding with limited resources — especially limited water or oxygen exchange in the root zone. This response is referred to as retrenchment, and it results in the tree reducing its live mass to better fit the resources available. It’s a strategy undertaken by the tree to reconsolidate energy and live growth in order to preserve itself.

Making use of tree response

As we become more attuned to tree response and more adept at assessing the implications of our observations, we’ll be better equipped to assess what defects a tree is harboring and better equipped to properly diagnose tree conditions and assess tree health. Recognizing and understanding tree response is an important step in achieving our goals of preserving trees, extending the life of trees, and helping trees reach their full potential in urban environments.