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Chapter 4 Tree biology.


1) Show how root, branch, and trunk biology should influence pruning strategies.

2) Determine where energy reserves are stored in trees.

3) Describe the tree maintenance practices that reduce energy storage capacity.

4) Describe compartmentalization of decay in trees.

5) Differentiate between good compartmentalizers of decay and poor compartmentalizers.

6) Show how certain pruning cuts can injure trees permanently.

7) Learn how to maximize energy reserves in trees.

8) Develop pruning strategies based on biology to maximize health.



Best management practices



Closure crack




Good compartmentalizer

Growth ring



Poor compartmentalizer

Radial crack


Reaction zone

Ring crack

Root defects







Walls 1, 2, 3, and 4




A basic understanding of tree and shrub biology can make it much easier to grasp the idea of how pruning can impact plant health. The relationships among the important parts of a plant are presented in this chapter. These basics are not taught in many biology or landscape courses because there are few places to find this information. For greater detail on woody plant biology, see Shigo (1991) or Lonsdale (1999).


Roots are usually the forgotten part of the tree. They typically comprise from one-third to one-fifth of the total dry weight of the tree. Their tips extend to about three times the edge of the canopy (Figure 4-1). Roots deflected by structures such as rocks, foundations, streets, and sidewalks can reduce the uniformity of the root system and decrease tree stability. Nursery containers can also deflect and deform root systems (Figure 4-2). Deformed root systems can result in unstable trees that can fall over. Roots damaged or severed during construction or utility installation can also result in tree instability due to loss of upright support, decay, and internal defects such as cracks.

Pruning a tree with root defects might help compensate for a deformed or injured root system. For example, a tree left in a nursery container too long might not develop roots on one entire side of the tree after it is planted in the landscape. This occurs because few lateral roots form on the outside portion of a curved root segment (Figure 4-2). Trees with one-sided root systems can fall over. Thinning the canopy of a tree with a deformed root system to reduce the force of the wind against it could allow the tree to remain standing longer. However, pruning the canopy of a tree with this root defect should never be considered a long-term solution to root deformities.


* Tip: Damaged or deformed roots can have a large impact on tree stability.

Roots perform functions other than stability. Some growth regulators are produced in the roots. Roots take up water and elements. They also form associations with organisms, such as mycorrhizae, that can help the tree in many ways. Roots fend off diseases and insect attack, and they store some of the energy produced in the leaves.


Water entering roots moves to the canopy through the xylem (Figure 4-3). Xylem makes up the woody part of the tree that begins on the inside of the cambium. It extends through to the pith or the trunk center. Xylem is made up of living and nonliving cells in roots, trunk, branches, and foliage. It can be thought of as a pipeline from roots to leaves. In dicot plants, a new layer of xylem is produced by the secondary growth system, the cambium, each year (or several times each year in some tropical trees) forming a new growth ring in trunks and branches. Xylem in foliage is produced by primary growing points in buds.

As water evaporates from the foliage (this process is called transpiration) through openings in leaves called stomata, it pulls adjoining water molecules with it. This pulling action helps draw water up the trunk and into the leaves. In addition, some trees may be capable of exerting a pumping action to push water up the tree. Most water moves up through the apoplast or the network of open, dead conducting elements in xylem. Elements (also called nutrients) such as magnesium and potassium move from the soil up to the foliage in the xylem cell sap.



Photosynthesis in foliage, twigs, and other green plant parts produces sugars (and other components) that are used by the tree to carry out its many functions. Sugars are moved about the plant in a layer of cells called the phloem. Phloem is made up of living cells located just outside the cambium. The cambium produces the phloem in trunks, branches, and roots. The tree usually expends energy moving sugars, growth regulators, proteins, and essential elements up and down the phloem to other locations in the plant.

Once sugar arrives at a location, it is used to carry out normal processes or it is stored. It is stored as starch in the network of living cell contents in the xylem called the symplast. Starch is a chain of sugars linked together. Starch is considered the money, or the energy, in the tree bank. The bank is the living xylem or wood in branches, stems, trunk, and roots. If there is less stored starch, there is less stored energy in the bank. Trees need stored starch to carry on normal functions, especially to break dormancy.


Rays are long groups or plates of living cells that extend from the phloem into the xylem toward the center of the trunk. On their way to the xylem, they cross the cambium. They can extend several inches or more up and down the trunks, branches, and roots. Sugars move from the phloem into the energy bank (xylem) through the rays. On healthy trees, rays are rich in starch.

Improper pruning such as topping (Figure 4-4), cutting roots, flush cutting, removing large branches, and injuring the trunk and main branches can make the energy bank considerably smaller. If the bank is made smaller, less energy can be stored. This makes the tree more susceptible to attack by agents such as insects and disease. Improper pruning can also make it tough for trees to recover from floods, droughts, or other stresses.



* Tip: Improper pruning techniques and mismanagement can reduce the available storage space for energy reserves.


One of the objectives of pruning is to develop and maintain strong architectural structure. In order to meet this objective, appropriate live stems and branches should be shortened or removed regularly. This should be done in a manner that does not measurably reduce energy storage capacity (the bank) of the tree. The bank becomes smaller because improper pruning reduces the volume of healthy wood that can store starch. This can lead to a tree starving itself if energy needs outpace reserves.

Injuries result from inappropriate pruning cuts, storms, ice, snow, animals, wind, excess weight, temperature extremes, trunk wounds, disease, chemicals, and other stresses. A tree reacts to injury by creating boundaries around it. The boundary-setting process that resists loss of normal wood function and resists the spread of discoloration and decay has been referred to as compartmentalization. Discoloration is the orderly response of the tree to microorganisms resulting in darkened wood but no strength loss. Decay is the orderly breakdown of tissue resulting in strength loss. The rate of the discoloration and decay process depends on the severity of the wound, position in the tree, size of the wound, time of year, species, tree age, and the types of infecting microorganisms.

Compartmentalization may be described using the CODIT (Compartmentalization Of Decay In Trees) model (Shigo, 1991). Four different boundaries (also called walls) in trees have been presented in CODIT: wall 1, wall 2, wall 3, and wall 4. Each forms in a different manner and protects the tree in unique ways (Figure 4-3). The walls are numbered in increasing order of their ability to retard movement of decay organisms. For example, wall 3 is stronger than wall 1. Wall 1 may or may not be present at the time of wounding. Walls 2 and 3 are present in the tree at all times. Wall 4 forms in response to injury.

Wall 1: Xylem vessels immediately above and below an injury plug with crystal-like tyloses and chemicals when a tree is injured. This plugging response forms wall 1. Some plugging may occur normally without injury. Plugging forms a weak boundary in some trees such as hackberry (Celtis) and poplars (Populus), but is stronger in others such as many of the oaks (Quercus) (Table 4-1). Because wall 1 is weak, decay in some trees can advance rapidly up and down the trunk from the injury resulting in long columns of decay and hollow branches and trunks.

Wall 2: The growth rings make up wall 2. The transition from one growth ring to the next retards advancing decay organisms. Decay organisms often have a tougher time moving across growth rings (wall 2) than they do up and down the stem (wall 1). The functioning of wall 2 can be demonstrated when you view a cross section of a trunk or branch that was injured previously (Figure 4-3a). A darkened region often appears to stop its advancement inward toward the pith at the boundary of one growth ring with the next. This is wall 2 working.

Wall 3: The rays make up wall 3. They have plenty of decay fighting capability because they are rich in starch. The rays (wall 3) retard decay from spreading around the trunk. Discoloration and decay have a tougher time moving across wall 3 than walls 1 and 2. The strength of wall 3 can be demonstrated by viewing a cross section of a trunk or branch injured several years ago. Notice that there is a clear demarcation between darkened tissue and normal light colored wood (Figure 4-3a). If walls 2 and 3 fail and decay organisms break through, the affected trunk or branch can become hollow. Wall 4 forms the outside edge of the hollow.

Wall 4: This is the strongest boundary that retards spread of discoloration and decay in trees. This reaction zone forms from the cambium along the edge of the outermost growth ring present at the time the tree was injured. It begins at the point where the tree was injured and it may extend all or part way around the tree (Figure 4-3). Wall 4 stays in the same position in the tree but may extend further around or up and down the trunk with time. It does not move out with the new cambium. There may be numerous wall 4s in a tree, depending on its wounding history. They often appear as crescentshaped dark lines when viewed on a wood cross section. Wall 4 forms the edge of a hollow.

Wall 4 develops in response to many different types of injuries (Table 4-2). It can take several years for wall 4 to reach the other side of the trunk--or it may never reach that far. Wall 4 extends above and below the injury essentially in the shape of a pipe. It may develop a few inches or many feet above and below the injury.

Wall 4 prevents discoloration and decay organisms from moving into the wood produced after the injury occurred. This means it is extremely difficult for discoloration or decay to move from inside wall 4 to the outside of wall 4. Although this task appears simple, it is vital to the longevity of trees. Imagine if decay organisms could spread into wood formed after injury--trees could not live to become old majestic masters.

The obvious advantage of wall 4 is that it retards decay, but there are two very important disadvantages. The first disadvantage is that sugars cannot move across wall 4; that is, sugars have a more difficult time moving in or out of the portion of the bank (xylem) surrounded by wall 4. As a result, some stored starch can get trapped in the rays and xylem located inside of wall 4 (Figure 4-3c). However, the starch is available to decay organisms. The tree may have wasted the effort required to produce the sugar and store it as starch. It earned the money (made the sugars), deposited the money in the bank (stored it as starch), then could not withdraw some of the money (starch was trapped inside). You can imagine how much stress this causes the tree by imagining your stress after you earned money, deposited it in a bank, and could not get some of it back. Creation of wall 4 also makes the energy bank smaller so less starch can be stored in the future. This occurs because wall 4 essentially shuts off new deposits of sugars into the walled off portion of the xylem.

The second disadvantage is that a crack can form along wall 4. This separation or delamination is called a ring crack and it may follow wall 4 all or part way around the trunk (Figure 4-3c). One or more secondary cracks, called radial cracks, can form from the ring crack along a ray. Another serious crack is the closure crack that occurs as the callus and woundwood attempt to grow over and close the wound. This crack often extends from the point of injury out to the current location of the bark. Sometimes this crack never closes. Even if it closes, the crack remains along with its associated weakness (Lonsdale, 1999). Cracks in trees cause weakness that can make them susceptible to breaking. In fact, cracks are probably of more concern than the decay that results from injury (Shigo, 1991).

Trees vary in their ability to form walls 1, 2, and 3 (Table 4-1). These walls are weak in trees that are poor compartmentalizers of decay. Hollow trunks result from weak walls. These three walls are stronger in trees that are good compartmentalizers. Following injury to a poor compartmentalizer, wall 4 may reach the opposite side of the trunk quickly, within a few years. Because walls 2 and 3 are stronger in a good compartmentalizer, wall 4 may only need to be produced part way around the trunk.


It is vital to care for trees in a manner that minimizes injury. Injured trees produce reaction zones, called wall 4s, to protect themselves, but this can cause serious problems for trees. The tree care management practices discussed next take into account the tree's internal response to treatment. As with medicine, just about every practice or treatment is a balance between advantages and disadvantages. The best management practice is the best available treatment, considering the benefits and drawbacks, based on current knowledge in the field.

Trees should not be topped or rounded over because this exposes wood to infection and initiates wall 4 and cracks in the wood. Wall 4 quickly forms completely around the cut stem following a topping cut. It can extend several inches or 20 feet or more below the cut depending on species of tree and cause root decay (Figure 4-5). This can trap stored starch inside the tree making it less available to the sprouts that result. Some of the energy needed for sprout growth may come from other portions of the tree resulting in even lowered reserves. One recent survey found that trees topped in the landscape break and fail more often than other trees (Karlovich et al. 2000). Some of this was probably due to cracks formed along wall 4s causing fractures and weakness in the wood.

On mature trees, perform canopy reduction (covered in Chapter 13) only after every other option has been considered because reduction cuts can initiate wall 4 and cracks. On young and medium-aged trees, reduction cuts can and should be used to reduce the length of codominant stems. This improves the structure and the potential life span of the tree. Reduction cuts on these younger trees are considered a best management practice because removing a codominant stem or large branch entirely (if it does not have a protection zone) could cause more problems than reducing its length.

If clearance is required under the canopy, raise the canopy slowly over a period of years, not all at once. Removing too many low branches at one time can initiate formation of wall 4s, decay, and cracks in the trunk. Gradual removal will allow the tree to adequately adjust to less live foliage. Do not remove too much live foliage all at once, especially on mature trees, because this can cause root problems, sprouts, wall 4 formation, stress, decline, or even death.

Energy reserves can be kept high in trees by removing the least amount of live branches necessary to accomplish the desired goal. Removing too many live branches can cause stress in trees by reducing sugar production and reducing storage capacity by initiating wall 4s. Many trees sprout in response to overpruning because they are attempting to replace the stored energy removed with pruning. However, live branch pruning is an essential ingredient to forming good structure, so it is necessary procedure in an urban tree care program.

* Tip: Keep energy reserves high by removing the minimum amount of live branches to accomplish your objectives.


High energy reserves in nursery trees mean rapid growth in the nursery. To keep energy levels high, live branches on the lower portion of the trunk are left on the tree but are shortened during the first few years of production. This keeps them small but allows them to produce sugars to feed the trunk and roots. The result is rapid increase in root growth and trunk diameter. However, if live low branches become too vigorous, a large wound results when they are removed prior to sale. This could cause cracks in the trunk that will have little effect in the nursery but could be the source of trunk cracks after the tree is planted in the landscape.


1) Atree capable of forming strong boundaries that resist decay can be referred to as a:

a. poor compartmentalizer.

b. good compartmentalizer.

c. healthy tree.

d. strong-structured tree.

2) What CODIT wall is responsible for preventing decay from spreading into new wood produced after injury?

a. wall 1

b. wall 2

c. wall 3

d. wall 4

3) What wall slows the spread of decay in toward the pith?

a. wall 1

b. wall 2

c. wall 3

d. wall 4

4) Which is the strongest wall?

a. wall 1

b. wall 2

c. wall 3

d. wall 4

5) What is the appropriate order of tissues in dicots moving from the bark to the pith?

a. cambium, phloem, xylem, cork cambium

b. xylem, cambium, phloem, cork cambium

c. cork cambium, phloem, cambium, xylem

d. phloem, cork cambium, cambium, xylem

6) A best management practice is:

a. the best treatment you can think of now.

b. a best technique used by business to manage people.

c. the practice or treatment you have used for the past ten years.

d. the best available treatment based on current knowledge.

7) What forms along wall 4 that can cause trees to fall apart or break?

a. a ring crack

b. a radial crack

c. a closure crack

d. included bark

Answers: b, d, b, d, c, d, a


1) Describe how starch can get trapped in the xylem and is make unavailable to the tree.

2) Describe to a colleague how hollow trunks form in trees.

3) From the tree's perspective, compare the advantages and disadvantages of wall 4.

4) Canopy reduction is performed to reduce tree size. What is a serious downside of this technique to consider when evaluating treatment alternatives?


1) Find a fresh woodpile with clean end cuts or make some clean cuts with a saw. Locate samples illustrating operation of walls 2, 3, and 4.

2) In a woodpile, find a wall 4 in cross section on a branch or trunk. Determine how many years ago the injury took place that caused formation of that wall 4. Can you find any other wall 4s on the same sample? Sometimes, there are many on the same sample.

3) In cross section, see if you can find a radial crack originating from a ring crack. Cut the sample into several cross sections to see how far the crack extends up or down the sample.
TABLE 4-1. Trees vary in ability to compartmentalize decay.

                   Poor compartmentalizers

Aesculus                            Myrica
Betula                              Peltophorum
Brachychiton                        Persea
Celtis                              Pinus virginiana
Delonix                             Populus
Eucalyptus (some)                   Prunus
Erythrina                           Quercus laurifolia, nigra,
                                      leavis, shumardii, palustris
Fagus                               Salix
Ficus benjamina                     Schinus

                   Good Compartmentalizers

Acer x freemanii, rubrum,           Robinia pseudoacacia
Albezia saman                       Taxus baccata
Bucida buserus                      Sweitenia
Bursera simaruba                    Tabebuia
Castanea sativa                     Ulmus americana
Pinus rigida
Quercus geminata, macrocarpa,
Quercus robur, rubra, virginiana

TABLE 4-2. Causes of wall 4 formation in trees.

Canopy reduction        Mechanical injury to roots, trunk, or branches
Dieback                 Removing large diameter branches
Drought                 Removing codominant stems
Flood                   Removing collars
Flush cuts              Storm damage
Improper pruning cuts   Topping or rounding over a tree
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Article Details
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Author:Gilman, Edward F.
Publication:An Illustrated Guide to Pruning, 2nd ed.
Geographic Code:1USA
Date:Jan 1, 2002
Previous Article:Chapter 3 Tree structure.
Next Article:Chapter 5 Pruning cuts.

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