to Pruning. An annotated version of a. PowerPoint presentation prepared upon request for classroom teaching material about how trees respond to pruning. This.
Tree Response to Pruning An annotated version of a PowerPoint presentation prepared upon request for classroom teaching material about how trees respond to pruning. This presentation can be used for workshops on tree care.
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How do trees respond to pruning?
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This slide illustrates a generic northern hardwood tree. The aboveground parts consist of green leaves where food is made by photosynthesis, branches which connect the foliage to the stem, and the main stem where wood is produced using food from the leaves and water and essential mineral elements from the roots belowground. As trees age and grow, the branches are lost as they die or are broken leaving a stub in which the wood is exposed to colonization by wooddestroying pathogens. The protective bark of the main stem can also be lost to produce a scar-type wound leaving wood exposed to colonization. Tree survival and wood quality depend on a protective system that can re-establish the protection lost when stubs and scars expose wood. We need to understand how trees work so we can maintain tree health and productivity over decades of time.
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This slide illustrates how to prune branches from trees to best preserve the protective system that limits the internal spread of wood-destroying pathogens. To understand the rationale for cutting along the A to B lines for dead branches and stubs (left), for live branches (right) and to avoid cutting behind the branch bark ridge and into the branch collar (as occurs in the older concept of flush cutting close to the main stem), let’s look at the internal anatomy and physiology of branches. Although hardwoods are used for illustrating the concept, the same principles hold true for conifers. 4
This is a well formed branch on a healthy young hardwood tree. The arrow indicates the branch bark ridge (BBR) where branchwood and stemwood intersect.
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At the beginning of the growing season, the branch grows first followed by the stem. The blue arrows indicate stemwood, the red arrow branchwood. When making a pruning cut, DO NOT CUT INTO STEMWOOD above and below the branch bark ridge. ONLY cut into branchwood to minimize the discoloration and decay of exposed wood.
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The result is an interlocking grain which makes the connection between branch and stem very strong to resist breakage.
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In some trees the BBR is vertical (VBBR) which indicates that the tree has codominant stems not a branch and a stem. This situation favors the inclusion of bark so that wood does not interlock and the tree is easily split open under stress as in an ice storm. Young trees with VBBRs should be removed from the stand when possible or one of the two stems cut at an early stage of development. In this case cutting into stemwood cannot be avoided, but make the cut as small as possible.
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If trees with VBBRs mature with included bark, they are likely to break causing a hazard to people and property. Watch the branch angle when deciding to prune. The steeper the angle, the more likely bark will be included at the stemwood/branchwood interface and cause a problem.
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If we infuse dye into branch wood, its path is mostly down the outer stem below the branch and not upward across the branch bark ridge. Water moves out and air moves in along the path indicated by the dye. Live cells in the drying and aerated wood begin to die and that partially dead wood becomes colonized by fungi and bacteria. However, if the branch is small, the live cells in wood can form a protective barrier as we shall see in the next slide.
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When young branches die or break outside of the BBR, a protective layer (see arrow) forms in exposed live sapwood of the branch. Live cells produce plugging and preserving substances to restore the protective layer formerly provided by bark so that the activity of wood-destroying fungi is limited to dead wood outside the living tree. Follow the line of bark through the protective layer formed in branch wood and back to bark again to see how the tree is protected until the exposed wood is overgrown and the bark layer is restored. 11
This type of protective layer forms when young branches are shed. Note the branch is ready to fall away leaving the BBR and branch collar intact. When pruning branches, care should be taken to preserve the natural protective system of tree as they grow to maturity.
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When the broken or dead branch is larger with less vital cells in the center, the protective layer is not complete and wood-destroying fungi can spread into the center of the tree. The tree responds over time to limit the outward spread into sound healthy wood by forming protective layers in stemwood (see arrow). This protective system is called, Compartmentalization of Decay in Trees, or “CODIT”.
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However, stubs of dead branches may become infected by a more aggressive type of wood-decay fungus which attacks both live sapwood and inner bark. These fungi are called canker-rot fungi and limit the effectiveness of the CODIT system to wall-off the infection by killing bark and enlarging the wound. The tree continues to limit the spread of infection, but more damage occurs.
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In some trees the damage can be quite extensive.
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In white pine we see a canker-rot fungus that has infected a branch.
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On the inside we see the initial stem infection well advanced in the center of the stem and a secondary infection is developing in live sapwood as bark is killed by the fungus.
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The result is multiple columns of infected wood which is called “ring rot”. This could be avoided if branches were shed or pruned at an earlier stage.
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Timely removal of dead stubs and living branches along the A to B line (WITHOUT cutting into stemwood as depicted by the A to X line) promotes formation of protective layers in branchwood that exclude harmful wood pathogens or confines damage to older wood in the core that is of little commercial value and unnecessary for tree health. Proper A to B cuts also reduce the possibility of canker-rots.
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If cuts go beyond the branch bark ridge and into the branch collar, colonization and infection is not limited to branchwood or the central core, but now wood of the main stem is exposed to wood-destroying pathogens. Once the bark of the main stem is removed, the wound becomes a scar type wound and no longer a stub type wound. The protective layer of the CODIT system now forms in stemwood, not branchwood, and the potential for the development of internal columns of discolored and decayed wood increases. 20
Scar type wounds have a highly variable response which we know is due to highly variable genetic resistance to infection among trees. Two logging scars of similar size made at the same time show very different responses. Some trees pruned flush may have sufficient resistance to limit damage as the tree on the right, but many lacking resistance will have greater damage. A proper pruning cut which does not unnecessarily expose stemwood by removing bark inside the BBR and branch collar reduces the potential for damage in either case. 21
In trees with steep branch angles, bark can be included with wood as BBR forms, making a weaker connection which can cause a split fork (red arrow) during an ice storm. This type of wound arising from a weak branch stem connection produces a deep scar exposing wood to infection leading to decay. This is unlike a broken branch which only exposes branchwood outside the stem (blue arrow). Proper pruning of the stub will reduce potential damage, but nothing can be done to help the split fork. Split forks can be mostly avoided, if trees with steeply angled BBRs are removed from young stands or pruned when branches are small. 22
Even after only three years post injury, here we see a lot of internal defect resulting from a split fork.
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Somewhat less damage occurs from a broken branch stub.
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When wood in the stem is exposed by a broken branch stub, the protective CODIT system will keep the wood-destroying infections contained within the central, less valuable part of the tree over the coming decades. After 40 years the clear sound wood is 75% of the volume, assuming the tree has not suffered repeated wounds, especially basal scars. Proper pruning would further enhance the health and value of the tree.
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If we infuse dye into the stem above a branch, its path is mostly downward into the central portion of the tree and does not move upward across wood associated with the branch bark ridge (BBR). As in branchwood, the path of the dye is the path of colonization leading to infection resulting in a central column of discolored and decaying wood as seen in the next slide.
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The broken top produced a stem stub, but the intact BBR prevent the incoming infection from spreading into the branch that became the new leader. As in the case of branch stubs, the anatomy and physiology of the tree provides a highly effective protective system which limits the spread of wood-destroying infections within the genetic limits inherited by the tree and the state of its environment.
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Decades have been spent studying wound response in many trees to learn how the tree protects itself from repeated injuries over a long life span. Recommendations about tree care have been based on this knowledge and not just empirical studies, although many of these have also been done. We see in this sugar maple 5 years after receiving a boring wound and inoculation with a wood decay pathogen, the live sapwood has been able to wall-off the resulting infection to protect the bulk of the stem. 28
We learned the importance of wood rays of live sapwood connecting to live inner bark (blue line) to bring additional nutrients to biosynthesize wood plugging and preserving substances. These substances comprise the dark, well-developed protective zone to limit the outward spread of pathogens toward the vascular cambial zone where live wood and bark are formed. The dead rays of the discolored wood (red line) cannot bring additional nutrients to enhance the inner column boundary layer so live cells have only stored food reserves, making for a less effective protective layer. 29
Columns of discolored and decaying wood that develop from a small bore-hole or scar type wound are much smaller than a combination of the two. Removing bark around the wound as in the case of canker-rots and flush cuts marked reduce the effectiveness of the CODIT protective system.
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When bore-holes close quickly (B), compartmentalization is highly effective (D). However, when the cambium dies back around the wound (A), compartmentalization is less effective (C) as in the case of canker-rots and flush cuts and experimental wounding (slide 30).
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There are always differences in wound response among trees. They may be highly resistant as the tree on the left, in which columns are discrete and do not coalesce, or highly susceptible as the tree on the right, in which columns coalesce into a large central column. In both cases infections do not spread into new wood formed after injury, so the trees survive and remain alive. However, the tree on the right will have low value and carry high risk because it is less likely to survive further injury. 32
Proper pruning along the A/B line gives the best possible outcome for an effective response given what we know about tree biology.
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Natural shedding of branches along with pruning based on our current understanding of tree biology should yield more trees like those on the right.
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The more you learn about tree response to wounding and infection, the more likely you are to make good decisions when deciding to prune trees.
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