Both Biology and Forestry Practices can make Landscape Pruning Clearer
Authored by Luke Lauby
Introduction
Pruning is the selective removal of a trees branches, buds or roots. It is commonly done in order to improve the shape, structure, and condition of any tree. Dr. Alex Shigo, often considered the father of modern arboriculture, said “many of the tree care practices that were developed centuries ago have changed little over the years. It has been a “take” relationship without any due consideration for the trees’ health.” He went on to say “practices are deep-rooted in attitudes” (10). One of these attitudes may be why pruning urban trees has been neglected. Possibly thought of as an option rather than a necessity, many tree owners do not prune. Homeowners may believe “Forested trees grow perfectly fine without regular pruning, so why should pruning be done on landscape trees.” The average person may not realize the benefits pruning can have on trees because we rarely get to see pruning being done in forests. Some forested trees are actually pruned to increase wood quality and tree form, while others naturally drop their branches (7). Natural pruning occurs on a regular basis. Death or slow shedding of branches occurs in forested trees due to competition for sunlight. Figure 1 shows a forested area where good natural pruning has occurred in the Longleaf Pine. It’s difficult to say how the neglect for landscape pruning began in our landscape tree culture, but the best solution is to have tree care professionals as well as tree owners understand the proper tree biology and how it applies to correct techniques when pruning. As we dive into this topic and comparison, it will be helpful to keep this in mind, pruning (done the right way) is an excellent and necessary skill to have. This web-page will answer the following questions:
Pruning is the selective removal of a trees branches, buds or roots. It is commonly done in order to improve the shape, structure, and condition of any tree. Dr. Alex Shigo, often considered the father of modern arboriculture, said “many of the tree care practices that were developed centuries ago have changed little over the years. It has been a “take” relationship without any due consideration for the trees’ health.” He went on to say “practices are deep-rooted in attitudes” (10). One of these attitudes may be why pruning urban trees has been neglected. Possibly thought of as an option rather than a necessity, many tree owners do not prune. Homeowners may believe “Forested trees grow perfectly fine without regular pruning, so why should pruning be done on landscape trees.” The average person may not realize the benefits pruning can have on trees because we rarely get to see pruning being done in forests. Some forested trees are actually pruned to increase wood quality and tree form, while others naturally drop their branches (7). Natural pruning occurs on a regular basis. Death or slow shedding of branches occurs in forested trees due to competition for sunlight. Figure 1 shows a forested area where good natural pruning has occurred in the Longleaf Pine. It’s difficult to say how the neglect for landscape pruning began in our landscape tree culture, but the best solution is to have tree care professionals as well as tree owners understand the proper tree biology and how it applies to correct techniques when pruning. As we dive into this topic and comparison, it will be helpful to keep this in mind, pruning (done the right way) is an excellent and necessary skill to have. This web-page will answer the following questions:
- How and why are trees pruned in forested areas?
- What is the difference between natural pruning and human pruning?
Figure 1: Longleaf Pine forest where the lower branches have undergone natural pruning (7)
The Science Behind Pruning
People need trees, they provide oxygen, filter air pollutants, and provide food and habitat for wildlife. To have urban trees in their best condition, they need to be cared for (12) and pruning is an important aspect of tree care and maintenance. It is important to understand the fundamentals of how trees respond to pruning. The purpose of pruning changes depending on whether you are managing a forest for timber production or own a few trees in your yard. For example, an emphasis on timber production in a forest, pruning might repair branches after storm damage, help maintain a single trunk for a tree and improve lumber quality (9). Also, in an apple orchard, you may prune for increased air flow to prevent the spread of pathogens and parasites (15). However, an average tree owner may prune to remove dead/decaying branches, increase the condition/strength of the tree, or enhance beauty as well as improve the safety for the surrounding area.
Natural Pruning, rarely occurs in urban landscape trees and it is helpful to understand how trees respond to pruning as well how their biology can help us better prune for both structure and function.
People need trees, they provide oxygen, filter air pollutants, and provide food and habitat for wildlife. To have urban trees in their best condition, they need to be cared for (12) and pruning is an important aspect of tree care and maintenance. It is important to understand the fundamentals of how trees respond to pruning. The purpose of pruning changes depending on whether you are managing a forest for timber production or own a few trees in your yard. For example, an emphasis on timber production in a forest, pruning might repair branches after storm damage, help maintain a single trunk for a tree and improve lumber quality (9). Also, in an apple orchard, you may prune for increased air flow to prevent the spread of pathogens and parasites (15). However, an average tree owner may prune to remove dead/decaying branches, increase the condition/strength of the tree, or enhance beauty as well as improve the safety for the surrounding area.
Natural Pruning, rarely occurs in urban landscape trees and it is helpful to understand how trees respond to pruning as well how their biology can help us better prune for both structure and function.
Compartmentalization: Pruning and Wound Response
Pruning for structure should ideally take place early in a tree’s life (10). After a cut has taken place, compartmentalization and more visibly, wound wood development can begin (Figure 2). Compartmentalization is a process of “walling off” or “sealing” the exposed surface so no pathogens or harmful bacteria can infect the wound (6). Compartmentalization can be most efficient when the pruning is done outside the branch collar, a technique referred to as Natural Target Pruning (Figure 3). As a tree is pruned and wound wood is formed, growth may be temporarily slowed as more energy is spent on sealing the wound. Trees are pruned for different reasons and respond differently depending on their genetics as well as the stress they are undergoing. In an apple orchard, for example, you may prune for increased air flow to prevent the spread of pathogens and parasites (15). Understanding different biological concepts like these better prepare us to not just prune effectively but also preserve the health of any tree and its longevity.
Pruning for structure should ideally take place early in a tree’s life (10). After a cut has taken place, compartmentalization and more visibly, wound wood development can begin (Figure 2). Compartmentalization is a process of “walling off” or “sealing” the exposed surface so no pathogens or harmful bacteria can infect the wound (6). Compartmentalization can be most efficient when the pruning is done outside the branch collar, a technique referred to as Natural Target Pruning (Figure 3). As a tree is pruned and wound wood is formed, growth may be temporarily slowed as more energy is spent on sealing the wound. Trees are pruned for different reasons and respond differently depending on their genetics as well as the stress they are undergoing. In an apple orchard, for example, you may prune for increased air flow to prevent the spread of pathogens and parasites (15). Understanding different biological concepts like these better prepare us to not just prune effectively but also preserve the health of any tree and its longevity.
Figure 2: Wound wood formation on a recently pruned tree (11).
Figure 3: Pruning should be done just outside the branch collar (11).
Recommendations/Implications
Up until the early 1980s, the “flush cut” was considered the proper method of pruning. A flush cut is done inside the branch collar completely flat against the trunk (Figure 4). It is an incorrect pruning technique and can be detrimental to the health of the tree. Making flush cuts increases the chance of pathogens to infect the tree because the wound is larger and therefore requires more time for new tissue to grow over the wound. However, with a better understanding of tree biology a new approach can be implemented. The Hamburg Tree Pruning System provides the basics of right and wrong ways to prune any urban tree. When pruning an individual tree, you should cut just outside the branch collar (approximately 2 inches) in order to speed wound response as well as decrease die back. In fact, as seen in Figure 3, pruning outside the branch collar decreases the likelihood of disease and decay as well (4).
Forested trees often are very clustered and concentrated making sunlight a very competitive resource for under story branches. These branches are sometimes naturally shed when they are too shaded and can’t provide enough sunlight for the tree. After the branch is shed, an abscission layer is formed similar to when leaves detach in the fall. There has been a large dispute over what exactly the correct pruning technique in forested areas is and how it relates to urban pruning. In production forestry, where natural pruning does not always occur, professional foresters may choose to enhance the timber quality and decrease risk of disease by pruning. They often have an economic outlook when it comes to pruning, focused on improving wood condition (2). Infection is a constant problem in forests and urban landscapes, it must have your attention when pruning (14). Knowing now how foresters prune and how trees behave in forested environments, you should not believe that landscape pruning is unnecessary. Routine care should be done on your landscape trees!
Up until the early 1980s, the “flush cut” was considered the proper method of pruning. A flush cut is done inside the branch collar completely flat against the trunk (Figure 4). It is an incorrect pruning technique and can be detrimental to the health of the tree. Making flush cuts increases the chance of pathogens to infect the tree because the wound is larger and therefore requires more time for new tissue to grow over the wound. However, with a better understanding of tree biology a new approach can be implemented. The Hamburg Tree Pruning System provides the basics of right and wrong ways to prune any urban tree. When pruning an individual tree, you should cut just outside the branch collar (approximately 2 inches) in order to speed wound response as well as decrease die back. In fact, as seen in Figure 3, pruning outside the branch collar decreases the likelihood of disease and decay as well (4).
Forested trees often are very clustered and concentrated making sunlight a very competitive resource for under story branches. These branches are sometimes naturally shed when they are too shaded and can’t provide enough sunlight for the tree. After the branch is shed, an abscission layer is formed similar to when leaves detach in the fall. There has been a large dispute over what exactly the correct pruning technique in forested areas is and how it relates to urban pruning. In production forestry, where natural pruning does not always occur, professional foresters may choose to enhance the timber quality and decrease risk of disease by pruning. They often have an economic outlook when it comes to pruning, focused on improving wood condition (2). Infection is a constant problem in forests and urban landscapes, it must have your attention when pruning (14). Knowing now how foresters prune and how trees behave in forested environments, you should not believe that landscape pruning is unnecessary. Routine care should be done on your landscape trees!
Figure 4: A flush cut wound that still has yet to be fully compartmentalized even after nine years (4).
Where and when to prune was debated, up until the model of Natural Target Pruning (similar to Hamburg study) came about and was adopted by foresters and arborists alike. In a study done in Finland on Norway Spruce determining the effects of pruning on visual grade defects, researchers found that pruning branches decreased the number of knots on a board by an impressive 10 fewer knots (1). This in turn, shows how much of a positive impact pruning, if done the right way, can have on a tree. It also can be helpful to see how much of a negative effect it can have if done the wrong way (flush cutting) (Figure 4). Understanding at least the basics of tree biology can help immensely in determining the right ways of pruning.
However sometimes either over-pruning and under-pruning can have negative effects in unexpected ways. For example, some studies have shown over-pruning to have very harmful effects on pavement quality. Over- pruning in close proximity to sidewalks or streets can lead to a reduction in the pavement performance (Figure 5). Over pruning can have a bigger impact than you think. It could cost a city or state a significant amount of money in repairing or repaving costs with the increased sunlight stress on the pavement (5). Pruning should be done when necessary, nothing more, nothing less. Trees should not be under pruned or over pruned. Gaining knowledge in tree biology can help enormously when deciding how much pruning is enough.
However sometimes either over-pruning and under-pruning can have negative effects in unexpected ways. For example, some studies have shown over-pruning to have very harmful effects on pavement quality. Over- pruning in close proximity to sidewalks or streets can lead to a reduction in the pavement performance (Figure 5). Over pruning can have a bigger impact than you think. It could cost a city or state a significant amount of money in repairing or repaving costs with the increased sunlight stress on the pavement (5). Pruning should be done when necessary, nothing more, nothing less. Trees should not be under pruned or over pruned. Gaining knowledge in tree biology can help enormously when deciding how much pruning is enough.
Figure 5: An entire street is shaded by two rows of trees increasing the pavement performance (5)
Conclusion
In order to prune competently, with the trees health in mind, tree biology must be taken into account. Knowing when and where to prune can help increase the condition and safety of your trees. Tree species, environmental factors, and stress all can affect how a tree reacts to pruning. It’s also important to realize that the majority of science itself isn’t black and white, there isn’t always an obvious or universal answer for something. Pruning often depends on the species you are dealing with, its genetics, and other variables that can complicate the decision making process. Tree biology can help us greatly in our ability to navigate these tough decisions. If you find the tree in a difficult location or position to prune yourself, absolutely contact a trained arborist. When in doubt, follow these simple guidelines:
1. Identify what branches/limbs you believe are necessary to prune
2. Always make your cut outside the branch collar (never flush cutting)
3. If questions still remain, contact a trained arborist for a consultation
I believe that by using these strategies and understanding how trees behave biologically in a forestry setting, the techniques you should use when pruning a landscape tree can become much clearer.
In order to prune competently, with the trees health in mind, tree biology must be taken into account. Knowing when and where to prune can help increase the condition and safety of your trees. Tree species, environmental factors, and stress all can affect how a tree reacts to pruning. It’s also important to realize that the majority of science itself isn’t black and white, there isn’t always an obvious or universal answer for something. Pruning often depends on the species you are dealing with, its genetics, and other variables that can complicate the decision making process. Tree biology can help us greatly in our ability to navigate these tough decisions. If you find the tree in a difficult location or position to prune yourself, absolutely contact a trained arborist. When in doubt, follow these simple guidelines:
1. Identify what branches/limbs you believe are necessary to prune
2. Always make your cut outside the branch collar (never flush cutting)
3. If questions still remain, contact a trained arborist for a consultation
I believe that by using these strategies and understanding how trees behave biologically in a forestry setting, the techniques you should use when pruning a landscape tree can become much clearer.
References
1. Effects of pruning in Norway spruce on tree growth and grading of sawn boards in Finland, Forestry: An International Journal of Forest Research, Harri Mäkinen, Erkki Verkasalo, Aili Tuimala; Volume 87, Issue 3, 1 July 2014, Pages 417–424
2. Pruning Wounds and Occlusion: A long standing Conundrum in Forestry Kevin L. O’Hara January 24, 2007, Journal of Forestry
3. Pruning Locust and Catalpa Trees for Timber B.O. Longyear, Press Bulletin No. 41 April 1907, The Agricultural Experiment Station, Fort Collins, CO
4. The Hamburg Tree Pruning System- A Framework for Pruning of Individual Trees, Dirk Dujesiefken and Horst Stobbe, 2002, Institute of Arboriculture, Hamburg, Germany
5. Effects of Street Tree Shade on Asphalt Concrete Pavement Performance E. Gregory Mcpherson, Jules Muchnick, Novmeber 2005, Journal of Arboriculture 31(6)
6. Pallardy, S. G. (2008). Physiology of woody plants.
7. Fenton, R.H. and Bond, A.R. (1964) The Silvics and Silviculture of Virginia Pine in Southern Maryland.
8. Compartamentalization Today Kevin T. Smith, Arboricultural Journal 2006, Vol. 29, pp. 173–184 © AB Academic Publishers 2006 Printed in Great Britain
9. Pruning Forest Trees Carol B. Trokey and E.A. McGinnes, Jr. University of Missouri Extension School of Natural Resources G5160, Reviewed October 1993
10. Tree pruning: a modern approach Hugh Morris, January 2013
11. “The Process of Compartmentalization in Trees.” Tree Compartmentalization Process | Cherokee Tree Care, www.cherokeetreecare.com/blog/the-process-of-compartmentalization-in-trees.
12. The Tree Book, a book for people on Urban Tree Care South Carolina Forestry Commission
13. News, O. (2013, July 17). Want to stay cooler in the summer? Try shade trees. Retrieved from https://www.farmanddairy.com/news/want-to-stay-cooler-in-the-summer-try-shade-trees/88634.html
14. Pruning Wounds and Occlusion: A Long-Standing Conundrum in Forestry Kevin O’Hara, Journal of Forestry, May 2007
15. Orchard management: with notes on varieties Fite, A. B. (Arra Burton), New Mexico State University Library, 1933-05
1. Effects of pruning in Norway spruce on tree growth and grading of sawn boards in Finland, Forestry: An International Journal of Forest Research, Harri Mäkinen, Erkki Verkasalo, Aili Tuimala; Volume 87, Issue 3, 1 July 2014, Pages 417–424
2. Pruning Wounds and Occlusion: A long standing Conundrum in Forestry Kevin L. O’Hara January 24, 2007, Journal of Forestry
3. Pruning Locust and Catalpa Trees for Timber B.O. Longyear, Press Bulletin No. 41 April 1907, The Agricultural Experiment Station, Fort Collins, CO
4. The Hamburg Tree Pruning System- A Framework for Pruning of Individual Trees, Dirk Dujesiefken and Horst Stobbe, 2002, Institute of Arboriculture, Hamburg, Germany
5. Effects of Street Tree Shade on Asphalt Concrete Pavement Performance E. Gregory Mcpherson, Jules Muchnick, Novmeber 2005, Journal of Arboriculture 31(6)
6. Pallardy, S. G. (2008). Physiology of woody plants.
7. Fenton, R.H. and Bond, A.R. (1964) The Silvics and Silviculture of Virginia Pine in Southern Maryland.
8. Compartamentalization Today Kevin T. Smith, Arboricultural Journal 2006, Vol. 29, pp. 173–184 © AB Academic Publishers 2006 Printed in Great Britain
9. Pruning Forest Trees Carol B. Trokey and E.A. McGinnes, Jr. University of Missouri Extension School of Natural Resources G5160, Reviewed October 1993
10. Tree pruning: a modern approach Hugh Morris, January 2013
11. “The Process of Compartmentalization in Trees.” Tree Compartmentalization Process | Cherokee Tree Care, www.cherokeetreecare.com/blog/the-process-of-compartmentalization-in-trees.
12. The Tree Book, a book for people on Urban Tree Care South Carolina Forestry Commission
13. News, O. (2013, July 17). Want to stay cooler in the summer? Try shade trees. Retrieved from https://www.farmanddairy.com/news/want-to-stay-cooler-in-the-summer-try-shade-trees/88634.html
14. Pruning Wounds and Occlusion: A Long-Standing Conundrum in Forestry Kevin O’Hara, Journal of Forestry, May 2007
15. Orchard management: with notes on varieties Fite, A. B. (Arra Burton), New Mexico State University Library, 1933-05