The Biology of Pruning

Bob Underwood

Dakota College at Bottineau

Underwood & Associates

• Education begins when you doubt something

• Education occurs when you resolve your doubts.

• We can not know what is wrong unless we know what right is!!

• Diesel vs Gas Engine

Principles of Arboriculture

• Arboriculture 3rd and 4th Editions

• Harris, Clark, Matheny

• Prentice Hall

“As tree development changes with time, so must tree care”

With your Forest?

• The Community Tree

“Tree Care is a long term, low intensity

process.”

“Tree Care is founded on the principles of plant health care.”

AMTOP PHC

Bronze Birch Borer - Symptom

The Culprit - Clean-up batter in Nature

NatureIn our yard

“Tree care applies general concepts to specific genotypes.”

Energy - Rise and Fall

What is it used for? Where is it created?Where is it stored?

Trees are a committed system - Like an airplane at takeoff!!!

Energy - Rise and Fall

• Like a Business, the Tree must make a profit or it fails? But there are expenses!!

“In tree care, an ounce of prevention is worth a pound of cure because

we have a limited ability to cure.”

“Good trees and tree care start with quality

plants.”

“Tree selection is founded on ‘right plant,

right place.’”

“know the place,

select the plant.”

“Arboricultural treatments can have

either positive benefits or negative

consequences.”

“Tree health and hazards are related, but

not equivalent.”

“Arboriculture and Forestry are related but not equivalent fields.”

“You become responsible, forever, for what you have tamed.”

From The Little Prince

• Mechanical Design of Trees:    Trees as structures must develop support tissue to not only support the combined weight of their own components, but also resist the forces of wind, snow, ice and rain. 

• Most of the research has been done by Claus Mattheck and his associates in Germany.  Their research is based on five general principles.

• Trees grow to evenly distribute the stress along their surface.– Trees adhere to the constant stress axiom.  Points where

changes in orientation occur are natural areas of weakness, such as the outer layers of the stem, the sapwood, which provides most of the support, except in older trees. 

• Over time there are no overloaded or under loaded points. – Again, the constant stress axiom

applies.  During severe storms there will be areas of weakness and for a period of time after growing conditions around a tree have undergone radical change, weakness may occur.

• Mechanical stress results in active cambium growth.– Wood production provides the support

and is fastest in a vigorous tree.  Where weakness or injury occurs, the tree actively adds wood to repair them.  We use this fact to add girth to bonsai trees by shaking them occasionally.

• A tree's internal anatomy is directly related to mechanical strength. – Wood and wood rays provide the mechanical strength.

• The manner in which a tree grows reveals the pattern of stress. – Mattheck says, "defect symptoms are a product of

repair growth."  He also says that "the body language of trees does not tell lies."

• Remember, trees grow in response to stress.  As they become larger and heavier, the base must become bigger to support them.  This widening near the base of the plant is referred to as taper. 

• The same occurs on branches. 

• If we know what causes this taper to develop, we can train stronger trees for the future. 

• Lateral branches encourage diameter growth and will reduce height growth.  Therefore, they should be left on a tree when it is young to develop a strong base, but kept under control as the tree ages.  This relatively easy, since laterals do not grow as rapidly as upright branches.

Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona

• Wounded or lost wood tissue is not replaced by a healing process.  Wounded or lost wood tissue is not replaced by a healing process.  Perennial plants react to injury by forming physical and chemical barriers to contain the injury and its effects.  Alex Shigo has termed this CODIT (compartmentalization of disease in trees).This is called compartmentalization since the tree forms a compartment around the damaged area to lessen its impact on the rest of the tree.  This compartment is six sided, formed by four walls: jury by forming physical and chemical barriers to contain the injury and its effects.  Alex Shigo has termed this CODIT .

• Wall 1 - The top and bottom This is the plugging of xylem cells and tracheids above and below the injured area. (weakest wall)

• Wall 2 - The inside limit Formed by the thick walled late season cells of an interior growth increment.

• Wall 3 - The two sides The radial xylem rays running horizontally between the bark and the interior of the tree.

• Wall 4 - The outer wall The new xylem formed after the wounding takes place. Chemically and anatomically different. (the strongest wall against spread of decay, but structurally weak, leading to ring shakes)  

• Walls 2 and 3 also undergo chemical changes that produce phenols in hardwoods and terpenes in softwoods to further limit decay.  

– (adapted from Shigo and Marx 1977

– Photo: A.L. Shigo

• Branch Attachment• Have you ever noticed that as you pull a branch

off a tree, it only rips down, never up or sideways?  This simple observation explains quite clearly how a branch is attached to the trunk, but was not noticed or understood by most people until the middle 1980's. 

• This attachment point must allow for the flow of water up to the leaves and the flow of photosynthetic products back down, while supporting an ever increasing mass. 

• As a branch grows, it adds successive new layers of xylem, these are in turn buried in the trunk by the layers of xylem being formed by the trunk.  They are two distinct generating systems and as such add another boundary to spread of disease within the tree.

Image Source - TREES, ASSOCIATES, and SHIGO (2 CD setMy photo

• We find that if the end is broken off a branch or a cut is made to the cambium, buds below this point developing into new branches will have a more upright growth than the norm. 

• If a shallow cut is made just below a bud, again to the cambium layer, the bud will develop a wider than normal branch attachment. 

• If a lateral with a sharp angle of attachment is removed from a young leader, the resulting new growth from several latent buds will have a wider angle of attachment.  With this information we can make better decisions on cuts when training young trees along the street or in our nurseries.

•    Purposes of pruning:  Before we remove a branch from a tree, we should know exactly why we are doing it and what the resulting action will be on the part of the tree. 

• We must know the natural form of the tree for it is hard to fight Mother Nature. 

• We must be observant so that we can determine how a tree of unknown growth habit has responded to the last time it was pruned and therefore what it is likely to do in the future. 

• The text looks at several reasons for pruning:  • Training young plants, • maintenance of health and appearance, • control of plant size, • influencing flowering, fruiting and vigor, • compensation for root loss, • invigoration of stagnating plants, • and increasing the value of conifers.  • While these are all standards of the industry, are

they all in the tree's best interest? 

• Training young plants is probably the closest to being truly beneficial for the tree, if it reduces future need for treatment. 

• If a tree has developed in its current environment, shouldn't the leaves be of the right type to utilize light most efficiently?  

• Shouldn't there be sufficient reaction wood and taper to withstand normal winds?

• Shouldn't the tree be vigorous enough to tolerate most common diseases and insects, eliminating the need for sprays? 

• If we put the right tree in the right spot originally, why would we need to control size? 

• If the roots have been cut off, do we reduce the energy producing component needed to replace them? 

• Why do trees produce large fruit crops occasionally, fear of death? 

• If we traumatically cut back a bush and it responds with a last ditch effort to put out leaves and produce energy, have we invigorated it? 

• What is vigor? 

• Pruning Responses:•   The initial response to pruning in most young trees and those with

few flowers is an invigoration of individual shoots (there's that i word again) if done during the growing season. 

• These shoots grow longer faster and have larger leaves.  The root-shoot ratio is definitely in their favor.  However, this growth is later in the  season using up more reserve while creating less for storage and the total leaf area is usually less than on an untrimmed tree. 

• This leads to a general dwarfing of the total plant. 

• If the pruning is done during the dormant season, the general dwarfing still occurs, but the top and roots are in balance by the end of the season and less of the obvious shoot growth is observed. 

• To reduce the growth rate of an individual branch, we generally shorten it, reducing the amount of energy available for growth.  (subordination pruning)

• Another concept to remember is that if we cut it to a horizontal side branch, these grow slower than upright shoots. 

• Time of Pruning:•    This varies from when the saw is sharp to the right time of the moon.  We

should always know the plant species, the condition it is in and what the desired results are before we start. 

• Light pruning of unwanted growth can be done when it is small. 

• Broken and dead branches may be removed at most anytime with no effect on plant vigor. 

• Generally it is best to prune deciduous trees during early dormancy, late dormancy may lead to "bleeding" which is much more painful to the customer than the tree. 

• We can pinch back tops and make small cuts to alter growth habit during the growing season. 

• Another consideration is if trees are more susceptible to specific insect pests or diseases at one time of the season or another (i.e.. Oak wilt prior to July 1 in Red Oaks). 

• If you wish to slow the growth of the plants, best results will come from pruning

in late spring or early summer, reducing leaf area for the longest period. 

• During the growing season, we can most easily determine what branches are hanging too low or are dead.  During this time we may be limited by the nesting activity of certain birds, whose presence is desired by the homeowner. 

•    The best general rule for flowering plants is to prune when they finish flowering .  This allows them the necessary time to set new buds for the next season.  It also may remove the fruit before it develops and actually saves this energy for the tree to put to other uses. 

• Flowering itself does not eat into a tree’s energy reserves to any great extent, since the energy required is already stored in the bud or in its immediate shoot.  This characteristic makes it very hard to reduce the size of flowers on a bonsai

tree to the extent that we can the limbs and leaves. 

• Closure of wounds may be faster if the pruning is done just prior or just following initiation of growth in the spring.  Within that growing season. 

• Wounds in the winter, spring or summer did not affect closure rates the following season, however those made in the autumn slowed closure by 20% in the first year.  There was no difference in following seasons. 

• Early pruning must be done carefully since the bark may tend to slip due to the very active cambium, and fall pruning may coincide with the sporulation of many fungi.

•    Pruning may stimulate new growth if done in late fall, leading to freeze damage.  This may also occur near a pruning cut if the temperature gets low enough.  This is something to consider on some species in colder climates.

•    Some trees are thinned in the fall to allow sunlight to penetrate for residences using solar energy for part of their heating.

• Did I create enough doubts to start education??

• Now your assignment is to resolve those doubts, between you, me and your peers.

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• Thanks as I ride off into the sunset, in my chauffeured Limousine!!!

Notes