Chapter 1: Tree Biology and Pruning Flashcards
trees
woody, compartmentalizing, long-lived perennial plants that grow to heights of a few to many feet (meters) on a single stem; the stem may divide low down, but it must do so above ground level; dominant vegetation over large areas of the world
arboriculture
the care of individual trees; an art and a science; combines skill and craft with knowledge and fact; propagation, planting, and management of trees so that people can coexist in sustainable environments
What is the foundation for the practice of arboriculture?
a thorough understanding of how trees grow and defend themselves and how people interact with them
tree biology
the study of structure and function and the relationship between them
sections of trees
- a crown with leaves and branches
- roots
- a trunk (sometimes called a bole or stem) that connects the crown and the roots
roots
the portion of a woody plant found below ground that provides stability, water, and nutrients to a woody plant; anchor the tree in the ground, absorb water and essential nutrients from the soil, transport those materials toward the trunk, and store starch reserves
xylem
water and nutrient-conducting tissue of the stem
leaves
the organs that make food for the tree via photosynthesis
photosynthesis
the process in green plants (and in some bacteria) by which light energy is used to form organic compounds from water and carbon dioxide; “putting together with light”
chloroplasts
cells where photosynthesis takes place; contain molecules of green, light-absorbing pigment chlorophyll
chlorophyll
traps light energy from sunlight and combines it with carbon dioxide from the air and water transported from the roots and produces carbohydrates in the form of sugars
What is a byproduct of photosynthesis?
oxygen; released into the atmosphere through stomata
photosynthate
general term for the products of photosynthesis; sugar product; carbohydrate
What is the energy source for plant respiration?
photosynthates or carbohydrates
sources
the leaves where photosynthates are produced
sinks
tree parts that use and/or store more photosynthate than they produce
phloem
plant vascular tissue that conducts photosynthates; situated to the inside of the bark; inner bark; outside of the xylem
wood rays
tissues that extend radially across the xylem and phloem of a tree; (outward to in = phloem to xylem)
energy reserves
form of starch or sugar; released and used as required by tree during normal biological functions; equivalent of animal fat
essential elements
nitrogen, potassium, sulfur, and iron
How do water, mineral elements, and micronutrients enter the tree?
osmosis via the absorbing roots
osmosis
diffusion of water throughout a semi-permeable membrane from a region of higher water potential to a region of lower water potential; passive process
pure water
highest potential
transpiration
the evaporation of water from plants; pulls water through the xylem to the leaves, where it is lost to the atmosphere through the stomata
axial transport
longitudinal movement of water in the xylem and photosynthates in the phloem
radial transport
horizontal movement in and out of wood and cells of different ages
ray cells
tissues that extend radially across the xylem and the phloem of a tree
rays
living channels of cells through which water, nutrients, and carbohydrates move laterally
reaction zone
a natural boundary formed by a tree to separate wood infected by disease organisms from healthy wood; important in the process of compartmentalization; limits the spread of decay following an injury or pruning event
Compartmentalization of Decay in Trees
CODIT; process whereby trees limit the spread of discoloration and decay; forms four barrier “walls” that limit the spread of decay in different planes within the tree; Wall 1 is formed when the tree responds to wounding by “plugging” the upper and lower vascular elements to limit vertical spread of decay; Wall 2 is formed by the last cells of the growth ring, limiting inward spread; Wall 3 is composed of ray cells that compartmentalize decay by limiting lateral spread; Wall 4 is the new growth ring that forms after the injury, the strongest wall
vertical wall
a layer of cells formed to resist upward and downward spread through the tree by plugging xylem vessels or tracheids; Wall 1 of CODIT; resists the spread up and down through the tree by plugging xylem vessels (in angiosperms) or tracheids (in gymnosperms)
horizontal wall
a layer of cells, formed by the last cells of the growth ring, that resists inward horizontal spread of decay; Wall 2 of CODIT; resists inward horizontal spread through the more compact summerwood cells and by depositing chemicals in active xylem cells
lateral/radial wall
a layer of cells composed of ray cells, that compartmentalizes decay by limiting lateral spread; Wall 3 of CODIT; inhibits lateral spread around the rings by activating ray cells to resist decay
barrier zone
a layer of wood formed immediately after injury, which resists the spread of microorganisms into new wood formed to the outside of the existing annual ring; Wall 4 of CODIT
strength of walls of CODIT
Wall 1 is the weakest barrier
Wall 4 is the strongest barrier; rarely fails; decay is kept out of new wood
Walls 1, 2, and 3 may fail sequentially if the tree cannot resist the spread of aggressive pathogens
pruning
cutting away unwanted parts of a plant; removing branches for defined reasons; the most common tree maintenance procedure; desirable and necessary for safety
common pruning goals
- improve tree structure or health
- accommodate human needs such as clearance pruning or hazard mitigation
- to increase light penetration
- to provide a view
- to correct or prevent a problem
line clearance
the process of removing vegetation around power lines; shape(s) may not always be aesthetically pleasing or appear natural
A300-2001
industry-developed standards of practice for tree care, approved by the American National Standards Institute; American National Standard for Tree Care Operations - Tree, Shrub and other Woody Plant Maintenance - Standard Practice (Pruning)
BS3998
British Standards Institute Recommendations for Tree Work; pruning guidelines in Britain
utility line-clearance pruning
the removal of vegetation around power lines for various management reasons, primarily to keep trees and branches clear from overhead power lines
What is the primary objective of utility line-clearance pruning?
to keep trees and/or their branches clear of overhead power lines
overhead power lines
electrical lines that have been placed on poles with insulators
power outages
interruptions to the electrical supply
structural failure
full or partial failure of a tree as a result of structural defects causing damage to lines or poles
electrical shorts
an interruption in the flow of electricity that could lead to a fire or an outage
On what basis should trees be pruned?
based on the species, growth rates, power line voltage, construction, and location of the tree in relation to the conductors
conductors
lines through which electricity can pass freely and thus be conducted
What is the goal of utility tree pruning?
to minimize and/or eliminate service interruptions, avoid equipment damage, comply with applicable legislation, and ensure that electric utilities meet their responsibilities to provide a continuous supply of quality electricity, safely and efficiently
Who should engage in line-clearance work?
only qualified line-clearance tree trimmers or line-clearance trainees
basis of proper pruning
branches should be cut back to an existing branch or stem (node) that is large enough to assume apical dominance and support the remaining stem
node
slightly enlarged portion of a stem where leaves and buds arise
drop-crotch
method for reducing the height of a tree
thinning
selective removal of unwanted branches and limbs to provide air or light penetration through the tree and to lighten the weight of the remaining branches
nodal cuts
the process of removing a branch back to an existing branch or stem
Examples of nodal cuts
drop-crotch and thinning
topping
cutting back a tree to buds, stubs, or laterals not large enough to assume apical dominance/terminal role; another term for heading back; a bad/unacceptable practice
internodal cuts
pruning cuts made in the region of a branch between two successive nodes
What are possible results of topping?
severely weaken or even kill some species; net loss of foliage can lead to dieback of the retained parts, which can in turn lead to extensive decay problems; regrowth may be structurally unstable and become hazardous to the targets around the tree such as overhead power lines, roads, sidewalks, and buildings
pollarding
a speciality pruning technique used on large-maturing trees that results in the development of callus at the cut ends of the branches; it is used to maintain trees at a specific height; form of internodal pruning; begins when tree is young; entails severely cutting back all branches to appropriate points between nodes and maintaining them at these points to produce pollard heads by frequent (annual or biannual) repruning
How to prevent branch splitting or bark tearing while pruning?
precut branches that are too large to support with one hand a short distance from the point of origin; remaining stub should then be removed to provide the final cut surface
natural/target pruning
the process of branch removal in which the cuts are made in relation to the positions of the branch collar and branch bark ridge; leaves a smooth surface with no jagged edges or torn barn
Where is the correct anatomical location for removing a branch?
at its point of origin close to the trunk or parent branch, without cutting into the branch bark ridge or branch collar, and without leaving a stub; does not damage trunk tissues; minimize damage to the tree by restricting the cut to branch tissues which are genetically programmed to produce barrier zones at this point; retains the branch reaction zone at the base which mimics the natural pruning methods one sees in trees when branches die and are naturally shed
branch bark ridge
top area of a tree’s crotch where the growth and development of the two adjoining limbs push the bark into a ridge
branch collar
area where a branch joins another branch or the trunk, which is created by the overlapping of xylem tissues year after year
wound dressings
compound applied to tree wounds or cuts, if necessary; paints thought to accelerate wound closure, protect against insects and diseases, and reduce decay
pruning principles/order of cuts
first cut undercuts the limb; the second cut removes the limb; the final cut should be just outside the branch collar to remove the resultant stub
shoot invigoration
increased growth of remaining unpruned shoots after pruning; growth after pruning takes place on fewer shoots, so unpruned parts tend to grow more than they would have without pruning
Principles to remember when pruning trees in proximity to overhead power lines
- leaf removal reduces photosynthesis and overall tree growth, in the short term
- epicormic shoots may be stimulated because of a loss of apical control and auxin production from the removed terminal growing point(s); these epicormic shoots require energy for growth, further reducing the tree’s energy reserves; new leaves will eventually replenish the loss through photosynthesis
- energy reserves are lost as the carbohydrate and starch stored in the branch are removed with it
- valuable stored carbohydrate/starch is used within the remaining parts of the tree, where walls of compartmentalization are formed to “lock up” or “seal off” pruning cuts from invasion by insect and disease organisms
Examples of times when trees experience stress
the period shortly after full leaf expansion in the spring, during severe droughts, and following construction injury
Pruning cuts should not exceed how much of the diameter of the parent stem?
1/3, as a general rule
Important items to consider when pruning and what that means for utility line-clearance pruning
- make as few cuts as possible to accomplish the pruning objective while considering the natural structure of the tree (entire limbs should be removed rather than hedging, shearing, or topping a tree)
- make the best possible cuts in line with target pruning principles (limbs should be removed at appropriate lateral branching points with properly made cuts)
- keep the wounds as small as possible (the smallest branches possible should be removed while still accomplishing the primary clearance objective)
utility pruning
pruning to remove branches from the vicinity of overhead electricity lines to provide clearance for a given period of time
clearance
the amount of open space between the end of the tree branch and the power line; defined prior to pruning
laterals
secondary or subordinate branches; the parent branch
What clearance should be prescribed?
while specific requirements for clearance distances vary, ideally the maximum clearance for the longest possible time; pruning cuts should be made at laterals outside the specified clearance zone
conductor sag
the phenomenon by which conductors expand in warm weather (and periods of high demand) and drop below their usual height
directional pruning
removing branches from a tree in such a way as to encourage new growth in a particular direction and away from overhead conductors; direct future growth away from the conductors as much as possible
Benefits of directional pruning
minimize regrowth toward the conductors and provide clearance for longer periods of time
Examples of pruning styles
- crown reduction
- side pruning
- overhanging
crown reduction
method of reducing the height or spread of a tree by performing appropriate pruning cuts; removes the main leader or leaders to a lower lateral branch no less than 1/3 the diameter of the original limb
leader
the primary terminal shoot or trunk of a tree
What kind of trees are best for crown reduction methods?
slow-growing species or those whose mature height will only marginally reach the power lines
side pruning
removal of branches on the conductor side of the tree to direct future growth away from the line; combines directional with natural/target pruning; provides good clearance for reasonable time; form of crown reduction except branches are shortened from the side rather than from the top
overhanging
tree branches that have grown above and over the power lines
Can overhanging branches be left in place?
Yes, provided they have strong branch attachments and are unlikely to fail in a windstorm or under the weight of snow, and have sufficient clearance to the conductors below
V-pruning
the removal of branches from the middle of the crown of a tree to provide clearance to conductors, which results in a V shape in the crown when viewed from one angle; no overhang above the conductors
through-pruning
the removal of branches from the middle of the crown of a tree to provide clearance to conductors; similar to V-pruning except that some branch overhang is left above the wires; finished job may resemble a tunnel through the crown; can leave the crown of the tree largely unaltered
Pruning options for trees located directly beneath low voltage and distribution power lines
v-pruning or through-pruning
pruning cycles
the time between pruning events set to ensure that maximum clearance between branches and conductors is maintained for the longest possible time; once every so many years
How to determine the time between pruning events?
based on the growth rates of the trees that are being pruned and not on arbitrary time scale dictated by the utility’s hardware maintenance cycle; depends on knowing
- the main species of trees that occur along the line
- the speed at which they grow, particularly after pruning, coupled with the utility’s required or desired amount of clearance
What to do with species that do not tolerate multiple pruning events?
consider removing and/or replacing such trees rather than subjecting them to multiple prunings
first cycle
the first time a tree is pruned for utility reasons
How to trim a tree in the first cycle?
as much branch material as appropriate, according to the situation and pertinent standards, should be removed by correct pruning practices; implement both natural/target and directional pruning methods to avoid unnecessary future problems
How much can you trim a mature tree?
no more than 1/4 of the crown (or 1/4 of the foliage on a single branch being cut to a lateral) should be removed within a growing season
90-3-90
a rule of thumb where 90% of the time three cuts can give 90% of the required clearance when pruning for utility lines; concept that 90% of the time the removal of 3 branches produces the majority of the clearance required
tree growth regulators
TGRs; a chemical that can be applied to trees that slows extension growth by shortening cell elongation; chemical substances that enhance or alter the growth and development process of the tree; increase or decrease normal growth, flowering, or fruiting
Why are TGRs used in utility work?
to control shoot growth of trees and other vegetation beneath utility lines; lengthening the time between cycles and saving money; reduce watersprout production and minimize sucker growth on trees
methods of growth inhibitors
sprayed on foliage, banded on the bark, applied to the soil, injected into the tree
gibberellin
a plant growth substance that controls cell elongation; naturally occurring cell-elongation hormone
How do some TGRs work?
by inhibiting the synthesis of gibberellin which can significantly reduce extension growth, thus lengthening the time between cycles and saving money
watersprout
an upright, adventitious shoot arising from the trunk or branches of a plant; incorrectly called a sucker
