Chapter 12: Tree Assessment and Risk Management Flashcards
risk management
systematic application of management policies, procedures, and practices for identifying, evaluating, treating, monitoring, and communicating risk; goal of balancing risk levels against other factors such as available time and money
risk assessment
process of evaluating what unexpected things could happen, how likely they are to happen, and what the likely outcomes are; in tree management, the systematic process to determine the level of risk posed by a tree, tree part, or group of trees; process of evaluating the likelihood that part or all of a tree will fail and cause damage and/or injury; identifies risk levels and recommends action
What factors does tree risk assessment consider?
the potential for tree failure, the environment that may contribute to failure, and what would be damaged and/or injured - the target
failure potential
in tree risk assessment, the professional assessment of the likelihood for a tree to fail within a defined period of time; likelihood that an entire tree or part of a tree will break a fall within a given time period
What to consider when evaluating a tree’s failure potential?
species, growth habit, defects, quality of branch attachments, condition of the root system, lean, and the history of the tree and site
load
general term used to indicate the magnitude of a force, bending moment, torque, pressure, etc. applied to a substance or material; or cargo, weight to be borne or conveyed
When does failure occur?
when the load experienced exceeds the structural capacity of a tree or part of tree or due to environmental conditions, such as storms with winds, snow, ice loading, lightning, and rainfall
What factors should be considered when assessing tree stability at a site?
- ) construction, grade changes, and trenching within the root zone
- ) removal of adjacent trees that previously served as a wind buffer
- ) replacement of sidewalks and root loss (root pruning)
- ) failure of nearby trees caused by root disease
- ) change in wind dynamics due to new structures on the site
target
a person, object, or structure that could be harmed (damaged or injured) by a tree or tree part in the event of a failure; location of target pruning; people and/or property that could be damaged or injured if the tree failed
structural defects
any naturally occurring or secondary conditions such as cavities, poor branch attachments, cracks, or decayed wood in the trunk, crown, or roots of a tree that my contribute to structural failure
What must you be familiar with to discern the difference between a minor defect and a significant weakness that could result in tree failure?
- ) tree species and typical modes of failure
- ) normal growth traits
- ) structure and form
- ) signs of decay
tree health vs. tree structure
not the same thing; a few thin layers of active xylem or phloem can keep a tree alive but structural soundness requires more wood volume; green, full-crowned, and essentially healthy trees can still fail due to structural defects
visual tree assessment
VTA; method of assessing the structural integrity of trees using external symptoms of mechanical stress (such as bulges, reactive growth, etc.) and defects (cracks, cavities, etc.); minimum standard for tree risk assessment observing conditions and characteristics 360 degrees around the tree
Steps in VTA
- ) look for dieback, gaps, or discoloration in the crown
- ) take note of any lean
- ) look for branches that extend beyond the rest of the crown
- ) examine the trunk taper
- ) inspect the trunk, the root collar, and the root zone
reaction wood
wood formed in leaning or crooked stems or on lower or upper sides of branches as the means of counteracting the effects of gravity; supports the tree in a leaning position
taper
change in diameter over the length of trunks, branches, and roots; a diameter that gradually decreases from the base to the tip; allows distribution of load such as the weight of the canopy and wind, along the length of the stem
What trees tend to have bad taper?
trees growing in closely set stands of trees; trees growing in the open tend to have good taper; stem is more likely to break where the stress is greatest
What general characteristics should be analyzed on individual parts of the tree for defects?
- ) the trunk flare - look for visible flare with solid, well-developed roots exhibiting no decay; lack of a trunk flare, discolored or loose bark, wounds, seams, and indentations created by circling roots are all defect indicators
- ) the trunk - look for wounds, cracks, decay, sloughing bark, codominant stems, and included bark
- ) major branches attachments - branches should be smaller (50% or less) than the trunk in diameter and spaced vertically along the trunk
codominant stems
forked stems nearly the same size in diameter, arising from a common junction and lacking a normal branch union
included bark
bark that becomes embedded in a crotch (union) between branch and trunk or between codominant stems; causes a weak structure
wood decay
a disease caused by certain fungi, which may or may not form conks or mushrooms
conk
fruiting body or nonfruiting body (sterile conk) of a fungus; often associated with decay
fruiting body
reproductive structure of a fungus; the presence of certain species may indicate decay in a tree
bracket
British English term for the fruiting body of a decay fungus
Types of fungal reproductive structures
conks, brackets, and mushrooms
What do fungal reproductive structures on a tree indicate?
internal decay; however a tree can have decay without the presence of conks or mushrooms on the outside
Types of fungal decay
white rot, brown rot, and soft rot; each characterized by the wood cellular and/or intercellular components that they remove
white rot
fungal decay of wood in which both cellulose and lignin are broken down; primarily decay the lignin within and between the cell walls in the wood; reduces wood stiffness; decayed wood appears white after the darker-colored lignin is decayed
Examples of white rot fungi?
Armillaria spp. and Ganoderma spp.
brown rot
fungal wood rot characterized by the breakdown of cellulose; most common on conifer trees (less common on deciduous trees); primarily decays the cellulose leaving behind the stiff lignin, thereby reducing the bending strength of the tree; makes the wood brittle and the decayed wood is dry and crumbles easily; remaining lignin is dark or brown in color after the cellulose is decayed
Examples of brown rot fungi?
Laetiporus and Phaeolus
soft rot
decay of plant tissues characterized by the breakdown of tissues within the cell walls; first degrades the cellulose portion of the wood
Example of soft rot fungi?
Kretzchmaria
How do trees react to white rot?
adapt to the loss of wood strength by laying down more new wood around the decayed area, which compensates for the strength loss in the existing wood
branch rot
branch decay located on large branches
trunk rot
decay located in the upper trunk
basal rot
decay of the lower trunk, trunk flare, or buttress roots; butt rot; decay at the base of the tree
structural root decay
develops from the bottom upward; visible symptoms of this rot may or may not develop in the crown
root rot
decay located in the roots
heartwood rot
any of several types of fungal decay of tree heartwood, often beginning with infected wounds in the living portions of wood tissue; heart rot; decay that starts in the heartwood (center) of the tree
sapwood rot
decay located in the sapwood; bark and/or cambium may be damaged or dead; signs of this classification of rot are usually numerous, but small, fruiting bodies along the bark’s surface are common; decay progresses from the outside of the branch toward the center
positive indicators of decay
means decay is present; presence of fruiting bodies (brackets, mushrooms, or conks) on the wood, open cavities, and visible decayed wood
potential indicators of decay
symptoms or signs that decay may be present that that further investigation is warranted; cracks, seams, bulges, and wounds from old pruning cuts, especially topping cuts
insects that indicate decay
carpenter ants are usually associated with decayed wood within the tree and bees will make hives in tree cavities
birds that indicate decay
cavity-nesting birds will nest in hollows or dig out decayed wood to create a hollow
What are trees fundamental strategies to react to decay?
compartmentalization - to contain the progress of decay;
growth - to outgrow it
root collar excavation
process of removing soil to expose and assess the root collar (root crown) of a tree
air excavation device
air excavator; devise that directs a jet of highly compressed air to excavate soil; used within the root zone of trees to avoid or minimize damage to the roots, or near underground structures such as pipes and wires to avoid or minimize damage to them
aerial lift device
mobile elevated work platform; to inspect the upper crown, probe cavities, and look for cracks and defects not visible from the ground
resonance testing
nondestructive means of determining the soundness or structural integrity of an object or material by determining the rate at which sound waves travel through it
Ways to detect extent of decay?
resonance testing, sounding with a hard plastic or other suitable mallet, inserting a measuring stick or probe in a cavity; drilling and using sound/resistance and color, texture, and odor of extracted wood as an indicator
cavity
open or closed hollow within a tree stem, usually associated with decay
increment borer
device used to take core samples from trees to determine age or detect problems, such as decay; bore into trunk or root tissue and extract a core of wood for further examination
decay detection devices
any device used to assess the presence (and often extent) of hidden internal decay
resistance recording drills
small, rotating needle pushed into the wood to record the resistance to wood penetration along the drilling path (decayed wood is softer and generates less resistance)
acoustic measurement devices
measure the time it takes for sound to travel between various points around the cross section of a tree; less invasive than drilling devises (sound waves pass through decayed wood slower than through sound wood)
tomogram
image generated by tomography; created by sending waves through an object; a computer then produces images of cross sections of the object by using information about how the waves change; color-coded “picture” of the inside of the tree at that section showing areas of decayed and sound wood
Types of decay-detecting equipment
resistance measuring devices, tomography, x-rays, and radar
What kind of variables influence the stability of trees with decay?
tree and pathogen species; tree age and health; canopy architecture, height, and weight distribution; exposure to wind, snow and ice; wood strength and toughness; root structure; and soil conditions
systematic procedure for assessment process
observations, measurements, and recommendations should be documented; good to include digital images as well; good to use an assessment form for consistency and thoroughness
What should your assessment form include?
tree genus, species, and common name; size, location, and general description; surroundings and description of any potential targets; environment of the tree such as prevailing winds, microclimates, surrounding trees, and other plants; history of the tree including maintenance and management practices
Who decides what level of risk is appropriate?
the property owner or manager
mitigation
in tree risk management, reducing, alleviating, or minimizing risk or harm (damage or injury); process of reducing failure potential
What is the primary recommendation that can reduce the likelihood of failure?
pruning to remove dead, broken, or high-risk branches or reduction pruning to even remove entire stems; other options are tree support systems, target removal, fencing off the area/posting signs
What are good management practices to which trees with cavities may respond favorably and improve in condition?
ensuring good soil health, moisture, and aeration, and applying organic mulch; if the tree has good vitality it may maintain structural integrity by producing new wood around an injury so filling cavities or attempting to remove decay is not usually a good idea
duty of care
legal obligation that requires an individual to use a reasonable standard of care when performing tasks that may potentially harm others; emphasizes the importance of documenting any potential hazards and risk levels discovered when assessing a tree, even if it is not for risk assessment
liability
something for which one is responsible; legal responsibility
negligence
failure to exercise due care; occurs when the arborist has the obligation or responsibility with regard to the tree (duty) and fails to act responsibly in providing the service (breach of duty) and injury or damage occurs (harm) because of the arborist’s action(s) or lack of action (causality)
breach of duty
the failure to act reasonably under the circumstances
standard of care
degree of care that a reasonable person should exercise in performing duty of care; a measurement used to assess whether an individual acted in a reasonable manner; degree of care that a reasonably prudent person should exercise in the same or similar circumstances; determined by courts
causation in fact
in determining legal liability, the test of the proximate cause and foreseeability of the consequences; damage, injury, and/or death can be traced back to the defendant’s action (or lack of it)
proximate cause
legal term that describes an act or omission that can be attributed as cause of loss and/or injury; injury was reasonably foreseeable
act of God
an occurrence due to natural causes that could not have been prevented by ordinary skill and foresight; defense used when trees fell/broke and caused damage, but is not always accepted, especially if the failure is a result of human intervention
