ISA Tree Biology and Physiology 3 — Questions and Answers

Question 1: What is the CODIT model and who developed it?

A computer model for predicting tree growth, developed by the US Forest Service
The Compartmentalization of Decay in Trees model, developed by Dr. Alex Shigo (Correct answer)
A classification system for tree diseases, developed by the International Society of Arboriculture
A computer-based diagnostic tool for identifying insect pests

Correct answer: The Compartmentalization of Decay in Trees model, developed by Dr. Alex Shigo

CODIT (Compartmentalization of Decay in Trees) was developed by Dr. Alex Shigo. It describes how trees actively wall off injured and infected tissue using four barrier walls to limit the spread of decay.

Dr. Alex Shigo's CODIT model, published in the 1970s-1980s after dissecting thousands of trees, revolutionized the understanding of how trees respond to wounds and decay. The model describes four walls of defense: Wall 1 (weakest) limits vertical spread by plugging xylem vessels above and below the wound with tyloses and phenolic compounds; Wall 2 limits inward spread at the annual ring boundary; Wall 3 limits lateral spread via ray parenchyma cells; Wall 4 (strongest, the barrier zone) is formed by the cambium after wounding, creating new wood with altered chemistry that is highly resistant to decay organisms. CODIT explains why flush cuts are harmful (they breach Wall 3) and why proper pruning cuts at the branch collar preserve the tree's defense system.

Question 2: What is the primary function of the vascular cambium in a tree?

Storing food reserves during winter dormancy
Producing new xylem (wood) cells inward and phloem (inner bark) cells outward, enabling secondary growth (Correct answer)
Transporting water from roots to leaves
Protecting the tree from insect attack

Correct answer: Producing new xylem (wood) cells inward and phloem (inner bark) cells outward, enabling secondary growth

The vascular cambium is a thin layer of actively dividing meristematic cells that produces xylem toward the inside and phloem toward the outside, enabling the tree to grow in diameter.

The vascular cambium is a single-cell-thick cylinder of meristematic tissue that extends from just below the terminal buds down through the trunk and into the roots. Through cell division, it produces xylem (water-conducting wood) toward the tree's center and phloem (food-conducting inner bark) toward the outside. This process, called secondary growth, is responsible for increases in trunk and branch diameter. The cambium is most active during the growing season, producing earlywood (large-celled, lighter) in spring and latewood (small-celled, darker) in summer, which together form one annual growth ring. The cambium is remarkably vulnerable — a thin band of living tissue that, if damaged around the full circumference (girdling), will kill the tree.

Question 3: How do deciduous trees prepare for winter dormancy at the cellular level?

They produce antifreeze proteins that prevent all cell freezing
They increase sugar concentrations in cells, lower the freezing point of cytoplasm, and move water from cells to intercellular spaces where ice formation is less damaging (Correct answer)
They completely dehydrate their tissues to eliminate any freezable water
They increase respiration rates to generate metabolic heat within tissues

Correct answer: They increase sugar concentrations in cells, lower the freezing point of cytoplasm, and move water from cells to intercellular spaces where ice formation is less damaging

Cold hardening involves increasing intracellular sugar concentrations to depress the freezing point, and promoting controlled dehydration where water moves out of cells to freeze in intercellular spaces, protecting the cell membranes from ice crystal damage.

Cold hardening (cold acclimation) is a complex physiological process triggered by shortening day length and falling temperatures in autumn. Trees convert starch reserves to sugars (sucrose, raffinose), which lower the cytoplasmic freezing point. Proteins called dehydrins stabilize cell membranes. Controlled dehydration moves water from inside cells to intercellular spaces through aquaporin channels. Ice that forms in intercellular spaces is less damaging than intracellular ice, which ruptures cell membranes and is lethal. Changes in cell membrane lipid composition increase fluidity at low temperatures. The process occurs in stages — initial hardening begins in early fall, with maximum hardiness reached in midwinter. De-hardening in spring is much faster, which is why late spring freezes can be so damaging.

Question 4: What is the cohesion-tension theory and what process does it explain?

How tree branches resist wind forces through flexible wood fibers
How water moves from roots to the top of tall trees through continuous water columns in the xylem, driven by transpiration pull (Correct answer)
How roots absorb minerals from the soil through ion exchange
How phloem transports sugars from leaves to roots

Correct answer: How water moves from roots to the top of tall trees through continuous water columns in the xylem, driven by transpiration pull

The cohesion-tension theory explains water transport in trees. Transpiration from leaf stomata creates negative pressure (tension) that pulls water upward through continuous columns in xylem vessels, held together by water's cohesive properties.

The cohesion-tension theory, also called the transpiration-cohesion-tension mechanism, is the accepted explanation for how water moves from roots to leaves in tall trees — sometimes over 300 feet against gravity with no mechanical pump. When stomata open for gas exchange, water evaporates from mesophyll cell walls into the leaf's air spaces and out through the stomata (transpiration). This creates negative pressure (tension) in the leaf's xylem. Because water molecules are strongly attracted to each other through hydrogen bonding (cohesion) and to xylem cell walls (adhesion), the tension is transmitted downward through an unbroken water column in the xylem, pulling water up from the roots. The system operates under enormous tension — up to -1.5 MPa in tall trees — and cavitation (column breakage) is a constant risk.

Question 5: What is the difference between heartwood and sapwood in terms of function?

Heartwood is living tissue that stores food; sapwood is dead structural tissue
Sapwood is the physiologically active outer wood that conducts water and stores reserves; heartwood is the non-conducting inner wood that provides structural support (Correct answer)
Heartwood conducts water faster than sapwood due to larger vessels
There is no functional difference; they differ only in color

Correct answer: Sapwood is the physiologically active outer wood that conducts water and stores reserves; heartwood is the non-conducting inner wood that provides structural support

Sapwood (outer rings) contains living ray parenchyma cells and functional xylem vessels that conduct water. Heartwood (inner rings) has undergone chemical transformation — cells are dead, vessels are blocked, but it provides critical structural support.

As a tree grows, the oldest sapwood gradually transitions to heartwood through a process called heartwood formation. Living parenchyma cells in the aging sapwood die, and their contents are converted to extractives — phenolic compounds, tannins, and other chemicals that darken the wood and make it more decay-resistant. Xylem vessels are blocked by tyloses (balloon-like growths from adjacent parenchyma cells). The result is heartwood that no longer conducts water or stores food but provides essential structural support and, through its extractive chemistry, resists decay better than sapwood. This is why hollow trees can survive — they have lost their heartwood but retain functional sapwood. However, the loss of heartwood significantly compromises structural integrity.

Question 6: What role do auxins play in tree growth and development?

They promote root growth exclusively and have no effect on shoots
They regulate apical dominance, promote cell elongation, stimulate cambial activity, and influence root initiation (Correct answer)
They are primarily responsible for leaf abscission in autumn
They function only as defense chemicals against herbivores

Correct answer: They regulate apical dominance, promote cell elongation, stimulate cambial activity, and influence root initiation

Auxins are versatile plant hormones that regulate apical dominance (suppressing lateral bud growth), promote cell elongation in shoots, stimulate the vascular cambium to produce new cells, and influence adventitious root formation.

Auxin (primarily indole-3-acetic acid, IAA) is produced mainly in shoot apical meristems, young leaves, and developing seeds. It moves basipetally (downward) through the phloem and parenchyma, creating concentration gradients that regulate numerous developmental processes. Apical dominance occurs because auxin from the terminal bud suppresses lateral bud growth; removing the terminal bud (as in heading cuts) removes this suppression, triggering prolific lateral sprouting. Auxin stimulates cell elongation in shoots (but inhibits it in roots at the same concentration — a key difference). It activates the vascular cambium to produce new xylem, which is why wound closure is faster above a wound (more auxin flowing down) than below. Synthetic auxins like IBA are used commercially to stimulate root formation on cuttings.

ISA Tree Biology and Physiology 3

What is the CODIT model and who developed it?