Certified Arborist Test Tree Biology and Structure 2 — Questions and Answers

Question 1: What is the primary function of the cambium layer in a tree?

Photosynthesis and sugar production
Producing new xylem and phloem cells (Correct answer)
Storing water and nutrients
Absorbing minerals from the soil

Correct answer: Producing new xylem and phloem cells

The cambium is a meristematic layer that produces new xylem (wood) to the inside and new phloem (bark) to the outside, enabling secondary growth and diameter increase.

The vascular cambium is a thin layer of undifferentiated cells located between the xylem and phloem. It is one of the most critical meristematic tissues in woody plants, responsible for secondary growth — the increase in girth that distinguishes trees from herbaceous plants. During each growing season, the cambium divides to produce secondary xylem (wood) on its inner face and secondary phloem (inner bark) on its outer face. Xylem cells differentiate into vessel elements, tracheids, fibers, and ray parenchyma, all serving structural and hydraulic functions. Phloem cells differentiate into sieve tubes and companion cells for sugar transport. The annual rings visible in cross-sections of temperate-zone trees are a direct product of cambial activity — earlywood (large, thin-walled cells) produced in spring and latewood (smaller, thick-walled cells) produced later in the season create the distinct banding pattern. Damage to the cambium — from mechanical wounds, pests, disease, or improper pruning — directly disrupts the tree's ability to produce new conductive tissue and compartmentalize wounds, which is why protecting the cambium is a central principle in arboricultural practice.

Question 2: Which of the following best describes the process of transpiration in trees?

Movement of sugars from leaves to roots via phloem
Loss of water vapor from leaf stomata driving sap ascent (Correct answer)
Conversion of CO2 to glucose during photosynthesis
Uptake of water by root hair cells through osmosis

Correct answer: Loss of water vapor from leaf stomata driving sap ascent

Transpiration is the evaporation of water from leaf stomata, which creates a tension that pulls water upward through the xylem from roots to leaves — the cohesion-tension mechanism.

Transpiration is the physiological process by which water absorbed by roots is transported upward through the xylem and eventually evaporates through tiny pores called stomata on leaf surfaces. It is the principal driver of the ascent of sap in trees. The cohesion-tension theory explains how this works: water molecules are highly cohesive, forming an unbroken column from roots to leaves. When water evaporates at the leaf surface, it creates a negative pressure (tension) that pulls the water column upward. This process is passive and requires no metabolic energy from the tree — it is driven entirely by solar energy causing evaporation. A mature tree can transpire hundreds of liters of water per day, making transpiration a critical component of the local water cycle and microclimate regulation. Stomata open during daylight to allow CO2 in for photosynthesis but simultaneously allow water out. Guard cells regulate stomatal aperture in response to light, CO2 concentration, temperature, and water availability. For arborists, understanding transpiration is essential when assessing drought stress, planning irrigation, evaluating transplant success, and understanding why large canopy removals can sometimes paradoxically increase water stress by disrupting the hydraulic architecture.

Question 3: What is the CODIT model primarily used to describe?

Carbon dioxide distribution through tree tissue
Compartmentalization of Decay in Trees (Correct answer)
Crown density index for tree vigor
Caliper and diameter increment tracking

Correct answer: Compartmentalization of Decay in Trees

CODIT (Compartmentalization of Decay in Trees), developed by Dr. Alex Shigo, describes the four walls of chemical and physical resistance trees form to contain decay following wounding.

The CODIT model — Compartmentalization of Decay in Trees — was developed by Dr. Alex Shigo at the USDA Forest Service and represents one of the most important conceptual advances in modern arboriculture. It describes how trees respond to wounds and infection by erecting chemical and physical barriers that limit the spread of pathogens and decay. The model identifies four walls of resistance. Wall 1 blocks upward and downward spread through vessels by forming tyloses and gels. Wall 2, the strongest of the axial walls, blocks inward spread through the growth rings present at the time of wounding. Wall 3 resists lateral spread through the rays. Wall 4 — the barrier zone — is the most significant and is formed by new cambial tissue after wounding, separating new wood laid down post-injury from any compromised wood. CODIT fundamentally changed how arborists approach pruning and wounding. It explained why flush cuts are harmful (they remove the branch protection zone and disrupt wall 4), why wound dressings are largely ineffective (they don't accelerate compartmentalization), and why larger wounds on older trees compartmentalize more slowly than smaller wounds on young, vigorous trees. Understanding CODIT guides decisions on pruning timing, wound treatment, cavity management, and tree removal thresholds. A tree's ability to compartmentalize successfully depends on its vigor, genetics, wound size, and the pathogen involved.

Question 4: In tree physiology, what is 'girdling' and what is its primary consequence?

Removal of lateral branches to redirect growth energy
Disruption of the phloem causing starvation of roots (Correct answer)
Pruning of adventitious roots to improve structural stability
Defoliation that reduces photosynthetic capacity temporarily

Correct answer: Disruption of the phloem causing starvation of roots

Girdling involves removal or damage to the phloem layer completely around a tree's circumference, preventing sugars from reaching roots and eventually causing root starvation and tree death.

Girdling refers to the complete interruption of the phloem around the entire circumference of a tree trunk or branch. Because phloem tissue carries photosynthetically produced sugars (primarily sucrose) downward from leaves to roots, roots are entirely dependent on this supply for energy and metabolic function. When the phloem is severed around the full circumference — whether by mechanical damage (wire, deer rubbing, lawn equipment), disease (Phytophthora collar rot), insects (emerald ash borer creating a 'girdle' of galleries), or intentional girdling for tree removal — the flow of carbohydrates is interrupted. Roots continue to respire and consume existing stored energy but receive no new supply. Over months to years depending on root reserves, the root system weakens, dies, and the tree ultimately collapses. Girdling roots are a chronic arboricultural problem where roots growing in a circular pattern around the base of the trunk gradually compress the phloem and cambium as both root and trunk expand, effectively creating a slow girdle. This is a common cause of unexplained decline in urban trees, particularly those planted too deep or in compacted soil that redirects root growth. Partial girdling — affecting only a portion of the circumference — can cause localized canopy dieback corresponding to the affected sector. Arborists must recognize girdling symptoms early, as intervention (root collar excavation, girdle correction) is only effective before extensive damage has occurred.

Question 5: What is the role of lenticels on tree bark?

Producing resin to repel insect borers
Allowing gas exchange between internal tissues and the atmosphere (Correct answer)
Anchoring epiphytes and mosses to the bark surface
Absorbing water during periods of low soil moisture

Correct answer: Allowing gas exchange between internal tissues and the atmosphere

Lenticels are small, porous openings in the bark of woody plants that allow oxygen and carbon dioxide to diffuse in and out of internal living tissues for cellular respiration.

Lenticels are raised, spongy structures visible on the bark of many trees and shrubs, formed from loosely arranged cells with prominent intercellular air spaces. They are the primary means by which living cells within the bark and outer wood exchange gases — oxygen in for aerobic respiration and CO2 out. Unlike leaves, bark does not have stomata. Lenticels fill this role by providing pathways through the otherwise gas-impermeable suberized cork cells of the outer bark. They are often visible as elongated horizontal slits, dots, or raised diamond-shaped patches and are particularly prominent on species like cherry (Prunus), birch (Betula), and elderberry (Sambucus). Lenticels have significant arboricultural relevance in several contexts. In flood-prone environments, trees under prolonged inundation suffer because lenticels become submerged, depriving internal tissues of oxygen. Some flood-tolerant species (like willows and bald cypress) can form adventitious lenticels on submerged parts to maintain gas exchange. Lenticel health also affects a tree's response to bark-invading pathogens — certain canker diseases and boring insects exploit lenticels as entry points. When trees are buried too deeply or mulched against the trunk, lenticel function is compromised. This is a key reason arborists recommend keeping mulch several inches away from the base of trees and avoiding deep planting — practices that directly affect bark respiration through lenticel occlusion.

Question 6: What is the primary purpose of tyloses in xylem vessels?

Conducting water more efficiently during drought stress
Plugging vessels to compartmentalize wounds and slow decay (Correct answer)
Storing non-structural carbohydrates for winter dormancy
Producing ethylene to signal stress responses

Correct answer: Plugging vessels to compartmentalize wounds and slow decay

Tyloses are balloon-like outgrowths from adjacent parenchyma cells that expand into xylem vessels, blocking them to restrict the spread of pathogens, air, and decay — part of the CODIT Wall 1 response.

Tyloses are intrusions of parenchyma cell protoplasts through the pit membranes into adjacent xylem vessels. When a tree is wounded or invaded by pathogens, parenchyma cells adjacent to vessel elements can expand their protoplasts through pit connections into the lumen of the vessel, eventually filling and plugging it completely. This process forms the basis of Wall 1 in the CODIT model — the restriction of upward and downward spread of decay through blocking of the vascular channels. By plugging vessels, the tree effectively limits how far water-borne pathogens, dissolved enzymes, and decay-causing fungi can travel through the vascular system. Tyloses are also responsible for the natural durability of heartwood in some species. White oak (Quercus alba), for example, has vessel elements filled with tyloses, making white oak heartwood highly resistant to water infiltration and decay — a characteristic that makes it valuable for cooperage (wine and whiskey barrels) because it holds liquids without leaking. Red oak lacks extensive tylosis and is thus unsuitable for watertight barrels. From an arboricultural perspective, the speed and completeness of tylosis formation indicates a tree's defensive capability. Young, vigorous trees form tyloses quickly, while stressed trees may fail to do so, leaving them vulnerable to systemic vascular diseases. Dutch elm disease and oak wilt both exploit xylem vessels, and understanding tylosis helps explain why some trees succumb rapidly while others mount effective compartmentalization.

Certified Arborist Test Tree Biology and Structure 2

What is the primary function of the cambium layer in a tree?