ISA Tree Biology & Physiology 3

Question 1

What is reaction wood and what are the two types found in trees?

Accepted Answer

Abnormal wood that forms in response to mechanical stress; tension wood in hardwoods (on upper side of leaning stems) and compression wood in conifers (on lower side)

Answer

Reaction wood forms in response to gravitational or mechanical loading. Hardwoods produce tension wood on the upper side of leaning stems (pulling it upright), while conifers produce compression wood on the lower side (pushing it upright). Both have altered cell structure and properties.

Question 2

How do trees allocate carbon resources under stress, and what does this mean for management decisions?

Accepted Answer

Trees prioritize maintenance respiration and defense over growth, meaning stressed trees have reduced capacity to compartmentalize wounds and respond to additional stressors like pruning

Answer

Under stress, trees shift carbon allocation from growth to maintenance and defense. This means stressed trees close wounds more slowly, are more vulnerable to secondary pathogens, and are less able to tolerate additional stress from heavy pruning or soil disturbance.

Question 3

What is apical dominance and how does removal of the apical meristem affect tree form?

Accepted Answer

Apical dominance is hormonal suppression of lateral bud growth by auxin from the terminal bud; removing it releases lateral buds, causing bushier growth or multiple leaders

Answer

The terminal (apical) bud produces auxin that flows downward and suppresses the growth of lateral buds below it. When the apical meristem is removed (by pruning, storm damage, or topping), this suppression is released, stimulating multiple lateral buds to grow vigorously.

Question 4

What is the role of the Casparian strip in root water uptake?

Accepted Answer

It is a waxy barrier in the endodermis that forces water and dissolved minerals to pass through living cells, allowing the root to selectively control which ions enter the xylem

Answer

The Casparian strip is a band of suberin and lignin in the radial and transverse walls of endodermal cells in roots. It blocks the apoplastic (cell wall) pathway, forcing water and solutes to cross the plasma membrane of endodermal cells, enabling selective ion control.

Question 5

How does phloem loading work and why is it important for understanding tree carbohydrate distribution?

Accepted Answer

Phloem loading is the active transport of sugars into sieve tube elements at source tissues, creating an osmotic gradient that drives the pressure flow of photoassimilates to sink tissues throughout the tree

Answer

At photosynthetic source tissues (primarily mature leaves), sugars are actively pumped into sieve tube elements of the phloem, creating high solute concentration that draws water in osmotically, generating positive pressure that drives flow toward lower-concentration sinks.

(ISA) ISA Certified Arborist Practice Test

(ISA) ISA Certified Arborist Practice Test

ISA Tree Biology & Physiology 3