18 Tree Biology Practice Questions & Answers

1. 1. What distinguishes the fine absorbing roots from the woody structural roots of an established tree?

   • A.Absorbing roots hold most of the tree's starch reserve through winter
   • B.Absorbing roots carry sugars downward from the crown into the soil
   • C.Absorbing roots anchor the tree and thicken through each growing season
   • D.Absorbing roots take up most water and minerals and are short-lived✓ Answer

   Absorption is done by fine non-woody roots and their root hairs, which are single epidermal cell extensions that live only days or weeks and are continually replaced. Anchorage, transport and the bulk of starch storage belong to the woody structural roots, which is why losing fine roots to drought or compaction shows up as crown stress long before anchorage is affected.

   Source: ISA Arborists' Certification Study Guide — root types and their functionsReport a problem with this question

2. 2. What does a mycorrhizal association provide to the tree?

   • A.A greatly enlarged absorbing surface that improves water and phosphorus uptake✓ Answer
   • B.A protective sheath that seals roots against soil moisture loss
   • C.A supply of sugars that supplements the tree's own photosynthesis
   • D.A nitrogen-fixing partnership that converts air nitrogen for the roots

   A mycorrhiza is a mutualism in which fungal hyphae, far finer than root hairs, explore a much larger volume of soil and deliver water and poorly mobile nutrients such as phosphorus to the root. The exchange runs the other way for carbon: the tree supplies the fungus with photosynthate, so the relationship costs the tree energy rather than supplying it.

   Source: ISA Arborists' Certification Study Guide — mycorrhizae and root-fungus associationsReport a problem with this question

3. 3. A ring of bark is removed all the way around a young trunk. Which transport is interrupted, and why?

   • A.Movement of water from the roots upward, because xylem sits just inside the bark
   • B.Movement of sugars from the leaves down, because phloem sits just inside the bark✓ Answer
   • C.Movement of water from the leaves downward, because xylem sits within the heartwood
   • D.Movement of sugars from the roots upward, because phloem sits inside the sapwood

   Phloem, the inner bark, is the outermost living conducting tissue and carries photosynthates from the leaves to roots and other sinks, so a complete ring of bark removal severs it immediately. Water continues to rise for a time in the xylem beneath the cut, which is why a girdled tree can look normal for a season before the starved root system fails.

   Source: ISA Arborists' Certification Study Guide — vascular tissues and translocation of photosynthatesReport a problem with this question

4. 4. A metal tag is fastened to a trunk 1.5 m above the ground. Ten years later, where is the tag?

   • A.Still at 1.5 m, because stems elongate only at their shoot tips✓ Answer
   • B.Well above 1.5 m, because new wood is added at the base of the trunk
   • C.Still at 1.5 m, because the cambium adds new wood only inward
   • D.Well above 1.5 m, because the whole trunk elongates as the tree grows

   Increase in height is primary growth and happens only at apical meristems in the shoot tips, so wood already formed never stretches or moves upward. The cambium adds girth at that point, so the tag stays at the same height and is gradually engulfed as the trunk thickens around it.

   Source: ISA Arborists' Certification Study Guide — primary growth at apical meristems versus secondary growth at the cambiumReport a problem with this question

5. 5. Why is the branch collar the correct place to end a branch removal cut?

   • A.It is where branch wood and trunk wood grow as one continuous unit
   • B.It holds the reserves the tree needs to seal the exposed wound
   • C.It contains overlapping trunk tissue and the branch protection zone✓ Answer
   • D.It carries the thinnest bark on the limb, so the wound closes fastest

   Branch and trunk wood are not continuous: trunk tissue grows around and overlaps the base of the branch each year, and at that junction the tree maintains a chemically protective branch protection zone. Cutting just outside the collar leaves that zone intact so decay stays in the branch stub, while a cut inside it opens a path straight into the trunk.

   Source: Shigo, Modern Arboriculture — branch attachment, branch bark ridge and the branch protection zoneReport a problem with this question

6. 6. Repeated heavy defoliation weakens a tree mainly because:

   • A.the phloem reverses direction and pulls sugars back to the leaves
   • B.stored starch is spent refoliating before new leaves can repay it✓ Answer
   • C.the wounds that the insect leaves let decay fungi enter the trunk
   • D.the roots stop respiring once the crown has lost its leaf surface

   Refoliation is funded entirely from starch held in living parenchyma of roots, trunk and branches, and the replacement leaves must operate for a long time before that withdrawal is repaid. Each additional defoliation draws the account down further, and because energy goes to maintenance and new growth ahead of defense and storage, the tree's ability to compartmentalize wounds falls with it.

   Source: ISA Arborists' Certification Study Guide — carbohydrate storage and energy allocationReport a problem with this question

7. 7. A mature tree in severe decline can keep losing energy reserves even while it is in full leaf. Why?

   • A.Its respiration demand exceeds what its reduced crown can produce✓ Answer
   • B.Its photosynthesis stops entirely once the tree has begun to decline
   • C.Its heartwood consumes the sugars that the crown continues to make
   • D.Its leaves lose sugars to the air through open stomata every night

   Net energy is what photosynthesis produces minus what respiration in all the living tissue consumes, and a large tree with a thin, small-leaved crown supports a great deal of living tissue on very little production. When that balance goes negative the shortfall comes out of stored starch, so reserves fall season after season even though the tree looks leafy.

   Source: ISA Arborists' Certification Study Guide — net energy balance of photosynthesis and respirationReport a problem with this question

8. 8. What moves water from the roots to the leaves of a tall tree?

   • A.Rhythmic contractions of the cambium, squeezing the sap upward
   • B.Tension from evaporation at the leaves, pulling a cohesive column✓ Answer
   • C.Capillary rise alone, driven by adhesion to the walls of vessels
   • D.Pressure generated in the roots, pushing water up through the xylem

   Under the cohesion-tension explanation, evaporation from leaf surfaces creates a negative pressure that is transmitted down an unbroken water column held together by cohesion between water molecules and adhesion to conduit walls. The column is therefore pulled from above rather than pushed from below; root pressure exists but is far too weak to lift water to the top of a tall tree.

   Source: ISA Arborists' Certification Study Guide — cohesion-tension theory of water movementReport a problem with this question

9. 9. Soil moisture drops sharply and a tree's stomata close. What is the immediate consequence?

   • A.Respiration halts because oxygen can no longer enter the leaf
   • B.Phloem transport reverses and sugars move back to the crown
   • C.Photosynthesis slows because carbon dioxide cannot enter the leaf✓ Answer
   • D.Root pressure rises and replaces the lost transpirational pull

   Stomata are the same doorway for water vapour leaving and carbon dioxide entering, so closing them to conserve water also shuts off the raw material for photosynthesis. That is why drought costs a tree carbohydrate production long before any wilting or scorch is visible, and why root loss produces the same effect even in moist soil.

   Source: ISA Arborists' Certification Study Guide — stomatal regulation, transpiration and water stressReport a problem with this question

10. 10. The central leader of a young conifer is broken out. What change in form should be expected?

    • A.Growth shifts entirely to the roots until a new leader has formed
    • B.The crown becomes narrower as the remaining branches turn upward
    • C.The tree keeps a single leader, because crown form is genetically fixed
    • D.Lateral branches compete to become leaders, giving a forked crown✓ Answer

    The terminal bud suppresses the growth of lateral buds below it, and that apical dominance is what produces the single-stemmed excurrent form typical of conifers. Remove the leader and the suppression is lifted, so several laterals turn upward and compete, producing codominant stems and a more decurrent, forked crown.

    Source: ISA Arborists' Certification Study Guide — apical dominance and excurrent versus decurrent formReport a problem with this question

11. 11. Two trees of one species, one young and one veteran, both need work. Why is the veteran managed more conservatively?

    • A.Its denser wood makes every pruning cut close far more slowly
    • B.Its reserves and its ability to wall off wounds are both lower✓ Answer
    • C.Its cambium has gone dormant and no longer produces new wood
    • D.Its root system has stopped growing and cannot replace lost tissue

    A veteran tree has slower shoot extension, a smaller ratio of live foliage to living tissue that must be maintained, and correspondingly thinner starch reserves, so it both closes wounds slowly and compartmentalizes them less effectively. Structural training that is appropriate on a young tree therefore becomes a small-dose, live-foliage-preserving approach on the veteran.

    Source: ISA Arborists' Certification Study Guide — tree developmental stages and management of mature and veteran treesReport a problem with this question

12. 12. What does a tree do with wood that has been wounded and infected?

    • A.It replaces the damaged wood with healthy new tissue in place
    • B.It dissolves the infected wood and reabsorbs the material
    • C.It pushes the damaged wood outward until the bark sheds it
    • D.It walls the damaged wood off and grows new wood over it✓ Answer

    Trees do not heal or regenerate injured wood; they compartmentalize it, setting chemical and physical boundaries around the damaged column and then adding new, clean wood outside those boundaries. Woundwood closes over the surface, but the injured and decayed wood stays permanently inside the trunk, which is why an old wound remains a structural consideration long after it looks closed.

    Source: Shigo and Marx, compartmentalization of decay in trees (CODIT)Report a problem with this question

13. 13. Among the four walls a tree forms around a wound, which is weakest and which is strongest?

    • A.Wall 4 is the weakest and Wall 1 is the strongest
    • B.Wall 1 is the weakest and Wall 4 is the strongest✓ Answer
    • C.Wall 3 is the weakest and Wall 2 is the strongest
    • D.Wall 2 is the weakest and Wall 3 is the strongest

    Wall 1 resists movement of decay up and down the stem by plugging conducting elements, and because those conduits run vertically it is the least effective of the four. Wall 4, the barrier zone laid down by the cambium after wounding, is the strongest, which is why decay from an old injury tends to spread along the stem rather than outward into wood formed after the wound.

    Source: Shigo and Marx, compartmentalization of decay in trees (CODIT) — the four wallsReport a problem with this question

14. 14. What is distinctive about the fourth wall compared with the other three?

    • A.It is formed from ray cells that were already present before the injury
    • B.It resists movement of decay up and down the stem above all else
    • C.It is the weakest of the four and yields first to advancing decay
    • D.It is laid down by the cambium after injury, separating old wood from new✓ Answer

    The first three walls are made by modifying tissue that already existed when the injury occurred, whereas the fourth is a barrier zone the cambium produces afterwards, separating all wood present at wounding from all wood formed later. It is the strongest of the four, but the discontinuity it creates can become a weak plane where cracking or ring shake develops.

    Source: Shigo and Marx, compartmentalization of decay in trees (CODIT) — the barrier zoneReport a problem with this question

15. 15. A pruning cut is made flush against the trunk. What is the biological consequence?

    • A.The wound closes faster because callus can form evenly on all sides
    • B.The branch protection zone is removed and decay reaches the trunk✓ Answer
    • C.The trunk lays down a barrier zone that seals the cut immediately
    • D.The cut heals over cleanly once a wound dressing is applied to it

    A flush cut removes the collar and with it the branch protection zone, so the boundary that would have confined decay to the branch base is gone and the wound opens directly into trunk tissue. The resulting injury is also larger and elongated, closes more slowly, and cannot be corrected by a dressing, since wound paints do not speed closure or stop decay.

    Source: Shigo, Modern Arboriculture, and ISA Best Management Practices for Tree Pruning — flush cuts and wound dressingsReport a problem with this question

16. 16. A leaning hardwood tree develops reaction wood. Where does it form and what does it do?

    • A.On the upper side, pulling the stem back toward upright✓ Answer
    • B.On the lower side, pushing the stem back toward upright
    • C.On the upper side, pushing the stem further from upright
    • D.On the lower side, holding the stem at its current angle

    Hardwoods form tension wood on the upper side of a leaning stem or branch, and it contracts as it matures so that it pulls the stem toward vertical. Conifers do the opposite, laying down compression wood on the lower side that pushes the stem up, so the side you see extra wood on tells you which group of tree you are looking at.

    Source: ISA Arborists' Certification Study Guide — reaction wood: tension wood in angiosperms, compression wood in gymnospermsReport a problem with this question

17. 17. Why should nursery stakes be removed once a newly planted tree is established?

    • A.Stakes attract lightning toward the trunk during storms
    • B.Movement in the wind builds trunk taper and stronger root anchorage✓ Answer
    • C.Stakes shade the lower trunk and slow the growth of new bark
    • D.Movement in the wind is what triggers the tree's spring bud break

    Trees respond to the mechanical strain of swaying by growing shorter, stouter and more tapered stems and by investing in root anchorage, a response known as thigmomorphogenesis. A tree held rigid by stakes never receives that signal, so it develops a slender, poorly tapered trunk and weak anchorage and is more likely to fail once the support finally comes off.

    Source: ISA Arborists' Certification Study Guide — thigmomorphogenesis, trunk taper and staking of newly planted treesReport a problem with this question

18. 18. A bulge of extra wood has formed on one side of a trunk above an old wound. What does this indicate?

    • A.The tree has stopped growing everywhere except at that point
    • B.The tree has restored the strength that the old wound removed
    • C.The tree is storing surplus starch in a swelling of the bark
    • D.The tree has added wood where load on the stem is highest✓ Answer

    Trees allocate secondary growth to the places where mechanical stress is greatest, so bulges, ribs and ridges are adaptive growth marking where the stem is working hardest. The swelling is a clue that an internal defect is being loaded, not evidence that the defect has been repaired, so it should prompt closer examination rather than reassurance.

    Source: ISA Arborists' Certification Study Guide — adaptive growth and the axiom of uniform stressReport a problem with this question

Practice questions based on the published knowledge domains of the ISA Certified Arborist examination outline and on standard arboriculture references. ISA and Certified Arborist are marks of the International Society of Arboriculture; this site is not affiliated with or endorsed by ISA. Answers here deliberately avoid pesticide products and rates, standard clause numbers and the dimensions recalled from them, regional species recommendations and local ordinances, all of which change and vary by place — always follow the pesticide label and your jurisdiction's applicator licensing, the tree-care safety standards in force, and the requirements of the authority having jurisdiction. Confirm current exam requirements with ISA before testing. About the ISA Certified Arborist credential →