Tree Biology and Physiology Cheat Sheet for ISA Candidates

Tree Biology is 10% of the ISA Certified Arborist exam, about 20 of the 200 questions, and it underpins the Diagnosis and Treatment and Pruning domains as well. The single most tested concept is CODIT: trees compartmentalize damage by walling it off, they do not heal wounds. That's why pruning cuts have to stay outside the branch collar instead of flush against the trunk.

Domain Review: This topic is one of the 10 domains covered on the ISA Certified Arborist exam.

Tree Biology is the engine under the hood. Understand how a tree actually works and you can reason out a lot of Pruning, Diagnosis and Treatment, and Tree Risk Management questions even when you don't remember the specific rule being tested.

Tree Biology on the ISA Exam: Why 10% Undersells It

Ten percent sounds small next to Pruning's 16% or Safe Work Practices' 15%. It's misleading. Diagnosis and Treatment (12%) is mostly biology applied to symptoms. Soil Management (12%) is mostly biology applied to roots. Tree Risk Management (11%) leans on reaction wood and decay behavior to judge failure potential. Get CODIT or xylem/phloem function wrong and you lose points across four domains that together make up close to half the exam.

The Big Three Processes

1. Photosynthesis: the tree makes food. Carbon dioxide, water, and light go in; sugar and oxygen come out, inside the leaf's chloroplasts.
2. Respiration: the tree burns that food to grow and maintain itself. This runs continuously, day and night, in every living cell, including roots buried in the dark.
3. Transpiration: the tree moves water. Water evaporates out through pores in the leaf called stomata, and that evaporative pull drags a column of water up from the roots, the same way pulling on the top of a straw draws liquid up from the bottom.

Photosynthesis only happens in daylight. Respiration never stops. A tree gains net energy only when photosynthesis is outpacing respiration, which is one reason heavy crown loss is so costly: you've cut the income side of the ledger while the expenses keep running.

Anatomy: The Pipeline

• Xylem (wood): carries water and dissolved minerals up from the roots. The outer, still-living rings that do this work are sapwood; the older rings toward the center, no longer conducting, are heartwood, there for structural support only.
• Phloem (inner bark): carries sugar down from the leaves to the roots and to any other tissue that needs it. Phloem sits just outside the cambium, under the outer bark.
• Cambium: a thin layer of actively dividing cells sandwiched between xylem and phloem. It produces new xylem to the inside and new phloem to the outside, which is how a trunk adds a growth ring every year.

Girdling, whether from a wire left on a stake, a string trimmer, or a root wrapped around the trunk, damages phloem first because phloem sits closest to the surface. A girdled tree can often still pull water up through undamaged xylem for a while, but the roots below the girdle stop receiving sugar and slowly starve. That's why a tree can look fine in the crown for a season or two after girdling starts, then decline hard once the root system runs out of reserves.

CODIT: How Trees Wall Off Damage

Trees don't heal the way animals do. Damaged wood doesn't get repaired or replaced. It gets sealed off, and stays sealed off, for the rest of the tree's life. That's the whole idea behind CODIT, compartmentalization of decay in trees:

• Wall 1 resists vertical spread, up and down the grain, by plugging the xylem vessels nearest the wound.
• Wall 2 resists inward spread, toward the center of the tree, using the existing growth rings as a barrier.
• Wall 3 resists lateral, side-to-side spread, using ray cells that run radially through the wood.
• Wall 4 is the strongest of the four: new wood the cambium lays down after the injury, forming a barrier between the wood that existed at the time of the wound and everything the tree grows afterward.

Wall 4 is the one worth remembering if you only remember one, because it's the reason the branch collar matters so much in pruning. The branch collar carries tissue specialized for laying down a strong Wall 4 response. Cut into it with a flush cut and you remove the tree's ability to compartmentalize that wound well, no matter how clean the cut looked walking away.

Reaction Wood

A leaning or wind-loaded tree grows extra wood to reposition itself, and which type it grows depends on what kind of tree it is.

• Hardwoods (broadleaf trees) grow tension wood on the upper side of the lean. Tension wood contracts as it matures, pulling the stem back toward vertical the way a cable under tension would.
• Conifers (softwoods) grow compression wood on the lower side of the lean. Compression wood pushes outward as it matures, propping the stem up from underneath the way a strut would.

Feature	Compression Wood	Tension Wood
Found in	Conifers (softwoods)	Hardwoods (broadleaf trees)
Forms on	Underside of the lean	Upper side of the lean
Mechanical action	Pushes and props the stem up	Pulls the stem back upright

Roots

Most of a tree's roots live in the top 12 to 18 inches of soil, spreading well beyond the drip line, the edge of the canopy. That shallow, wide footprint is the whole reason soil compaction and construction damage are such a big deal: the roots doing most of the water and nutrient absorption aren't buried deep where equipment can't reach them. They're right where foot traffic, grading, and trenching happen.

Mycorrhizal fungi partner with fine roots and extend the root system's effective reach well beyond what the roots could cover alone, in exchange for sugar. Don't kill that relationship with a fungicide drench you didn't need. If a struggling tree's real problem turns out to be compacted or contaminated soil rather than something above ground, you'll find more of that reasoning in the soil management domain.

Biology in the Field: Three Scenarios Worth Knowing

The exam likes to describe a situation and make you supply the biological reason behind it. Three come up often enough to work through.

A hardwood is leaning hard after a storm

You're called out to a mature oak that's now leaning noticeably after a wind event, still alive, roots apparently intact. In the months since, you'd expect to find tension wood forming on the upper side of the lean, denser and slightly different in grain from the wood around it, as the tree works to pull itself back toward vertical. If this were a leaning pine instead of an oak, you'd be looking for compression wood on the underside doing the opposite job, propping rather than pulling. Getting this backwards, hardwood versus conifer, upper side versus lower side, is one of the more common ways this topic trips people up on the exam.

A maple has a root wrapped around its own trunk

A stem girdling root has grown across the base of the trunk instead of away from it, and as both the root and the trunk thicken, the root slowly cuts into the trunk's own phloem at that contact point. Because phloem carries sugar down to the roots, the tissue on the far side of the girdle, including the roots below it, gets progressively cut off from its food supply, even while the crown may still look reasonably full. Left alone, the girdling point typically gets worse every year as both pieces of wood grow thicker against each other, which is why arborists correct stem girdling roots while they're still small and easy to cut, rather than waiting for obvious crown decline to show up.

A driveway gets poured inside the drip line

A contractor compacts soil and pours concrete over what used to be root zone for a mature tree, well inside the drip line where most of the absorbing roots were living. Compacted soil has less pore space, which means less oxygen for roots that need it for respiration, the same process running in every living cell of the tree. Roots suffocating for oxygen can't take up water or minerals well even when the soil itself still has moisture in it, and the mycorrhizal relationships in that soil get disrupted along with everything else. The tree may not show obvious crown symptoms right away, which is exactly why protecting root zones before construction starts matters more than treating the damage afterward.

Summary

Biology is logical once you connect it to consequence. Pruning removes sugar factories (leaves). Compaction suffocates sugar burners (roots). Girdling cuts the pipeline between the two. Every practice question in this domain is really asking whether you can trace that chain from cause to effect.

The Big Three Processes at a Glance

Process	What It Does	Where
Photosynthesis	Converts light, water, and CO₂ into sugars	Leaves
Respiration	Burns sugars to release energy, continuously	All living cells
Transpiration	Water loss through stomata; drives water uptake	Leaves

Frequently Asked Questions

How much of the ISA exam is tree biology, and why does it matter more than that number suggests?

Tree Biology is formally 10% of the exam, about 20 of the 200 questions, but the same concepts, CODIT, xylem and phloem function, root behavior, resurface inside Diagnosis and Treatment, Soil Management, and Tree Risk Management.

What does CODIT mean?

Compartmentalization of decay in trees. Trees respond to wounding by forming chemical and physical boundaries that wall off damaged tissue. They do not regenerate or heal the damaged wood. They isolate it and grow new wood around it instead.

Do trees heal their wounds?

No. Trees compartmentalize rather than heal. New tissue grows over a wound, but the damaged wood inside stays exactly as damaged as it was the day it was wounded. This is why cutting into the branch collar is so harmful: it breaches the boundary that limits how far decay can spread.

What is the difference between xylem and phloem?

Xylem transports water and minerals upward from the roots, through sapwood, the outer living rings of wood. Phloem transports sugars produced in the leaves down to the roots and everywhere else that needs them. Girdling damages phloem first, since it sits closer to the surface, which is why it starves a tree from the roots up rather than the crown down.

What is reaction wood?

Wood a tree forms to reposition a leaning stem or branch. Conifers form compression wood on the underside of the lean to prop it up; hardwoods form tension wood on the upper side to pull it back upright. Which type forms on which side, in which kind of tree, is commonly tested.

Where are most tree roots located?

In the top 12 to 18 inches of soil, extending well beyond the drip line. This shallow, wide distribution is why compaction and construction damage within the root zone are so consequential. The roots doing most of the absorbing work simply aren't deep enough to be out of reach.

What is a stem girdling root, and why is it dangerous?

A root that has grown across the trunk's base instead of away from it. As both pieces thicken, the root cuts into the trunk's phloem at the contact point, slowly cutting off sugar supply to the roots on the far side of the girdle. It's dangerous because it worsens every year as both grow, and because the crown can still look healthy for a while before decline becomes obvious.

What's the difference between sapwood and heartwood?

Sapwood is the outer, still-living wood actively conducting water up from the roots. Heartwood is the older wood toward the center of the trunk that no longer conducts water and exists purely for structural support. Both are xylem. The difference is function and age, not a different tissue type.

Why does soil compaction hurt tree roots?

Compacted soil has less pore space, which means less oxygen available for root respiration, the same continuous process that keeps every living cell in the tree running. Roots that can't respire properly also struggle to take up water and minerals even when soil moisture is adequate, and it disrupts the mycorrhizal fungi that would otherwise help.

How do you tell whether a leaning tree is a hardwood or a conifer just from its reaction wood?

Check which side of the lean the reaction wood is on. Tension wood on the upper side means you're looking at a hardwood pulling itself upright. Compression wood on the underside means you're looking at a conifer propping itself up from below. Mixing up which side goes with which growth habit is one of the more common ways this concept trips people up on the exam.