Tree Biology and Physiology Cheat Sheet for ISA Candidates

Domain Review: This topic is one of the [10 ISA exam domains](/blog/isa-arborist-exam-domains-explained) you must master.

Tree Biology (10%) is the engine under the hood. If you understand how a tree works, you can guess the answer to almost any pruning or diagnosis question.

The Big Three Processes

1. Photosynthesis: Making food.
   • Inputs: CO2 + Water + Light.
   • Outputs: Sugar (Energy) + Oxygen.
2. Respiration: Using food.
   • The tree burns the sugar to grow. This happens 24/7 (roots respire all night).
3. Transpiration: Moving water.
   • Water evaporates from leaves (stomata), pulling a column of water up from the roots like a straw.

Anatomy: The Pipeline

• Xylem (Wood): Carries Water UP from roots. (Think "Xy-High").
• Phloem (Inner Bark): Carries Sugar DOWN (and around) from leaves. (Think "Phloem-Food").
• Cambium: The thin layer between Xylem and Phloem that makes new cells.

CODIT (Compartmentalization Of Decay In Trees)

Trees don't "heal" (restore tissue); they "seal" (wall off damage).

• Wall 1: Resists vertical spread (Plugging vessels).
• Wall 2: Resists inward spread (growth rings).
• Wall 3: Resists lateral spread (Ray cells).
• Wall 4: The Strongest Wall. New wood that grows after the injury to seal it off.

Reaction Wood

Trees grow special wood to hold themselves up when leaning.

• Broadleaf (Hardwoods): Grow Tension Wood on the upper side of the lean (Pulling it up).
• Conifers (Softwoods): Grow Compression Wood on the lower side of the lean (Propping it up).

Roots

• Absorbing Roots: Tiny, fine roots near the surface (top 6-12 inches) that drink water/nutrients.
• Mycorrhizae: A beneficial fungus that infects roots and increases their ability to drink water. DO NOT kill this with fungicide!

Summary

Biology is logical. Pruning removes "sugar factories" (leaves). Compaction kills "sugar burners" (roots). Connect every practice back to biology.