Tree Biology and Health: Growth, Defense, and Site Diagnosis
Tree Biology and Health tests the physiology behind tree responses: how trees grow, defend wounds, obtain water and minerals, and decline when site conditions change.
Core rules and concepts
- Secondary growth comes from the vascular cambium: xylem inward, phloem outward. Roots and shoots grow from meristems. Root systems often extend well beyond the crown, and most fine roots are in the upper 12–18 inches (30–45 cm) of soil.
- Photosynthesis supplies carbohydrates; transpiration pulls water through xylem. Stored carbohydrates in stems and roots fuel defense and refoliation. Pruning removes stored energy and photosynthetic area, so timing and dose matter.
- Trees compartmentalize decay through CODIT walls-plugging, reaction zone, and barrier zone-rather than “healing.” Wounds close by callus. Flush cuts and topping injure the defense zone.
- Soil compaction, grade change, and burial of the root flare reduce oxygen and water. Diagnose abiotic disorders before treating pests; fertilizer is not a cure for construction damage.
- Plant at correct depth with the root flare at grade, a wide mulch ring, and no mulch volcano.
Worked example
A 25-year-old red maple at a park edge has sparse chlorotic foliage and dieback. The lawn was regraded three years ago, raising soil 8 inches around the trunk. The arborist finds a buried root flare, compacted soil, and no pest outbreak.
Reasoning: the primary disorder is abiotic-root oxygen loss and a buried flare-not a nutrient deficiency. Fertilizing would not fix the grade. The plan is to excavate the flare, remove excess soil, aerate and mulch the root zone, prevent further compaction, and monitor. Correct soil pH or phosphorus only if a soil test supports it after the physical fix.
Traps that cost marks
- Believing topping invigorates a tree.
- Calling flush cuts “healing cuts.”
- Mulching against the trunk in a volcano.
- Fertilizing before diagnosing the cause.
- Assuming roots stop at the dripline.