Tree Biology
Core concepts
Trees grow only at meristems: apical meristems add length, the vascular cambium adds diameter (xylem inward, phloem outward), and the cork cambium produces periderm. Only the outermost sapwood conducts water; inner heartwood provides support and stored reserves. Roots are shallow and wide - most fine roots sit in the top 30–45 cm (12–18 in) of soil, and structural roots commonly extend 1.5–3 times the canopy radius. Carbohydrates from leaves travel in phloem to roots and storage tissues; water and minerals rise in xylem, driven largely by transpiration.
Trees do not heal; they compartmentalize. Shigo's CODIT model names four walls: wall 1 (plugged vessels, weakest), wall 2 (plugged tracheids and resin), wall 3 (rays), and wall 4, the barrier zone formed by the cambium after wounding - generally the strongest. Reaction wood (tension wood in angiosperms, compression wood in conifers) lets leaning stems right themselves.
Traps
- 'Trees heal wounds' - false; they wall off decay, and it can persist behind the barrier zone.
- Reversing wall strength: wall 4 > wall 3 > wall 2 > wall 1.
- Mixing up xylem (water up) and phloem (sugars, both directions).
- Assuming roots mirror the crown or stop at the drip line.
Rule of thumb
For a 60 cm (24 in) DBH tree, expect structural roots within roughly 9–18 m (30–60 ft) and most absorbing roots in the top 45 cm. Protect that volume, not just the drip line.