Forestry
What the forestry paper tests
The Forestry section tests silviculture, forest ecology, measurement, nursery practice, conservation and agroforestry. You should be able to compute basal area, standing volume and tree density, interpret DBH and form factor, and match silvicultural systems to objectives.
Core rules
- DBH is diameter at breast height, conventionally 1.3 m above ground; measure diameter, not radius, and keep it in metres for volume formulae.
- Basal area per tree = pi/4 x d^2. Stand basal area = sum of tree basal areas per unit area, often m2/ha.
- Standing volume = basal area x height x form factor. The form factor corrects for taper; 0.5 is a common approximation, not a universal constant.
- Even-aged systems use clear-felling, seed-tree or shelterwood; uneven-aged systems use selection. Regeneration method must match shade tolerance and seed source.
- In nurseries, root-collar diameter and seedling height are better stock-quality indicators than age alone.
Worked example
A tree has DBH 30 cm and height 20 m; use form factor 0.5. Volume? d = 0.30 m. Basal area = pi/4 x (0.30)^2 = 0.0707 m2. Volume = 0.0707 x 20 x 0.5 = 0.707 m3. If DBH were mistakenly used as 30 (cm) without conversion, the answer would be 100 times too large. If the question gives radius, square it directly; do not divide by 4 again.
Traps
Using diameter in centimetres in the volume formula; confusing radius and diameter; treating form factor as 1.0; measuring DBH at the stump; adding basal areas without converting plot size to hectares; assuming selection cutting suits all shade-intolerant species.