Site Index Calculators

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Site index (SI) is the most widely used measure of forest site productivity — the expected height (m or ft) of dominant and codominant trees in a stand at a specified reference age, typically 50 or 100 years. Site index is determined by measuring the heights and ages of dominant trees and using species-specific site index curves or equations to estimate the expected height at base age. High site index indicates a productive site (deep, fertile, moist soil; favorable climate); low site index indicates a poor site (shallow, rocky, dry). Site index is used to select species, estimate timber yield, determine rotation length, and value forest land.

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Site Index Formula and Measurement

SI = f(height, age) — derived from species-specific height-age growth curves. Two approaches: (1) Read SI from published curves: plot the point (age, height) on a species-specific site index curve diagram; read off SI from the curve that passes through that point. (2) Use a polynomial equation: SI = h × (base_age / age)^b (base age correction).

Measuring Site Index in the Field

Select 3–5 dominant or codominant trees (tallest trees in the stand, not suppressed). Measure: total height with a clinometer or laser hypsometer. Age: core with increment borer at breast height (1.3 m); count rings; add correction for growth from ground to 1.3 m (typically 5–10 years). Apply species-specific SI equation or curve. Average the individual tree SI values.

Typical Site Index Values (50-year base, feet)

  • Douglas-fir excellent site: SI₅₀ > 130 ft (~40 m)
  • Douglas-fir average site: SI₅₀ 90–110 ft (27–34 m)
  • Douglas-fir poor site: SI₅₀ < 70 ft (21 m)
  • Ponderosa pine good site: SI₅₀ 80–100 ft
  • Loblolly pine excellent: SI₅₀ > 90 ft

Site Index in Yield Models

Volume yield tables and growth models use SI as the primary driver of growth. Higher SI → faster volume growth → shorter rotation to target volume. SI is also the primary variable in regional forest inventory models (FIA in the US).

Glossary

Site Index (SI)
The height of dominant trees at a reference age (50 or 100 years); the primary measure of forest site productivity; integrates soil, climate, and topographic factors; used in yield models and silvicultural planning.
Dominant/Codominant Trees
The tallest trees in a stand representing site productivity potential; used for site index measurement; not suppressed, damaged, or open-grown trees, which would give misleading SI values.
Rotation Age
The planned age for harvesting a stand; often set to the CMAI age (culmination of mean annual increment); shorter on high-SI sites; affected by economic factors (timber price, discount rate).

Frequently Asked Questions

Site index is the expected height of dominant and codominant trees in a forest stand at a specified reference age (usually 50 or 100 years depending on the species and region). It integrates all soil, climate, and topographic factors affecting tree growth into a single number. Procedure: (1) Measure the height and age of dominant/codominant trees in the stand. (2) Use species-specific site index curves or equations to estimate what height those trees would reach at base age. (3) Report SI in feet or meters at the base age. High SI (e.g., SI₅₀ = 130 ft for Douglas-fir): excellent productivity — deep, fertile, moist soil. Low SI (e.g., SI₅₀ = 60 ft): poor productivity — shallow, rocky, dry, or nutrient-poor site.

Steps: (1) Select 3–5 dominant or codominant trees — the tallest trees in the overstory canopy that represent the site's productive potential. Do not use suppressed, damaged, or open-grown trees. (2) Measure total height with a clinometer, Vertex, or laser instrument. (3) Measure tree age by extracting an increment core at breast height (1.3 m) and counting annual growth rings. Add the number of years to grow from the ground to 1.3 m (typically 5–15 years depending on species and site). (4) Apply species-specific site index equation or plot on site index curves. (5) Average SI across 3–5 trees. Accuracy: typically ±5–10 SI units (±1.5–3 m); increment core errors and height measurement errors are the main sources of uncertainty.

Site index directly measures tree growth — it integrates all factors affecting productivity (soil depth, texture, drainage, fertility, climate, aspect) into a single, biologically meaningful number. Soil description: qualitative; multiple properties need to be combined somehow; relationships between soil properties and tree growth are complex and species-specific; two sites with identical soil profiles may have very different productivity due to microclimate, frost pockets, or aspect. Site index: quantitative; species-specific curves exist; directly enters yield tables and growth models; measured in the field in 1–2 hours. Limitation: site index assumes the current stand reflects site potential — recently disturbed stands, plantations on converted agricultural land, or very young stands may give misleading SI values.

Rotation age (the age at which to harvest) is often set to the age of culmination of mean annual increment (CMAI) — when average annual volume growth per year is maximized. On high-SI sites: trees grow faster; CMAI is reached earlier (shorter rotation). On low-SI sites: trees grow slower; CMAI is reached later (longer rotation). Example: Douglas-fir SI₅₀ = 130 ft → CMAI at ~55–65 years; SI₅₀ = 90 ft → CMAI at ~75–85 years. Economic rotation may differ from biological rotation (CMAI): include stumpage price, silvicultural costs, and discount rate. High-SI sites justify intensive management (planting, thinning, fertilization) because the investment pays off faster.