Timber Yield Calculators

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Timber yield refers to the volume of merchantable wood that can be harvested from a forest stand, typically expressed in cubic meters per hectare (m³/ha) or board feet per acre. Yield depends on species composition, stand age, site quality (site index), stocking density, and management history. Forest managers use yield tables, variable-density growth models, and empirical equations to project future stand volume and plan harvest schedules. Sustainable yield management ensures that harvests do not exceed long-term productivity, maintaining forest carbon stocks and biodiversity.

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Site Index

Site index (SI) is the most important measure of forest site quality, defined as the average height of dominant and co-dominant trees at a reference age (commonly 50 or 100 years). Higher SI indicates better growing conditions — more productive soils, favorable climate, adequate moisture. SI is determined from height-age curves and is used as the primary input variable in yield models.

Normal Yield Tables

Normal yield tables relate stand age and SI to volume per hectare under the assumption of full stocking. Entries are in the form: for a Douglas-fir stand with SI 30 at age 60, the normal yield is approximately 600 m³/ha. Actual yields are corrected with stocking density (basal area / normal basal area) to produce empirical or managed yield estimates.

Stand Basal Area

Basal area (BA) is the cross-sectional area of all tree stems per unit of land area, measured at breast height (1.3 m). BA (m²/ha) = Σ π(DBH/2)²/10,000. BA is the primary measure of stand density and a key driver of volume yield. Normal stocking for managed conifer stands typically ranges from 20–40 m²/ha depending on species and age.

Mean Annual Increment (MAI)

MAI = cumulative volume / stand age. MAI rises to a maximum (MAI_max) then declines as the stand matures. Harvesting at MAI_max age maximizes average annual volume production — this is the basis of the rotation age concept in even-aged forest management.

Glossary

Site Index (SI)
The average height of dominant trees at a standard reference age (usually 50 or 100 years); the primary measure of forest site productivity for timber production.
Mean Annual Increment (MAI)
Cumulative stand volume divided by stand age; reaches a maximum at the biologically optimal rotation age for sustained timber production.
Basal Area (BA)
The sum of cross-sectional areas of all tree stems per unit land area (m²/ha), measured at breast height; a key measure of stand density and predictor of timber volume.

Frequently Asked Questions

Site index (SI) is the average height of dominant and co-dominant trees at a standard reference age — typically 50 years for hardwoods and some conifers, or 100 years for long-lived conifers. It is the primary measure of site productivity for timber production. Higher SI means faster growth and greater volume yield. SI is determined by measuring tree height and age (from increment borings) and looking up the corresponding SI from species-specific height-age curves.

Mean annual increment (MAI) = cumulative stand volume / stand age. It rises from zero, reaches a maximum, then declines as tree growth slows in older stands. The rotation age that maximizes long-term timber production is the age at which MAI peaks. Harvesting before this age leaves wood volume on the table; harvesting after this age means slower average growth. MAI is a fundamental concept in sustained-yield forest management for optimizing timber output per unit area over time.

Basal area (m²/ha) is the sum of cross-sectional areas of all tree stems at breast height. Volume is estimated by multiplying basal area by stand height and a form factor: Volume (m³/ha) ≈ BA × Lorey's height × form factor. Alternatively, regression equations specific to species and region use BA and dominant height as predictors. Field measurement uses a basal area prism (wedge prism) or angle gauge to rapidly estimate BA without measuring every tree.

Timber volume is the total wood volume currently standing in a forest — a static measure at one point in time, usually expressed as merchantable volume in m³/ha above a minimum diameter. Timber yield refers to the volume that can be harvested over a period of time — a dynamic production concept incorporating growth and removal. Sustainable yield means harvesting no more than the net annual increment (growth minus natural mortality) so that volume is maintained or increased over time.